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

Using function length

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 Firefox for Android Opera for Android Safari on iOS Samsung Internet Node.js length Chrome Full support 1 Edge Full support 12 Firefox Full support 1 IE Full support 4 Opera Full support 3 Safari Full support 1 WebView Android Full support 1 Chrome Android Full support 18 Firefox Android Full support 4 Opera Android Full support 10.1 Safari iOS Full support 1 Samsung Internet Android Full support 1.0 nodejs Full support Yes Configurable: true Chrome Full support 43 Edge Full support 12 Firefox Full support 37 IE No support No Opera Full support 30 Safari No support No WebView Android Full support 43 Chrome Android Full support 43 Firefox Android Full support 37 Opera Android Full support 30 Safari iOS No support No Samsung Internet Android Full support 4.0 nodejs Full support Yes Legend Full support Full support No support No support

See also