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The
syntax In linguistics, syntax ( ) is the study of how words and morphemes combine to form larger units such as phrases and sentences. Central concerns of syntax include word order, grammatical relations, hierarchical sentence structure (constituenc ...
of
JavaScript JavaScript (), often abbreviated as JS, is a programming language and core technology of the World Wide Web, alongside HTML and CSS. Ninety-nine percent of websites use JavaScript on the client side for webpage behavior. Web browsers have ...
is the set of rules that define a correctly structured JavaScript program. The examples below make use of the log function of the console object present in most browsers for standard text output. The JavaScript
standard library In computer programming, a standard library is the library (computing), library made available across Programming language implementation, implementations of a programming language. Often, a standard library is specified by its associated program ...
lacks an official standard text output function (with the exception of document.write). Given that JavaScript is mainly used for client-side scripting within modern
web browser A web browser, often shortened to browser, is an application for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's scr ...
s, and that almost all Web browsers provide the alert function, alert can also be used, but is not commonly used.


Origins

Brendan Eich Brendan Eich ( ; born July 4, 1961) is an American computer programmer and technology executive. He created the JavaScript programming language and co-founded the Mozilla project, the Mozilla Foundation, and the Mozilla Corporation. He serve ...
summarized the ancestry of the syntax in the first paragraph of the JavaScript 1.1 specification as follows:


Basics


Case sensitivity

JavaScript is case sensitive. It is common to start the name of a constructor with a capitalized letter, and the name of a function or variable with a lower-case letter. Example: var a = 5; console.log(a); // 5 console.log(A); // throws a ReferenceError: A is not defined


Whitespace and semicolons

Unlike in C, whitespace in JavaScript source can directly impact
semantics Semantics is the study of linguistic Meaning (philosophy), meaning. It examines what meaning is, how words get their meaning, and how the meaning of a complex expression depends on its parts. Part of this process involves the distinction betwee ...
. Semicolons end statements in JavaScript. Because of automatic semicolon insertion (ASI), some statements that are well formed when a newline is parsed will be considered complete, as if a semicolon were inserted just prior to the newline. Some authorities advise supplying statement-terminating semicolons explicitly, because it may lessen unintended effects of the automatic semicolon insertion. There are two issues: five tokens can either begin a statement or be the extension of a complete statement; and five restricted productions, where line breaks are not allowed in certain positions, potentially yielding incorrect parsing. The five problematic tokens are the open parenthesis "(", open bracket " /", plus "+", and minus "-". Of these, the open parenthesis is common in the /code>", slash "/", plus "+", and minus "-". Of these, the open parenthesis is common in the immediately invoked function expression pattern, and open bracket occurs sometimes, while others are quite rare. An example: a = b + c (d + e).foo() // Treated as: // a = b + c(d + e).foo(); with the suggestion that the preceding statement be terminated with a semicolon. Some suggest instead the use of ''leading'' semicolons on lines starting with '(' or '[', so the line is not accidentally joined with the previous one. This is known as a defensive semicolon, and is particularly recommended, because code may otherwise become ambiguous when it is rearranged. For example: a = b + c ;(d + e).foo() // Treated as: // a = b + c; // (d + e).foo(); Initial semicolons are also sometimes used at the start of JavaScript libraries, in case they are appended to another library that omits a trailing semicolon, as this can result in ambiguity of the initial statement. The five restricted productions are return, throw, break, continue, and post-increment/decrement. In all cases, inserting semicolons does not fix the problem, but makes the parsed syntax clear, making the error easier to detect. return and throw take an optional value, while break and continue take an optional label. In all cases, the advice is to keep the value or label on the same line as the statement. This most often shows up in the return statement, where one might return a large object literal, which might be accidentally placed starting on a new line. For post-increment/decrement, there is potential ambiguity with pre-increment/decrement, and again it is recommended to simply keep these on the same line. return a + b; // Returns undefined. Treated as: // return; // a + b; // Should be written as: // return a + b;


Comments

Comment syntax is the same as in Comment (computer programming)">Comment syntax is the same as in C++, Swift (programming language)">Swift Swift or SWIFT most commonly refers to: * SWIFT, an international organization facilitating transactions between banks ** SWIFT code * Swift (programming language) * Swift (bird), a family of birds It may also refer to: Organizations * SWIF ...
and many other languages. // a short, one-line comment /* this is a long, multi-line comment about my script. May it one day be great. */ /* Comments /* may not be nested */ Syntax error */


Variables

Variables in standard JavaScript have no Type system">type attached, so any value (each ''value'' has a type) can be stored in any variable. Starting with ES6, the 6th version of the language, variables could be declared with var for function scoped variables, and let or const which are for
block level variables. Before ES6, variables could only be declared with a var statement. Values assigned to variables declared with const cannot be changed, but their properties can. var should no longer be used since let and const are supported by modern browsers. A variable's Identifier (computer languages)">identifier An identifier is a name that identifies (that is, labels the identity of) either a unique object or a unique ''class'' of objects, where the "object" or class may be an idea, person, physical countable object (or class thereof), or physical mass ...
must start with a letter, underscore (_), or dollar sign ($), while subsequent characters can also be digits (0-9). JavaScript is case sensitive, so the uppercase characters "A" through "Z" are different from the lowercase characters "a" through "z". Starting with JavaScript 1.5, ISO 8859-1 or Unicode letters (or \uXXXX Unicode escape sequences) can be used in identifiers. In certain JavaScript implementations, the at sign (@) can be used in an identifier, but this is contrary to the specifications and not supported in newer implementations.


Scoping and hoisting

Variables declared with var are lexically scoped at a function level, while ones with let or const have a block level scope. Since declarations are processed before any code is executed, a variable can be assigned to and used prior to being declared in the code. This is referred to as ', and it is equivalent to variables being forward declared at the top of the function or block. With var, let, and const statements, only the declaration is hoisted; assignments are not hoisted. Thus a statement in the middle of the function is equivalent to a declaration statement at the top of the function, and an assignment statement at that point in the middle of the function. This means that values cannot be accessed before they are declared; forward reference is not possible. With var a variable's value is undefined until it is initialized. Variables declared with let or const cannot be accessed until they have been initialized, so referencing such variables before will cause an error. Function declarations, which declare a variable and assign a function to it, are similar to variable statements, but in addition to hoisting the declaration, they also hoist the assignment – as if the entire statement appeared at the top of the containing function – and thus forward reference is also possible: the location of a function statement within an enclosing function is irrelevant. This is different from a function expression being assigned to a variable in a var, let, or const statement. So, for example, var func = function() // declaration is hoisted only function func() // declaration and assignment are hoisted Block scoping can be produced by wrapping the entire block in a function and then executing it – this is known as the immediately-invoked function expression pattern – or by declaring the variable using the let keyword.


