The length property indicates the number of parameters expected by the function.

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Property attributes of Function.length Writable no Enumerable no Configurable yes

Description

length is a property of a function object, and indicates how many arguments the function expects, i.e. the number of formal parameters. This number excludes the rest parameter and only includes parameters before the first one with a default value. By contrast, arguments.length is local to a function and provides the number of arguments actually passed to the function.

Data property of the Function constructor

The Function constructor is itself a Function object. Its length data property has a value of 1. The property attributes are: Writable: false , Enumerable: false , Configurable: true .

Property of the Function prototype object

The length property of the Function prototype object has a value of 0.

Examples

console.log(Function.length); /* 1 */ console.log((function() {}).length); /* 0 */ console.log((function(a) {}).length); /* 1 */ console.log((function(a, b) {}).length); /* 2 etc. */ console.log((function(...args) {}).length); // 0, rest parameter is not counted console.log((function(a, b = 1, c) {}).length); // 1, only parameters before the first one with // a default value is counted

Specifications

Browser compatibility

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. Update compatibility data on GitHub Desktop Mobile Server Chrome Edge Firefox Internet Explorer Opera Safari Android webview Chrome for Android Edge Mobile Firefox for Android Opera for Android Safari on iOS Samsung Internet Node.js Basic support Chrome Full support Yes Edge Full support Yes Firefox Full support 1 IE Full support Yes Opera Full support Yes Safari Full support Yes WebView Android Full support Yes Chrome Android Full support Yes Edge Mobile Full support Yes Firefox Android Full support 4 Opera Android Full support Yes Safari iOS Full support Yes Samsung Internet Android Full support Yes nodejs Full support Yes Configurable: true Chrome ? Edge ? Firefox Full support 37 IE ? Opera ? Safari ? WebView Android ? Chrome Android ? Edge Mobile ? Firefox Android Full support 37 Opera Android ? Safari iOS ? Samsung Internet Android ? nodejs ? Legend Full support Full support Compatibility unknown Compatibility unknown

See also