freeCodeCamp/guide/english/javascript/standard-objects/function/function-length/index.md

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Function Length

Function Length

The length property on the function object holds the number of arguments expected by the function when called.

function noArgs() { }

function oneArg(a) { }

console.log(noArgs.length); // 0

console.log(oneArg.length); // 1

ES2015 Syntax

ES2015, or ES6 as it is commonly called, introduced the rest operator and default function parameters. Both of these additions change the way the length property works.

If either the rest operator or default parameters are used in a function declaration the length property will only include the number of arguments before a rest operator or a default parameter.

function withRest(...args) { }

function withArgsAndRest(a, b, ...args) { }

function withDefaults(a, b = 'I am the default') { }

console.log(withRest.length); // 0

console.log(withArgsAndRest.length); // 2

console.log(withDefaults.length); // 1

More Information on Function.length can be found on Mozilla's MDN Docs.