Declaration and assignment

Variables declared outside a scope are
global Global may refer to: General *Globe, a spherical model of celestial bodies *Earth, the third planet from the Sun Entertainment * ''Global'' (Paul van Dyk album), 2003 * ''Global'' (Bunji Garlin album), 2007 * ''Global'' (Humanoid album), 198 ...
. If a variable is declared in a higher scope, it can be accessed by child scopes. When JavaScript tries to resolve an identifier, it looks in the local scope. If this identifier is not found, it looks in the next outer scope, and so on along the ''scope chain'' until it reaches the ''global scope'' where global variables reside. If it is still not found, JavaScript will raise a ReferenceError exception. When assigning an identifier, JavaScript goes through exactly the same process to retrieve this identifier, except that if it is not found in the ''global scope'', it will create the "variable" in the scope where it was created. As a consequence, a variable never declared will be global, if assigned. Declaring a variable (with the keyword var) in the ''global scope'' (i.e. outside of any function body (or block in the case of let/const)), assigning a never declared identifier or adding a property to the ''global object'' (usually ''window'') will also create a new global variable. Note that JavaScript's ''strict mode'' forbids the assignment of an undeclared variable, which avoids global namespace pollution.


Examples

Here are some examples of variable declarations and scope: var x1 = 0; // A global variable, because it is not in any function let x2 = 0; // Also global, this time because it is not in any block function f() f(); console.log(z); // This line will raise a ReferenceError exception, because the value of z is no longer available for (let i = 0; i < 10; i++) console.log(i); console.log(i); // throws a ReferenceError: i is not defined for (const i = 0; i < 10; i++) console.log(i); // throws a TypeError: Assignment to constant variable for (const i of ,2,3 console.log(i); //will not raise an exception. i is not reassigned but recreated in every iteration const pi; // throws a SyntaxError: Missing initializer in const declaration


Primitive data types

The JavaScript language provides six primitive data types: * Undefined * Number * BigInt * String * Boolean * Symbol Some of the primitive data types also provide a set of named values that represent the extents of the type boundaries. These named values are described within the appropriate sections below.


Undefined

The value of "undefined" is assigned to all
uninitialized variable In computing, an uninitialized variable is a variable (programming), variable that is declared but is not set to a definite known value before it is used. It will have ''some'' value, but not a predictable one. As such, it is a programming error an ...
s, and is also returned when checking for object properties that do not exist. In a Boolean context, the undefined value is considered a false value. Note: undefined is considered a genuine primitive type. Unless explicitly converted, the undefined value may behave unexpectedly in comparison to other types that evaluate to false in a logical context. let test; // variable declared, but not defined, ... // ... set to value of undefined const testObj = ; console.log(test); // test variable exists, but value not ... // ... defined, displays undefined console.log(testObj.myProp); // testObj exists, property does not, ... // ... displays undefined console.log(undefined

null); // unenforced type during check, displays true console.log(undefined

null); // enforce type during check, displays false Note: There is no built-in language literal for undefined. Thus is not a foolproof way to check whether a variable is undefined, because in versions before ECMAScript 5, it is legal for someone to write . A more robust approach is to compare using . Functions like this will not work as expected: function isUndefined(x) // like this... function isUndefined(x) // ... or that second one function isUndefined(x) // ... or that third one Here, calling isUndefined(my_var) raises a if is an unknown identifier, whereas does not.


Number

Numbers are represented in binary as
IEEE 754 The IEEE Standard for Floating-Point Arithmetic (IEEE 754) is a technical standard for floating-point arithmetic originally established in 1985 by the Institute of Electrical and Electronics Engineers (IEEE). The standard #Design rationale, add ...
floating point doubles. Although this format provides an accuracy of nearly 16 significant digits, it cannot always exactly represent real numbers, including fractions. This becomes an issue when comparing or formatting numbers. For example: console.log(0.2 + 0.1

0.3); // displays false console.log(0.94 - 0.01); // displays 0.9299999999999999 As a result, a routine such as the method should be used to round numbers whenever they ar
formatted for output
Numbers may be specified in any of these notations: 345; // an "integer", although there is only one numeric type in JavaScript 34.5; // a floating-point number 3.45e2; // another floating-point, equivalent to 345 0b1011; // a binary integer equal to 11 0o377; // an octal integer equal to 255 0xFF; // a hexadecimal integer equal to 255, digits represented by the ... // ... letters A-F may be upper or lowercase There is also a numeric separator, (the underscore), introduced in ES2021: // Note: Wikipedia syntax does not support numeric separators yet 1_000_000_000; // Used with big numbers 1_000_000.5; // Support with decimals 1_000e1_000; // Support with exponents // Support with binary, octals and hex 0b0000_0000_0101_1011; 0o0001_3520_0237_1327; 0xFFFF_FFFF_FFFF_FFFE; // But users cannot use them next to a non-digit number part, or at the start or end _12; // Variable is not defined (the underscore makes it a variable identifier) 12_; // Syntax error (cannot be at the end of numbers) 12_.0; // Syntax error (does not make sense to put a separator next to the decimal point) 12._0; // Syntax error 12e_6; // Syntax error (next to "e", a non-digit. Does not make sense to put a separator at the start) 1000____0000; // Syntax error (next to "_", a non-digit. Only 1 separator at a time is allowed The extents +∞, −∞ and NaN (Not a Number) of the number type may be obtained by two program expressions: Infinity; // positive infinity (negative obtained with -Infinity for instance) NaN; // The Not-A-Number value, also returned as a failure in ... // ... string-to-number conversions Infinity and NaN are numbers: typeof Infinity; // returns "number" typeof NaN; // returns "number" These three special values correspond and behave as the IEEE-754 describes them. The Number constructor (used as a function), or a unary + or -, may be used to perform explicit numeric conversion: const myString = "123.456"; const myNumber1 = Number(myString); const myNumber2 = +myString; When used as a constructor, a numeric ''wrapper'' object is created (though it is of little use): const myNumericWrapper = new Number(123.456); However, NaN is not equal to itself: const nan = NaN; console.log(NaN

NaN); // false console.log(NaN

NaN); // false console.log(NaN !

NaN); // true console.log(nan !

nan); // true // Users can use the isNaN methods to check for NaN console.log(isNaN("converted to NaN")); // true console.log(isNaN(NaN)); // true console.log(Number.isNaN("not converted")); // false console.log(Number.isNaN(NaN)); // true


BigInt

BigInts can be used for arbitrarily large
integer An integer is the number zero (0), a positive natural number (1, 2, 3, ...), or the negation of a positive natural number (−1, −2, −3, ...). The negations or additive inverses of the positive natural numbers are referred to as negative in ...
s. Especially whole numbers larger than 253 - 1, which is the largest number JavaScript can reliably represent with the Number primitive and represented by the Number.MAX_SAFE_INTEGER constant. When dividing BigInts, the results are truncated.


