What is the length of the function in js

Preface to the table of contents
Why
How much is it?
Number of formal parameters
Default parameter
Residual parameter
Summarize

Preface

Today, I will tell you about length of function, and how to calculate it. I hope everyone can learn something from it and consolidate the foundation.

Why

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

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

To tell the truth, I didn't answer this question at first. Actually, I know that the interviewer wants to test the toString method on the Number prototype, but I am stuck in the problem of what is the length of the toString function. That's why today's article came into being

How much is it?

Number of formal parameters

Let's take a 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 function has as many formal parameters as length. But is this really the case? Keep looking down

Default parameter

If there are default parameters, what is the length of the function?

function fn1 (name) {}

function fn2 (name = ' Lin 3 Heart ') {}

function fn3 (name, age = 22) {}

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

function fn5(name = ' Lin 3 Heart ', 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 length of function is the number of parameters before the first one has the default value

Residual parameter

In the formal parameters of the function, there are residual parameters. If there are residual parameters, how will it be calculated?

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 concluding, publish 123 ['toString']. length + 123 =? The answer is 124

To sum up, length is an attribute value of a function object, which refers to how many parameters the function has to pass in, that is, the number of formal parameters. The number of formal parameters does not include the number of remaining parameters, but only the number of parameters before the first one has the default value