function expression
BaselineWidely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
Thefunction keyword can be used to define a function inside an expression.
You can also define functions using thefunction declaration or thearrow syntax.
Try it
const getRectArea = function (width, height) { return width * height;};console.log(getRectArea(3, 4));// Expected output: 12Syntax
function (param0) { statements}function (param0, param1) { statements}function (param0, param1, /* …, */ paramN) { statements}function name(param0) { statements}function name(param0, param1) { statements}function name(param0, param1, /* …, */ paramN) { statements}Note:Anexpression statement cannot begin with the keywordfunction to avoid ambiguity with afunction declaration. Thefunction keyword only begins an expression when it appears in a context that cannot accept statements.
Parameters
nameOptionalThe function name. Can be omitted, in which case the function isanonymous. The name is only local to the function body.
paramNOptionalThe name of a formal parameter for the function. For the parameters' syntax, see theFunctions reference.
statementsOptionalThe statements which comprise the body of the function.
Description
Afunction expression is very similar to, and has almost the same syntax as, afunction declaration. The main difference between afunction expression and afunction declaration is thefunction name, which can be omitted infunction expressions to createanonymous functions. Afunction expression can be used as anIIFE (Immediately Invoked Function Expression) which runs as soon as it is defined. See also the chapter aboutfunctions for more information.
Function expression hoisting
Function expressions in JavaScript are not hoisted, unlikefunction declarations. You can't use function expressions before you create them:
console.log(notHoisted); // undefined// Even though the variable name is hoisted,// the definition isn't. so it's undefined.notHoisted(); // TypeError: notHoisted is not a functionvar notHoisted = function () { console.log("bar");};Named function expression
If you want to refer to the current function inside the function body, you need to create a named function expression. This name is then local only to the function body (scope). This avoids using the deprecatedarguments.callee property to call the function recursively.
const math = { factorial: function factorial(n) { console.log(n); if (n <= 1) { return 1; } return n * factorial(n - 1); },};math.factorial(3); // 3;2;1;If a function expression is named, thename property of the function is set to that name, instead of the implicit name inferred from syntax (such as the variable the function is assigned to).
Unlike declarations, the name of the function expressions is read-only.
"use strict";function foo() { foo = 1;}foo();console.log(foo); // 1(function foo() { foo = 1; // TypeError: Assignment to constant variable.})();Examples
Using function expression
The following example defines an unnamed function and assigns it tox. The function returns the square of its argument:
const x = function (y) { return y * y;};Using a function as a callback
More commonly it is used as acallback:
button.addEventListener("click", function (event) { console.log("button is clicked!");});Using an Immediately Invoked Function Expression (IIFE)
IIFEs are a common pattern used to execute arbitrarily many statements in their own scope (and possibly return a value), in a location that requires a single expression. Many traditional use cases of IIFEs have been obsoleted by new syntax features such asmodules andblock-scoped declarations. IIFEs themselves are more commonly written witharrow functions now, but the idea remains the same. In general, IIFEs look like this:
// standard IIFE(function () { // statements…})();// IIFE with arguments(function (a, b) { console.log(a + b);})(1, 2); // logs 3// IIFE being used to initialize a variableconst value = (() => { const randomValue = Math.random(); if (randomValue > 0.5) { return "heads"; } return "tails";})();Here, we introduce several use cases with examples.
Avoid polluting the global namespace in script code
The top-level scope of all scripts are shared, which could include many functions and global variables from different files, so to avoid name conflicts, it's important to limit the number of globally declared names (this is greatly mitigated inmodules, but sometimes limiting the scope of temporary variables is still useful, especially when the file is very long). If we have some initialization code that we don't need to use again, we could use the IIFE pattern, which is better than using a function declaration or a function expression because it ensures that the code is only run here and once.
// top-level of a script (not a module)var globalVariable = (() => { // some initialization code let firstVariable = something(); let secondVariable = somethingElse(); return firstVariable + secondVariable;})();// firstVariable and secondVariable cannot be accessed outside of the function body.The module pattern
We would also use IIFE to create private and public variables and methods. For a more sophisticated use of the modulepattern and other use of IIFE, you could see the book Learning JavaScript Design Patterns by Addy Osmani.
const makeWithdraw = (balance) => ((copyBalance) => { let balance = copyBalance; // This variable is private const doBadThings = () => { console.log("I will do bad things with your money"); }; doBadThings(); return { withdraw(amount) { if (balance >= amount) { balance -= amount; return balance; } return "Insufficient money"; }, }; })(balance);const firstAccount = makeWithdraw(100); // "I will do bad things with your money"console.log(firstAccount.balance); // undefinedconsole.log(firstAccount.withdraw(20)); // 80console.log(firstAccount.withdraw(30)); // 50console.log(firstAccount.doBadThings); // undefined; this method is privateconst secondAccount = makeWithdraw(20); // "I will do bad things with your money"console.log(secondAccount.withdraw(30)); // "Insufficient money"console.log(secondAccount.withdraw(20)); // 0For loop with var before ES6
We could see the following use of IIFE in some old code, before the introduction of the block-scopedlet andconst declarations. With the statementvar, we have only function scopes and the global scope.Suppose we want to create 2 buttons with the texts Button 0 and Button 1 and when we clickthem, we would like them to alert 0 and 1. The following code doesn't work:
for (var i = 0; i < 2; i++) { const button = document.createElement("button"); button.innerText = `Button ${i}`; button.onclick = function () { console.log(i); }; document.body.appendChild(button);}console.log(i); // 2When clicked, both Button 0 and Button 1 alert 2 becausei is global,with the last value 2. To fix this problem before ES6, we could use the IIFE pattern:
for (var i = 0; i < 2; i++) { const button = document.createElement("button"); button.innerText = `Button ${i}`; button.onclick = (function (copyOfI) { return function () { console.log(copyOfI); }; })(i); document.body.appendChild(button);}console.log(i); // 2When clicked, Buttons 0 and 1 alert 0 and 1. The variablei is globally defined. Using the statementlet, we could simply do:
for (let i = 0; i < 2; i++) { const button = document.createElement("button"); button.innerText = `Button ${i}`; button.onclick = function () { console.log(i); }; document.body.appendChild(button);}console.log(i); // Uncaught ReferenceError: i is not defined.When clicked, these buttons alert 0 and 1.
Control flow statements in expression positions
IIFEs enable us to use language constructs such asswitch in an expression.
someObject.property = (() => { switch (someVariable) { case 0: return "zero"; case 1: return "one"; default: return "unknown"; }})();This approach can be especially useful in scenarios where you want to make a variableconst, butare forced to uselet orvar during initialization:
let onlyAssignedOnce;try { onlyAssignedOnce = someFunctionThatMightThrow();} catch (e) { onlyAssignedOnce = null;}Using IIFEs, we can make the variableconst:
const onlyAssignedOnce = (() => { try { return someFunctionThatMightThrow(); } catch (e) { return null; }})();Specifications
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-function-definitions |