What is the length of a function in js?

Preface

Today I will tell you how to calculate the length of a function. I hope everyone can learn something from it and consolidate their basics.

Why

Why did I think of this knowledge point? Because last night, in a group, a classmate was discussing an interview question from ByteDance.

123['toString'].length + 123 = ?

To be honest, I couldn't answer this question at first. Actually, I knew that the interviewer wanted to test the toString method on the Number prototype, but I was stuck on the difficult question of what is the length of the toString function. That’s why I have this article today.

How much is it?

Number of parameters

Let's look at the following example

function fn1 () {}

function fn2 (name) {}

function fn3 (name, age) {}

console.log(fn1.length) // 0
console.log(fn2.length) // 1
console.log(fn3.length) // 2

It can be seen that the length is the same as the number of parameters of the function. But is this really the case? Continue reading

Default Parameters

What would be the length of the function if there were default parameters?

function fn1 (name) {}

function fn2 (name = '林三心') {}

function fn3 (name, age = 22) {}

function fn4 (name, age = 22, gender) {}

function fn5(name = '林三心', age, gender) { }

console.log(fn1.length) // 1
console.log(fn2.length) // 0
console.log(fn3.length) // 1
console.log(fn4.length) // 1
console.log(fn5.length) // 0

It shows that the length of a function is the number of parameters before the first one with a default value.

Rest parameters

There are also rest parameters in the function's formal parameters. How are they calculated if there are rest parameters?

function fn1(name, ...args) {}

console.log(fn1.length) // 1

It can be seen that the remaining parameters are not included in the calculation of length

Summarize

Before we conclude, let me first announce that the answer to 123['toString'].length + 123 = ? is 124.

To summarize, length is a property value of a function object, which refers to how many parameters must be passed into the function, that is, the number of formal parameters. The number of formal parameters does not include the remaining parameters, only the number of parameters before the first one with a default value.

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