Summary and sharing of knowledge points related to js functions

In js, the reference type is a data structure, including 5 reference types, namely Object, data, Array, re. Please enter the code gExp, Function. Today we will talk about the data structure of Function.

In js, functions are actually objects, and each function is an instance of the Function type. Like other reference types, they have properties and methods.

1. There are several ways to define functions

1) Function declaration syntax definition

  function sum(num1,num2){
  return num1+num2;
 }

2) Function expression

Note that there is a semicolon after the function expression;

var sum=function(num1,num2){
   return num1+num2;
}

Use the Function constructor

The constructor can receive any number of parameters, but the last parameter is always regarded as the function body, while the previous parameters enumerate the parameters

var sum=new Function("num1","num2","return num1+num2")

Of course The third method is unfriendly and not recommended.

The function name is just a pointer to the function, so the function name is no different from other variables containing object pointers. That is to say, a function may have multiple names.
What is the difference between function declaration and function expression here?

When the parser loads data into the execution environment, it treats function declarations and function expressions differently. The parser first reads the function declaration and makes the function declaration code callable before any code. This is what we usually call function declaration hoisting. Function expressions are different. They must wait until the function parser executes the code block where they are located before they are actually executed. Compare the following 2 examples

console.log(sum(10,10));

function sum(n1,n2){
  return n1+n2;
}

以上代码会正常执行

再看！

console.log(sum(10,10));

var sum=function(n1,n2){
  return n1+n2;
}

以上执行器就会报错！

2. The function is not overloaded

If two functions with the same name are declared, The result is that the later function overwrites the earlier function.

function sun(a){
 return a+100;
}
function sun(a){
 return a+200;
}

var result=sum(2)//202

3. The internal attributes of the function

include this,arguments

The details will not be described again

4. Function attributes and methods

The function has two attributes, length and prototype

length indicates the number of named parameters the function hopes to receive.

function a(name){
 //todo
}
function b(name,age){
 //todo
}
function c(){
 //todo
}

a.length //1
b.length //2
c.length //0

The prototype attribute is very important for reference types and will be explained specifically, so I won’t go into details here.

Each function contains 2 non-inherited methods apply and call

The purpose of these two methods is to call the function in a specific scope, which is actually to set the function The value referred to by this in the body.

first. The apply() method receives two parameters, one is the scope in which the function runs, and the other is the parameter array. The second parameter can be an instance of the array or arguments.

function sum(num1,num2){
  return num1+num2;
}

function test1(a,b){
  return sum.apply(this,arguments);
}

function test2(c,d){
  return sum.apply(this.[c,d]);
}

console.log(test1(1,1)) //2

console.log(test2(1,1)) //2

There is not much difference between the call method and the apply method. The difference is that the second parameter of the call method must be passed one by one.

The biggest advantage of using call and apply to expand the scope is that the object is not coupled to any method

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