String

A string in JavaScript is a sequence of characters. In JavaScript, strings can be created directly (as literals) by placing the series of characters between double (") or single (') quotes. Such strings must be written on a single line, but may include escaped newline characters (such as \n). The JavaScript standard allows the backquote character (`, a.k.a. grave accent or backtick) to quote multiline literal strings, as well as embedded expressions using the syntax $. const greeting = "Hello, World!"; const anotherGreeting = 'Greetings, people of Earth.'; const aMultilineGreeting = `Warm regards, John Doe.` // Template literals type-coerce evaluated expressions and interpolate them into the string. const templateLiteral = `This is what is stored in anotherGreeting: $.`; console.log(templateLiteral); // 'This is what is stored in anotherGreeting: 'Greetings, people of Earth.'' console.log(`You are $ to view this web page`); Individual characters within a string can be accessed using the method (provided by ). This is the preferred way when accessing individual characters within a string, because it also works in non-modern browsers: const h = greeting.charAt(0); In modern browsers, individual characters within a string can be accessed (as strings with only a single character) through the same notation as arrays: const h = greeting However, JavaScript strings are immutable: greeting = "H"; // Fails. Applying the equality operator ("

") to two strings returns true, if the strings have the same contents, which means: of the same length and containing the same sequence of characters (case is significant for alphabets). Thus: const x = "World"; const compare1 = ("Hello, " + x

"Hello, World"); // Here compare1 contains true. const compare2 = ("Hello, " + x

"hello, World"); // Here compare2 contains ... // ... false since the ... // ... first characters ... // ... of both operands ... // ... are not of the same case. Quotes of the same type cannot be nested unless they are escaped. let x = '"Hello, World!" he said.'; // Just fine. x = ""Hello, World!" he said."; // Not good. x = "\"Hello, World!\" he said."; // Works by escaping " with \" The constructor creates a string object (an object wrapping a string): const greeting = new String("Hello, World!"); These objects have a method returning the primitive string wrapped within them: const s = new String("Hello !"); typeof s; // Is 'object'. typeof s.valueOf(); // Is 'string'. Equality between two objects does not behave as with string primitives: const s1 = new String("Hello !"); const s2 = new String("Hello !"); s1

s2; // Is false, because they are two distinct objects. s1.valueOf()

s2.valueOf(); // Is true.


Boolean

JavaScript JavaScript (), often abbreviated as JS, is a programming language and core technology of the World Wide Web, alongside HTML and CSS. Ninety-nine percent of websites use JavaScript on the client side for webpage behavior. Web browsers have ...
provides a
Boolean data type In computer science, the Boolean (sometimes shortened to Bool) is a data type that has one of two possible values (usually denoted ''true'' and ''false'') which is intended to represent the two truth values of logic and Boolean algebra. It is na ...
with and literals. The operator returns the string for these
primitive types In computer science, primitive data types are a set of basic data types from which all other data types are constructed. Specifically it often refers to the limited set of data representations in use by a particular processor, which all compiled ...
. When used in a logical context, , , , , , and the empty string () evaluate as due to automatic type conversion. All other values (the complement of the previous list) evaluate as , including the strings , and any object.


Type conversion

Automatic type coercion by the equality comparison operators (

 and !=) can be avoided by using the type checked comparison operators (

 and !

). When type conversion is required, JavaScript converts , , , or operands as follows: ;: The string is converted to a number value. JavaScript attempts to convert the string numeric literal to a Number type value. First, a mathematical value is derived from the string numeric literal. Next, this value is rounded to nearest Number type value. ;: If one of the operands is a Boolean, the Boolean operand is converted to 1 if it is , or to 0 if it is . ;: If an object is compared with a number or string, JavaScript attempts to return the default value for the object. An object is converted to a primitive String or Number value, using the or methods of the object. If this fails, a runtime error is generated.


Boolean type conversion

Douglas Crockford advocates the terms "truthy" and "falsy" to describe how values of various types behave when evaluated in a logical context, especially in regard to edge cases. The binary logical operators returned a Boolean value in early versions of JavaScript, but now they return one of the operands instead. The left–operand is returned, if it can be evaluated as : , in the case of conjunction: (a && b), or , in the case of
disjunction In logic, disjunction (also known as logical disjunction, logical or, logical addition, or inclusive disjunction) is a logical connective typically notated as \lor and read aloud as "or". For instance, the English language sentence "it is ...
: (a , , b); otherwise the right–operand is returned. Automatic type coercion by the comparison operators may differ for cases of mixed Boolean and number-compatible operands (including strings that can be evaluated as a number, or objects that can be evaluated as such a string), because the Boolean operand will be compared as a numeric value. This may be unexpected. An expression can be explicitly cast to a Boolean primitive by doubling the logical negation operator: (), using the function, or using the conditional operator: (c ? t : f). // Automatic type coercion console.log(true

2 ); // false... true → 1 !

2 ← 2 console.log(false

2 ); // false... false → 0 !

2 ← 2 console.log(true

1 ); // true.... true → 1

1 ← 1 console.log(false

0 ); // true.... false → 0

0 ← 0 console.log(true

"2"); // false... true → 1 !

2 ← "2" console.log(false

"2"); // false... false → 0 !

2 ← "2" console.log(true

"1"); // true.... true → 1

1 ← "1" console.log(false

"0"); // true.... false → 0

0 ← "0" console.log(false

"" ); // true.... false → 0

0 ← "" console.log(false

NaN); // false... false → 0 !

NaN console.log(NaN

NaN); // false...... NaN is not equivalent to anything, including NaN. // Type checked comparison (no conversion of types and values) console.log(true

1); // false...... data types do not match // Explicit type coercion console.log(true

!!2); // true.... data types and values match console.log(true

!!0); // false... data types match, but values differ console.log( 1 ? true : false); // true.... only ±0 and NaN are "falsy" numbers console.log("0" ? true : false); // true.... only the empty string is "falsy" console.log(Boolean()); // true.... all objects are "truthy" The new operator can be used to create an object wrapper for a Boolean primitive. However, the operator does not return for the object wrapper, it returns . Because all objects evaluate as , a method such as , or , must be used to retrieve the wrapped value. For explicit coercion to the Boolean type, Mozilla recommends that the function (without ) be used in preference to the Boolean object. const b = new Boolean(false); // Object false const t = Boolean(b); // Boolean true const f = Boolean(b.valueOf()); // Boolean false let n = new Boolean(b); // Not recommended n = new Boolean(b.valueOf()); // Preferred if (0 , , -0 , , "" , , null , , undefined , , b.valueOf() , , !new Boolean() , , !t) else if ([] && && b && typeof b

"object" && b.toString()

"false")


Symbol

New in ECMAScript6. A ''Symbol'' is a unique and immutable identifier. Example: let x = Symbol(1); const y = Symbol(1); x

y; // => false const symbolObject = ; const normalObject = ; // since x and y are unique, // they can be used as unique keys in an object symbolObject = 1; symbolObject = 2; symbolObject // => 1 symbolObject // => 2 // as compared to normal numeric keys normalObject = 1; normalObject = 2; // overrides the value of 1 normalObject // => 2 // changing the value of x does not change the key stored in the object x = Symbol(3); symbolObject // => undefined // changing x back just creates another unique Symbol x = Symbol(1); symbolObject // => undefined There are also ''well known symbols''. One of which is Symbol.iterator; if something implements Symbol.iterator, it is iterable: const x = , 2, 3, 4 // x is an Array x ymbol.iterator

Array.prototype ymbol.iterator // and Arrays are iterable const xIterator = x ymbol.iterator); // The ymbol.iteratorfunction should provide an iterator for x xIterator.next(); // xIterator.next(); // xIterator.next(); // xIterator.next(); // xIterator.next(); // xIterator.next(); // // for..of loops automatically iterate values for (const value of x) // Sets are also iterable: ymbol.iteratorin Set.prototype; // true for (const value of new Set( apple', 'orange')


Native objects

The JavaScript language provides a handful of native objects. JavaScript native objects are considered part of the JavaScript specification. JavaScript environment notwithstanding, this set of objects should always be available.


