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function.length in JavaScript

·2 mins

Recently I found a function.length in the real world pacakge. I did not know what it was. To my surprise, it is a legal property given a function. It gives the number of parameters expected by the function.

const func1 = () => console.log('hello');
const func2 = (a,b,c) => console.log('world');
func1.length; // 0
func2.length; // 3

How is this useful? I have never needed to know the number of parameters a function needs before calling it. Turns out some libraries use this length property to do some method overloading. 🤯

That is pretty smart way of doing this without using just one big object in the first parameter.

const myFunc = (callback) => {
    const a = 1, b = 2, c = 3;
    const verifyArityArgsMap = {
        1: [a],
        2: [a b],
        3: [a,b,c],
    const arity = callback.length; // Here using length to check how many parameter to use.
    const verifyArgs = verifyArityArgsMap[arity];


But there is a caveat. With JavaScript, there is always a catch. Let's see the spec on MDN.

This number excludes the rest parameter and only includes parameters before the first one with a default value.

const func3 = (a, b = 1) => console.log('I am weird');
const func4 = (...args) => console.log('I do not have a length');
console.log(func3.length); // 1
console.log(func4.length); // 0

Real world example #

🧐 The wild code I was looking at that uses length for method overloadding.


Should you use this? #

I don’t know, depends on your situation. But the answer is probably not. That’s all. I hope you find this interesting.