Editing JavaScript (section)

== Syntax ==
{{Main|JavaScript syntax}}
[[Variable (computer science)|Variables]] in JavaScript can be defined using either the <code>var</code>,<ref>{{cite web | url=https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Statements/var | title=var – JavaScript | publisher=The [[Mozilla Developer Network]] | access-date=December 22, 2012 | archive-date=December 23, 2012 | archive-url=https://web.archive.org/web/20121223162713/https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Statements/var | url-status=live }}</ref> <code>let</code><ref name="moz_let">{{cite web |title=let |url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let |website=MDN web docs |publisher=Mozilla |access-date=June 27, 2018 |ref=moz_let |archive-date=May 28, 2019 |archive-url=https://web.archive.org/web/20190528140803/https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let |url-status=live }}</ref> or <code>const</code><ref name="moz_const">{{cite web |title=const |url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const |website=MDN web docs |publisher=Mozilla |access-date=June 27, 2018 |ref=moz_const |archive-date=June 28, 2018 |archive-url=https://web.archive.org/web/20180628044054/https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const |url-status=live }}</ref> keywords.  Variables defined without keywords will be defined at the global scope.

Arrow functions were first introduced in [[w:ECMAScript#6th Edition – ECMAScript 2015|6th Edition – ECMAScript 2015]]. They shorten the syntax for writing functions in JavaScript. Arrow functions are anonymous, so a variable is needed to refer to them in order to invoke them after their creation, unless surrounded by parenthesis and executed immediately.

Here is an example of JavaScript syntax.
<syntaxhighlight lang="javascript" start="1">
// Declares a function-scoped variable named `x`, and implicitly assigns the
// special value `undefined` to it. Variables without value are automatically
// set to undefined.
// var is generally considered bad practice and let and const are usually preferred.
var x;

// Variables can be manually set to `undefined` like so
let x2 = undefined;

// Declares a block-scoped variable named `y`, and implicitly sets it to
// `undefined`. The `let` keyword was introduced in ECMAScript 2015.
let y;

// Declares a block-scoped, un-reassignable variable named `z`, and sets it to
// a string literal. The `const` keyword was also introduced in ECMAScript 2015,
// and must be explicitly assigned to.

// The keyword `const` means constant, hence the variable cannot be reassigned
// as the value is `constant`.
const z = "this value cannot be reassigned!";

// Declares a global-scoped variable and assigns 3.  This is generally considered
// bad practice, and will not work if strict mode is on.
t = 3;

// Declares a variable named `myNumber`, and assigns a number literal (the value
// `2`) to it.
let myNumber = 2;

// Reassigns `myNumber`, setting it to a string literal (the value `"foo"`).
// JavaScript is a dynamically-typed language, so this is legal.
myNumber = "foo";
</syntaxhighlight>

Note the [[Comment (computer programming)|comments]] in the examples above, all of which were preceded with two [[Slash (punctuation)|forward slashes]].

More examples can be found at the [[wikibooks:JavaScript/Syntax examples|Wikibooks page on JavaScript syntax examples]].