Array

An Array is a JavaScript object prototyped from the Array constructor specifically designed to store data values indexed by integer keys. Arrays, unlike the basic Object type, are prototyped with methods and properties to aid the programmer in routine tasks (for example, join, slice, and push). As in the C family, arrays use a zero-based indexing scheme: A value that is inserted into an empty array by means of the push method occupies the 0th index of the array. const myArray = []; // Point the variable myArray to a newly ... // ... created, empty Array myArray.push("hello World"); // Fill the next empty index, in this case 0 console.log(myArray ; // Equivalent to console.log("hello World"); Arrays have a length property that is guaranteed to always be larger than the largest integer index used in the array. It is automatically updated, if one creates a property with an even larger index. Writing a smaller number to the length property will remove larger indices. Elements of Arrays may be accessed using normal object property access notation: myArray // the 2nd item in myArray myArray 1" The above two are equivalent. It is not possible to use the "dot"-notation or strings with alternative representations of the number: myArray.1; // syntax error myArray 01" // not the same as myArray Declaration of an array can use either an Array literal or the Array constructor: let myArray; // Array literals myArray = , 2 // length of 2 myArray = , 2, // same array - Users can also have an extra comma at the end // It is also possible to not fill in parts of the array myArray = , 1, /* hole */, /* hole */, 4, 5 // length of 6 myArray = , 1, /* hole */, /* hole */, 4, 5, // same array myArray = , 1, /* hole */, /* hole */, 4, 5, /* hole */, // length of 7 // With the constructor myArray = new Array(0, 1, 2, 3, 4, 5); // length of 6 myArray = new Array(365); // an empty array with length 365
Arrays An array is a systematic arrangement of similar objects, usually in rows and columns. Things called an array include: {{TOC right Music * In twelve-tone and serial composition, the presentation of simultaneous twelve-tone sets such that the ...
are implemented so that only the defined elements use memory; they are " sparse arrays". Setting and only uses space for these two elements, just like any other object. The length of the array will still be reported as 58. The maximum length of an array is 4,294,967,295 which corresponds to 32-bit binary number (11111111111111111111111111111111)2. One can use the object declaration literal to create objects that behave much like associative arrays in other languages: const dog = ; dog color" // results in "brown" dog.color; // also results in "brown" One can use the object and array declaration literals to quickly create arrays that are associative, multidimensional, or both. (Technically, JavaScript does not support multidimensional arrays, but one can mimic them with arrays-of-arrays.) const cats = cats "size"]; // results in "large" const dogs = ; dogs spot""size"]; // results in "small" dogs.rover.color; // results in "brown"


Date

A Date object stores a signed millisecond count with zero representing 1970-01-01 00:00:00 UT and a range of ±108 days. There are several ways of providing arguments to the Date constructor. Note that months are zero-based. new Date(); // create a new Date instance representing the current time/date. new Date(2010, 2, 1); // create a new Date instance representing 2010-Mar-01 00:00:00 new Date(2010, 2, 1, 14, 25, 30); // create a new Date instance representing 2010-Mar-01 14:25:30 new Date("2010-3-1 14:25:30"); // create a new Date instance from a String. Methods to extract fields are provided, as well as a useful toString: const d = new Date(2010, 2, 1, 14, 25, 30); // 2010-Mar-01 14:25:30; // Displays '2010-3-1 14:25:30': console.log(d.getFullYear() + '-' + (d.getMonth() + 1) + '-' + d.getDate() + ' ' + d.getHours() + ':' + d.getMinutes() + ':' + d.getSeconds()); // Built-in toString returns something like 'Mon 1 March, 2010 14:25:30 GMT-0500 (EST)': console.log(d);


Error

Custom error messages can be created using the Error class: throw new Error("Something went wrong."); These can be caught by try...catch...finally blocks as described in the section on #Exception_handling, exception handling.


Math

The object contains various math-related constants (for example, ) and functions (for example, cosine). (Note that the object has no constructor, unlike or . All its methods are "static", that is "class" methods.) All the trigonometric functions use angles expressed in
radian The radian, denoted by the symbol rad, is the unit of angle in the International System of Units (SI) and is the standard unit of angular measure used in many areas of mathematics. It is defined such that one radian is the angle subtended at ...
s, not degrees or grads.


Regular expression

/expression/.test(string); // returns Boolean "string".search(/expression/); // returns position Number "string".replace(/expression/, replacement); // Here are some examples if (/Tom/.test("My name is Tom")) console.log("Hello Tom!"); console.log("My name is Tom".search(/Tom/)); //

11 (letters before Tom) console.log("My name is Tom".replace(/Tom/, "John")); //

"My name is John"


Character classes

// \d - digit // \D - non digit // \s - space // \S - non space // \w - word char // \W - non word // - one of // - one not of // - - range if (/\d/.test('0')) console.log('Digit'); if (/ -9.test('6')) console.log('Digit'); if (/ 3579.test('1')) console.log('Odd number'); if (/\S\S\s\S\S\S\S/.test('My name')) console.log('Format OK'); if (/\w\w\w/.test('Tom')) console.log('Hello Tom'); if (/ -zA-Z.test('B')) console.log('Letter');


Character matching

// A...Z a...z 0...9 - alphanumeric // \u0000...\uFFFF - Unicode hexadecimal // \x00...\xFF - ASCII hexadecimal // \t - tab // \n - new line // \r - CR // . - any character // , - OR if (/T.m/.test('Tom')) console.log ('Hi Tom, Tam or Tim'); if (/A, B/.test("A")) console.log ('A or B');


Repeaters

// ? - 0 or 1 match // * - 0 or more // + - 1 or more // - exactly n // - n or more // - n or less // - range n to m if (/ab?c/.test("ac")) console.log("OK"); // match: "ac", "abc" if (/ab*c/.test("ac")) console.log("OK"); // match: "ac", "abc", "abbc", "abbbc" etc. if (/ab+c/.test("abc")) console.log("OK"); // match: "abc", "abbc", "abbbc" etc. if (/abc/.test("abbbc")) console.log("OK"); // match: "abbbc" if (/abc/.test("abbbc")) console.log("OK"); // match: "abbbc", "abbbbc", "abbbbbc" etc. if (/abc/.test("abc")) console.log("OK"); // match: "abc", "abbc", "abbbc"


Anchors

// ^ - string starts with // $ - string ends with if (/^My/.test("My name is Tom")) console.log ("Hi!"); if (/Tom$/.test("My name is Tom")) console.log ("Hi Tom!");


Subexpression

// ( ) - groups characters if (/water(mark)?/.test("watermark")) console.log("Here is water!"); // match: "water", "watermark", if (/(Tom), (John)/.test("John")) console.log("Hi Tom or John!");


Flags

// /g - global // /i - ignore upper/lower case // /m - allow matches to span multiple lines console.log("hi tom!".replace(/Tom/i, "John")); //

