1. Closures

The closure in js is a function (a closed package structure or space that is not open to the outside world)

1. Problems to be solved by closures

• The data inside the function cannot be accessed outside the function

• To be solved The problem is that the data inside the function needs to be accessed indirectly from the outside

2. Basic structure

 function outer(){
        var data = "數(shù)據(jù)";        return function(){
            return data;
        }
    }

 function outer(){
    var data = "數(shù)據(jù)";     return {
         getData:function(){
             return data;
         },
         setData:function(value){
             data = value;             return data;
         }
     }
 }

3. Application of closure

1) Use Closure to solve the timer problem

由于js是單線程執(zhí)行的，會(huì)先執(zhí)行主任務(wù)，然后執(zhí)行次要任務(wù)(包括setTimeOut和setInterval中的回調(diào)函數(shù)中的代碼)

For example:

for(var i = 0 ; i < 10; i++){
    setTimeout(function(){
         console.log(i);
     },0);
 }

will not print out 1~10 as expected, but print out 10 10 because the for loop will not be executed until it is executed. setTimeout callback function, if the time is up, execute

Solution:

 for(var i = 0; i< 3; i++){   function foo(j){
        return function(){
            console.log(j);
        };
    }    var f = foo(i);
    setTimeout(f, 0);
}

It will be printed as 1 2 3

2) Use closure to save the environment

Closures share the same function definition, but save different lexical environments

function makeSizer(size) {
  return function() {
    document.body.style.fontSize = size + 'px';
  };
}var size12 = makeSizer(12);var size14 = makeSizer(14);var size16 = makeSizer(16);

document.getElementById('size-12').onclick = size12;
document.getElementById('size-14').onclick = size14;
document.getElementById('size-16').onclick = size16;

The text will change when clicked12, 14, 16

But if you change the writing:

function makeSizer(size) {
    document.body.style.fontSize = size + 'px';
}

If written like this, the text size is all 12, because they share the same lexical environment. After the first one is executed, the following and the previous ones share the same lexical environment

3) Create Use closures when objects or classes

When creating a new object or class, methods should usually be associated with the object's prototype rather than being defined in the object's constructor. The reason is that this will cause the method to be reassigned every time the constructor is called (that is, every object is created).

For example, we can write like this:

function MyObject(name, message) {
  this.name = name.toString();  this.message = message.toString();
}
MyObject.prototype.getName = function() {
  return this.name;
};
MyObject.prototype.getMessage = function() {
  return this.message;
};

2. Prototype

Related recommendations:

Detailed explanation of the use of js closure-js tutorial

Detailed introduction to the in-depth analysis of Javascript closure and code implementation method

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