"hi John!" console.log("ratatam".replace(/ta/, "tu")); //

"ratutam" console.log("ratatam".replace(/ta/g, "tu")); //

"ratutum"


Advanced methods

my_array = my_string.split(my_delimiter); // example my_array = "dog,cat,cow".split(","); // my_array

dog","cat","cow" my_array = my_string.match(my_expression); // example my_array = "We start at 11:30, 12:15 and 16:45".match(/\d\d:\d\d/g); // my_array

11:30","12:15","16:45"


Capturing groups

const myRe = /(\d-\d-\d) (\d:\d:\d)/; const results = myRe.exec("The date and time are 2009-09-08 09:37:08."); if (results) else console.log("Did not find a valid date!");


Function

Every function in JavaScript is an instance of the Function constructor: // x, y is the argument. 'return x + y' is the function body, which is the last in the argument list. const add = new Function('x', 'y', 'return x + y'); add(1, 2); // => 3 The add function above may also be defined using a function expression: const add = function(x, y) ; add(1, 2); // => 3 In ES6, arrow function syntax was added, allowing functions that return a value to be more concise. They also retain the this of the global object instead of inheriting it from where it was called / what it was called on, unlike the function() expression. const add = (x, y) => ; // values can also be implicitly returned (i.e. no return statement is needed) const addImplicit = (x, y) => x + y; add(1, 2); // => 3 addImplicit(1, 2) // => 3 For functions that need to be hoisted, there is a separate expression: function add(x, y) add(1, 2); // => 3 Hoisting allows users to use the function before it is "declared": add(1, 2); // => 3, not a ReferenceError function add(x, y) A function instance has properties and methods. function subtract(x, y) console.log(subtract.length); // => 2, arity of the function (number of arguments) console.log(subtract.toString()); /* "function subtract(x, y) " */


Operators

The '+' operator is overloaded: it is used for string concatenation and arithmetic addition. This may cause problems when inadvertently mixing strings and numbers. As a unary operator, it can convert a numeric string to a number. // Concatenate 2 strings console.log('He' + 'llo'); // displays Hello // Add two numbers console.log(2 + 6); // displays 8 // Adding a number and a string results in concatenation (from left to right) console.log(2 + '2'); // displays 22 console.log('$' + 3 + 4); // displays $34, but $7 may have been expected console.log('$' + (3 + 4)); // displays $7 console.log(3 + 4 + '7'); // displays 77, numbers stay numbers until a string is added // Convert a string to a number using the unary plus console.log(+'2'

2); // displays true console.log(+'Hello'); // displays NaN Similarly, the '*' operator is overloaded: it can convert a string into a number. console.log(2 + '6'*1); // displays 8 console.log(3*'7'); // 21 console.log('3'*'7'); // 21 console.log('hello'*'world'); // displays NaN


Arithmetic

JavaScript supports the following binary arithmetic operators: JavaScript supports the following unary arithmetic operators: let x = 1; console.log(++x); // x becomes 2; displays 2 console.log(x++); // displays 2; x becomes 3 console.log(x); // x is 3; displays 3 console.log(x--); // displays 3; x becomes 2 console.log(x); // displays 2; x is 2 console.log(--x); // x becomes 1; displays 1 The modulo operator displays the remainder after division by the modulus. If negative numbers are involved, the returned value depends on the operand. const x = 17; console.log(x%5); // displays 2 console.log(x%6); // displays 5 console.log(-x%5); // displays -2 console.log(-x%-5); // displays -2 console.log(x%-5); // displays 2 To always return a non-negative number, users can re-add the modulus and apply the modulo operator again: const x = 17; console.log((-x%5+5)%5); // displays 3 Users could also do: const x = 17; console.log(Math.abs(-x%5)); // also 3


Assignment

Assignment of primitive types let x = 9; x += 1; console.log(x); // displays: 10 x *= 30; console.log(x); // displays: 300 x /= 6; console.log(x); // displays: 50 x -= 3; console.log(x); // displays: 47 x %= 7; console.log(x); // displays: 5 Assignment of object types /** * To learn JavaScript objects... */ const object_1 = ; // assign reference of newly created object to object_1 let object_2 = ; let object_3 = object_2; // object_3 references the same object as object_2 does object_3.a = 2; message(); // displays 1 2 2 object_2 = object_1; // object_2 now references the same object as object_1 // object_3 still references what object_2 referenced before message(); // displays 1 1 2 object_2.a = 7; // modifies object_1 message(); // displays 7 7 2 object_3.a = 5; // object_3 does not change object_2 message(); // displays 7 7 5 object_3 = object_2; object_3.a=4; // object_3 changes object_1 and object_2 message(); // displays 4 4 4 /** * Prints the console.log message */ function message()


Destructuring assignment

In Mozilla's JavaScript, since version 1.7, destructuring assignment allows the assignment of parts of data structures to several variables at once. The left hand side of an assignment is a pattern that resembles an arbitrarily nested object/array literal containing l-lvalues at its leaves that are to receive the substructures of the assigned value. let a, b, c, d, e; , b, c= , 4, 5 console.log(`$,$,$`); // displays: 3,4,5 e = ; const arr = []; ( = e); console.log(`$,$,$`); // displays: 5,6,Baz,,,Content [a, b] = [b, a]; // swap contents of a and b console.log(a + ',' + b); // displays: 6,5 , b, c= , 4, 5 // permutations , b, c= , c, a console.log(`$,$,$`); // displays: 4,5,3


Spread/rest operator

The ECMAScript 2015 standard introduced the "..." array operator, for the related concepts of "spread syntax" and "rest parameters". Object spreading was added in ECMAScript 2018. Spread syntax provides another way to destructure arrays and objects. For arrays, it indicates that the elements should be used as the parameters in a function call or the items in an array literal. For objects, it can be used for merging objects together or overriding properties. In other words, "..." transforms " ..foo/code>" into " oo[0 foo[1">.html" ;"title="oo[0">oo[0 foo[1 foo[2">">oo[0<_a>_foo[1.html" ;"title=".html" ;"title="oo[0">oo[0 foo[1">.html" ;"title="oo[0">oo[0 foo[1 foo[2", and "this.bar(...foo);" into "this.bar(foo foo foo[2]);", and "" into . const a = , 2, 3, 4 // It can be used multiple times in the same expression const b = ..a, ...a // b = , 2, 3, 4, 1, 2, 3, 4 // It can be combined with non-spread items. const c = , 6, ...a, 7, 9 // c = , 6, 1, 2, 3, 4, 7, 9 // For comparison, doing this without the spread operator // creates a nested array. const d = , a // d = 1, 2, 3, 4 , 2, 3, 4 // It works the same with function calls function foo(arg1, arg2, arg3) // Users can use it even if it passes more parameters than the function will use foo(...a); // "1:2:3" → foo(a a a a ; // Users can mix it with non-spread parameters foo(5, ...a, 6); // "5:1:2" → foo(5, a a a a 6); // For comparison, doing this without the spread operator // assigns the array to arg1, and nothing to the other parameters. foo(a); // "1,2,3,4:undefined:undefined" const bar = ; // This would copy the object const copy = ; // copy = ; // "b" would be overridden here const override = ; // override = When ... is used in a function ''declaration'', it indicates a rest parameter. The rest parameter must be the final named parameter in the function's parameter list. It will be assigned an Array containing any arguments passed to the function in excess of the other named parameters. In other words, it gets "the rest" of the arguments passed to the function (hence the name). function foo(a, b, ...c) foo(1, 2, 3, 4, 5); // "3" → c = , 4, 5foo('a', 'b'); // "0" → c = [] Rest parameters are similar to Javascript's arguments object, which is an array-like object that contains all of the parameters (named and unnamed) in the current function call. Unlike arguments, however, rest parameters are true Array objects, so methods such as .slice() and .sort() can be used on them directly.


Comparison

Variables referencing objects are equal or identical only if they reference the same object: const obj1 = ; const obj2 = ; const obj3 = obj1; console.log(obj1

obj2); //false console.log(obj3

obj1); //true console.log(obj3

obj1); //true See also String.


Logical

JavaScript provides four logical operators: * unary
negation In logic, negation, also called the logical not or logical complement, is an operation (mathematics), operation that takes a Proposition (mathematics), proposition P to another proposition "not P", written \neg P, \mathord P, P^\prime or \over ...
(NOT = !a) * binary
disjunction In logic, disjunction (also known as logical disjunction, logical or, logical addition, or inclusive disjunction) is a logical connective typically notated as \lor and read aloud as "or". For instance, the English language sentence "it is ...
(OR = a , , b) and conjunction (AND = a && b) * ternary conditional (c ? t : f) In the context of a logical operation, any expression evaluates to true except the following: * Strings: "", '', * Numbers: 0, -0, NaN, * Special: null, undefined, * Boolean: false. The Boolean function can be used to explicitly convert to a primitive of type Boolean: // Only empty strings return false console.log(Boolean("")

false); console.log(Boolean("false")

true); console.log(Boolean("0")

true); // Only zero and NaN return false console.log(Boolean(NaN)

false); console.log(Boolean(0)

false); console.log(Boolean(-0)

false); // equivalent to -1*0 console.log(Boolean(-2)

true); // All objects return true console.log(Boolean(this)

true); console.log(Boolean()

true); console.log(Boolean([])

true); // These types return false console.log(Boolean(null)

false); console.log(Boolean(undefined)

false); // equivalent to Boolean() The NOT operator evaluates its operand as a Boolean and returns the negation. Using the operator twice in a row, as a double negative, explicitly converts an expression to a primitive of type Boolean: console.log( !0

Boolean(!0)); console.log(Boolean(!0)

!!1); console.log(!!1

Boolean(1)); console.log(!!0

Boolean(0)); console.log(Boolean(0)

!1); console.log(!1

Boolean(!1)); console.log(!""

Boolean(!"")); console.log(Boolean(!"")

!!"s"); console.log(!!"s"

Boolean("s")); console.log(!!""

Boolean("")); console.log(Boolean("")

!"s"); console.log(!"s"

Boolean(!"s")); The ternary operator can also be used for explicit conversion: console.log([]

false); console.log([] ? true : false); // “truthy”, but the comparison uses [].toString() console.log(

false); console.log( true : false); // toString()

"0" console.log("0"

false); console.log("0"? true : false); // "0" → 0 ... (0

0) ... 0 ← false console.log(

true); console.log( true : false); // toString()

"1" console.log("1"

true); console.log("1"? true : false); // "1" → 1 ... (1

1) ... 1 ← true console.log( != true); console.log( true : false); // toString()

"2" console.log("2" != true); console.log("2"? true : false); // "2" → 2 ... (2 != 1) ... 1 ← true Expressions that use features such as post–incrementation (i++) have an anticipated
side effect In medicine, a side effect is an effect of the use of a medicinal drug or other treatment, usually adverse but sometimes beneficial, that is unintended. Herbal and traditional medicines also have side effects. A drug or procedure usually use ...
. JavaScript provides short-circuit evaluation of expressions; the right operand is only executed if the left operand does not suffice to determine the value of the expression. console.log(a , , b); // When a is true, there is no reason to evaluate b. console.log(a && b); // When a is false, there is no reason to evaluate b. console.log(c ? t : f); // When c is true, there is no reason to evaluate f. In early versions of JavaScript and JScript, the binary logical operators returned a Boolean value (like most C-derived programming languages). However, all contemporary implementations return one of their operands instead: console.log(a , , b); // if a is true, return a, otherwise return b console.log(a && b); // if a is false, return a, otherwise return b Programmers who are more familiar with the behavior in C might find this feature surprising, but it allows for a more concise expression of patterns like null coalescing: const s = t , , "(default)"; // assigns t, or the default value, if t is null, empty, etc.


Logical assignment


Bitwise

JavaScript supports the following binary bitwise operators: Examples: const x = 11 & 6; console.log(x); // 2 JavaScript supports the following unary bitwise operator:


Bitwise Assignment

JavaScript supports the following binary assignment operators: Examples: let x=7; console.log(x); // 7 x<<=3; console.log(x); // 7->14->28->56


String

Examples: let str = "ab" + "cd"; // "abcd" str += "e"; // "abcde" const str2 = "2" + 2; // "22", not "4" or 4.


??


Control structures


Compound statements

A pair of curly brackets and an enclosed sequence of statements constitute a compound statement, which can be used wherever a statement can be used.


If ... else

if (expr) else if (expr2) else


Conditional (ternary) operator

The conditional operator creates an expression that evaluates as one of two expressions depending on a condition. This is similar to the ''if'' statement that selects one of two statements to execute depending on a condition. I.e., the conditional operator is to expressions what ''if'' is to statements. const result = condition ? expression : alternative; is the same as: if (condition) else Unlike the ''if'' statement, the conditional operator cannot omit its "else-branch".


Switch statement

The syntax of the JavaScript
switch statement In computer programming languages, a switch statement is a type of selection control mechanism used to allow the value of a variable or expression to change the control flow of program execution via search and map. Switch statements function ...
is as follows: switch (expr) * break; is optional; however, it is usually needed, since otherwise code execution will continue to the body of the next case block. This ''fall through'' behavior can be used when the same set of statements apply in several cases, effectively creating a
disjunction In logic, disjunction (also known as logical disjunction, logical or, logical addition, or inclusive disjunction) is a logical connective typically notated as \lor and read aloud as "or". For instance, the English language sentence "it is ...
between those cases. * Add a break statement to the end of the last case as a precautionary measure, in case additional cases are added later. * String literal values can also be used for the case values. * Expressions can be used instead of values. * The default case (optional) is executed when the expression does not match any other specified cases. * Braces are required.


For loop

The syntax of the JavaScript
for loop In computer science, a for-loop or for loop is a control flow Statement (computer science), statement for specifying iteration. Specifically, a for-loop functions by running a section of code repeatedly until a certain condition has been satisfi ...
is as follows: for (initial; condition; loop statement) or for (initial; condition; loop statement(iteration)) // one statement


For ... in loop

The syntax of the JavaScript for ... in loop is as follows: for (var property_name in some_object) * Iterates through all enumerable properties of an object. * Iterates through all used indices of array including all user-defined properties of array object, if any. Thus it may be better to use a traditional for loop with a numeric index when iterating over arrays. * There are differences between the various Web browsers with regard to which properties will be reflected with the for...in loop statement. In theory, this is controlled by an internal state property defined by the ECMAscript standard called "DontEnum", but in practice, each browser returns a slightly different set of properties during introspection. It is useful to test for a given property using . Thus, adding a method to the array prototype with may cause for ... in loops to loop over the method's name.


While loop

The syntax of the JavaScript while loop is as follows: while (condition)


Do ... while loop

The syntax of the JavaScript do ... while loop is as follows: do while (condition);


With

The with statement adds all of the given object's properties and methods into the following block's scope, letting them be referenced as if they were local variables. with (document) ; * Note the absence of before each invocation. The semantics are similar to the with statement of Pascal. Because the availability of with statements hinders program performance and is believed to reduce code clarity (since any given variable could actually be a property from an enclosing ), this statement is not allowed in ''strict mode''.


Labels

JavaScript supports nested labels in most implementations. Loops or blocks can be labeled for the break statement, and loops for continue. Although goto is a reserved word, goto is not implemented in JavaScript. loop1: for (let a = 0; a < 10; ++a) //end of loop1 block1: goto block1; // Parse error.


Functions

A function is a block with a (possibly empty) parameter list that is normally given a name. A function may use local variables. If a user exits the function without a return statement, the value is returned. function gcd(number1, number2) console.log(gcd(60, 40)); // 20 //In the absence of parentheses following the identifier 'gcd' on the RHS of the assignment below, //'gcd' returns a reference to the function itself without invoking it. let mygcd = gcd; // mygcd and gcd reference the same function. console.log(mygcd(60, 40)); // 20 Functions are first class objects and may be assigned to other variables. The number of arguments given when calling a function may not necessarily correspond to the number of arguments in the function definition; a named argument in the definition that does not have a matching argument in the call will have the value (that can be implicitly cast to false). Within the function, the arguments may also be accessed through the object; this provides access to all arguments using indices (e.g. ), including those beyond the number of named arguments. (While the arguments list has a .length property, it is ''not'' an instance of ; it does not have methods such as , , etc.) function add7(x, y) ; add7(3); // 11 add7(3, 4); // 9 Primitive values (number, boolean, string) are passed by value. For objects, it is the reference to the object that is passed. const obj1 = ; const obj2 = ; function foo(p) foo(obj1, 3); // Does not affect obj1 at all. 3 is additional parameter console.log(`$ $`); // writes 1 3 Functions can be declared inside other functions, and access the outer function's local variables. Furthermore, they implement full closures by remembering the outer function's local variables even after the outer function has exited. let t = "Top"; let bar, baz; function foo() foo(); baz("baz arg"); bar(); // "baz arg" (not "foo var") even though foo() has exited. console.log(t); // Top An ''anonymous function'' is simply a function without a name and can be written either using function or arrow notation. In these equivalent examples an anonymous function is passed to the map function and is applied to each of the elements of the array. ,2,3map(function(x) { return x*2;); //returns ,4,6 ,2,3map((x) => { return x*2;}); //same result A ''generator function'' is signified placing an * after the keyword ''function'' and contains one or more ''yield'' statements. The effect is to return a value and pause execution at the current state. Declaring an generator function returns an iterator. Subsequent calls to ''iterator.next()'' resumes execution until the next ''yield''. When the iterator returns without using a yield statement there are no more values and the done property of the iterator is set to true. With the exception of iOS devices from Apple, generators are not implemented for browsers on mobile devices. function* generator() { yield "red"; yield "green"; yield "blue"; } let iterator=generator(); let current; while(current=iterator.next().value) console.log(current); //displays red, green then blue console.log(iterator.next().done) //displays true


Async/await


Objects

For convenience, types are normally subdivided into ''primitives'' and ''objects''. Objects are entities that have an identity (they are only equal to themselves) and that map property names to values ("slots" in
prototype-based programming Prototype-based programming is a style of object-oriented programming in which behavior reuse (known as inheritance) is performed via a process of reusing existing objects that serve as prototypes. This model can also be known as ''prototypal'', ...
terminology). Objects may be thought of as
associative arrays In computer science, an associative array, key-value store, map, symbol table, or dictionary is an abstract data type that stores a collection of (key, value) pairs, such that each possible key appears at most once in the collection. In math ...
or hashes, and are often implemented using these data structures. However, objects have additional features, such as a prototype chain, which ordinary associative arrays do not have. JavaScript has several kinds of built-in objects, namely Array, Boolean, Date, Function, Math, Number, Object, RegExp and String. Other objects are "host objects", defined not by the language, but by the runtime environment. For example, in a browser, typical host objects belong to the DOM (window, form, links, etc.).


Creating objects

Objects can be created using a constructor or an object literal. The constructor can use either a built-in Object function or a custom function. It is a convention that constructor functions are given a name that starts with a capital letter: // Constructor const anObject = new Object(); // Object literal const objectA = {}; const objectA2 = {}; // A != A2, {}s create new objects as copies. const objectB = {index1: 'value 1', index2: 'value 2'}; // Custom constructor (see below) Object literals and array literals allow one to easily create flexible data structures: const myStructure = { name: { first: "Mel", last: "Smith" }, age: 33, hobbies: chess", "jogging"}; This is the basis for
JSON JSON (JavaScript Object Notation, pronounced or ) is an open standard file format and electronic data interchange, data interchange format that uses Human-readable medium and data, human-readable text to store and transmit data objects consi ...
, which is a simple notation that uses JavaScript-like syntax for data exchange.


Methods

A method is simply a function that has been assigned to a property name of an object. Unlike many object-oriented languages, there is no distinction between a function definition and a method definition in object-related JavaScript. Rather, the distinction occurs during function calling; a function can be called as a method. When called as a method, the standard local variable ' is just automatically set to the object instance to the left of the "". (There are also ' and ' methods that can set ' explicitly—some packages such as jQuery do unusual things with '.) In the example below, Foo is being used as a constructor. There is nothing special about a constructor - it is just a plain function that initializes an object. When used with the ' keyword, as is the norm, ' is set to a newly created blank object. Note that in the example below, Foo is simply assigning values to slots, some of which are functions. Thus it can assign different functions to different instances. There is no prototyping in this example. function px() { return this.prefix + "X"; } function Foo(yz) { this.prefix = "a-"; if (yz > 0) { this.pyz = function() { return this.prefix + "Y"; }; } else { this.pyz = function() { return this.prefix + "Z"; }; } this.m1 = px; return this; } const foo1 = new Foo(1); const foo2 = new Foo(0); foo2.prefix = "b-"; console.log("foo1/2 " + foo1.pyz() + foo2.pyz()); // foo1/2 a-Y b-Z foo1.m3 = px; // Assigns the function itself, not its evaluated result, i.e. not px() const baz = {"prefix": "c-"}; baz.m4 = px; // No need for a constructor to make an object. console.log("m1/m3/m4 " + foo1.m1() + foo1.m3() + baz.m4()); // m1/m3/m4 a-X a-X c-X foo1.m2(); // Throws an exception, because foo1.m2 does not exist.


Constructors

Constructor functions simply assign values to slots of a newly created object. The values may be data or other functions. Example: Manipulating an object: function MyObject(attributeA, attributeB) { this.attributeA = attributeA; this.attributeB = attributeB; } MyObject.staticC = "blue"; // On MyObject Function, not object console.log(MyObject.staticC); // blue const object = new MyObject('red', 1000); console.log(object.attributeA); // red console.log(object.attributeB); // 1000 console.log(object.staticC); // undefined object.attributeC = new Date(); // add a new property delete object.attributeB; // remove a property of object console.log(object.attributeB); // undefined The constructor itself is referenced in the object's prototype's ''constructor'' slot. So, function Foo() {} // Use of 'new' sets prototype slots (for example, // x = new Foo() would set x's prototype to Foo.prototype, // and Foo.prototype has a constructor slot pointing back to Foo). const x = new Foo(); // The above is almost equivalent to const y = {}; y.constructor = Foo; y.constructor(); // Except x.constructor

y.constructor; // true x instanceof Foo; // true y instanceof Foo; // false // y's prototype is Object.prototype, not // Foo.prototype, since it was initialized with // {} instead of new Foo. // Even though Foo is set to y's constructor slot, // this is ignored by instanceof - only y's prototype's // constructor slot is considered. Functions are objects themselves, which can be used to produce an effect similar to "static properties" (using C++/Java terminology) as shown below. (The function object also has a special prototype property, as discussed in the "Inheritance" section below.) Object deletion is rarely used as the scripting engine will garbage collect objects that are no longer being referenced.


Inheritance

JavaScript supports inheritance hierarchies through prototyping in the manner of
Self In philosophy, the self is an individual's own being, knowledge, and values, and the relationship between these attributes. The first-person perspective distinguishes selfhood from personal identity. Whereas "identity" is (literally) same ...
. In the following example, the class inherits from the class. When is created as , the reference to the base instance of is copied to . Derive does not contain a value for , so it is retrieved from ''when is accessed''. This is made clear by changing the value of , which is reflected in the value of . Some implementations allow the prototype to be accessed or set explicitly using the slot as shown below. function Base() { this.anOverride = function() { console.log("Base::anOverride()"); }; this.aBaseFunction = function() { console.log("Base::aBaseFunction()"); }; } function Derived() { this.anOverride = function() { console.log("Derived::anOverride()"); }; } const base = new Base(); Derived.prototype = base; // Must be before new Derived() Derived.prototype.constructor = Derived; // Required to make `instanceof` work const d = new Derived(); // Copies Derived.prototype to d instance's hidden prototype slot. d instanceof Derived; // true d instanceof Base; // true base.aBaseFunction = function() { console.log("Base::aNEWBaseFunction()"); }; d.anOverride(); // Derived::anOverride() d.aBaseFunction(); // Base::aNEWBaseFunction() console.log(d.aBaseFunction

Derived.prototype.aBaseFunction); // true console.log(d.__proto__

base); // true in Mozilla-based implementations and false in many others. The following shows clearly how references to prototypes are ''copied'' on instance creation, but that changes to a prototype can affect all instances that refer to it. function m1() { return "One"; } function m2() { return "Two"; } function m3() { return "Three"; } function Base() {} Base.prototype.m = m2; const bar = new Base(); console.log("bar.m " + bar.m()); // bar.m Two function Top() { this.m = m3; } const t = new Top(); const foo = new Base(); Base.prototype = t; // No effect on foo, the *reference* to t is copied. console.log("foo.m " + foo.m()); // foo.m Two const baz = new Base(); console.log("baz.m " + baz.m()); // baz.m Three t.m = m1; // Does affect baz, and any other derived classes. console.log("baz.m1 " + baz.m()); // baz.m1 One In practice many variations of these themes are used, and it can be both powerful and confusing.


Exception handling

JavaScript includes a try ... catch ... finally
exception handling In computing and computer programming, exception handling is the process of responding to the occurrence of ''exceptions'' – anomalous or exceptional conditions requiring special processing – during the execution of a program. In general, an ...
statement to handle run-time errors. The try ... catch ... finally statement catches exceptions resulting from an error or a throw statement. Its syntax is as follows: try { // Statements in which exceptions might be thrown } catch(errorValue) { // Statements that execute in the event of an exception } finally { // Statements that execute afterward either way } Initially, the statements within the try block execute. If an exception is thrown, the script's control flow immediately transfers to the statements in the catch block, with the exception available as the error argument. Otherwise the catch block is skipped. The catch block can , if it does not want to handle a specific error. In any case the statements in the finally block are always executed. This can be used to free resources, although memory is automatically garbage collected. Either the catch or the finally clause may be omitted. The catch argument is required. The Mozilla implementation allows for multiple catch statements, as an extension to the ECMAScript standard. They follow a syntax similar to that used in
Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...
: try { statement; } catch (e if e

"InvalidNameException") { statement; } catch (e if e

"InvalidIdException") { statement; } catch (e if e

"InvalidEmailException") { statement; } catch (e) { statement; } In a browser, the event is more commonly used to trap exceptions. onerror = function (errorValue, url, lineNr) {...; return true;};


Native functions and methods


eval (expression)

Evaluates the first parameter as an expression, which can include assignment statements. Variables local to functions can be referenced by the expression. However, represents a major security risk, as it allows a bad actor to execute arbitrary code, so its use is discouraged. > (function foo() { ... var x = 7; ... console.log("val " + eval("x + 2")); ... })(); val 9 undefined


See also

* Comparison of JavaScript-based source code editors *
JavaScript JavaScript (), often abbreviated as JS, is a programming language and core technology of the World Wide Web, alongside HTML and CSS. Ninety-nine percent of websites use JavaScript on the client side for webpage behavior. Web browsers have ...


References


Further reading

* Danny Goodman: ''JavaScript Bible'', Wiley, John & Sons, . * David Flanagan, Paula Ferguson: ''JavaScript: The Definitive Guide'', O'Reilly & Associates, . * Thomas A. Powell, Fritz Schneider: ''JavaScript: The Complete Reference'', McGraw-Hill Companies, . * Axel Rauschmayer: ''Speaking JavaScript: An In-Depth Guide for Programmers'', 460 pages, O'Reilly Media, 25 February 2014, .
free online edition
* Emily Vander Veer: ''JavaScript For Dummies, 4th Edition'', Wiley, .


External links


A re-introduction to JavaScript - Mozilla Developer Center

JavaScript Loops
* ECMAScript standard references


Interactive JavaScript Lessons - example-based

JavaScript on About.com: lessons and explanation

JavaScript Training
* Mozilla Developer Center Core References for JavaScript version
1.5
an


Mozilla JavaScript Language Documentation
{{DEFAULTSORT:Javascript Syntax
syntax In linguistics, syntax ( ) is the study of how words and morphemes combine to form larger units such as phrases and sentences. Central concerns of syntax include word order, grammatical relations, hierarchical sentence structure (constituenc ...
Articles with example JavaScript code Programming language syntax