ES6 learning function (strict mode, higher-order function, closure)

1、 The role of strict mode  

1. Eliminated JS Grammatical irrationality 、 Not rigorous , Reduced some weird behavior
2. Eliminate some insecurity of code running , Ensure the safety of code operation .
3. Improve compiler efficiency , Increase running speed .
4. Disabled in ECMAScript Some of the syntax that may be defined in future versions of . For example, some reserved words ：import,class...

Can be applied to The whole script or Individual functions in .

2、 Changes in strict patterns

1、this Point to the problem

//1. use var Statement 
var num = 10;
console.log(num);// Report errors 
//2. You cannot delete declared variables at any time 
//delete num;
//3. In a function in the global scope this yes undefined（ No window）
function fn(){
    console.log(this);
}
//4. Constructors do not add new call ,this Will report a mistake 
function Star(){
    this.sex = 'nv';
}
//5. In the timer this Or point to window
setTimeout(function(){
    console.log(this);
},1000);

2、 Changes in functions

//1. There can be no formal parameters with duplicate names 
function fun(a,a){
    console.log(a+a);
}
fun(1,2);// Report errors 
//2. You cannot declare a function within a code block that is not a function 
if(ture)
{
    function(){

    }// Report errors 
}

JavaScript | MDN

3、 Higher order function

Higher order functions are functions that operate on other functions , It receives function As Parameters , Or output the function as a return value .

Function is also a data type , It can also be used as a parameter , Pass to another parameter to use .

// Higher order function - Function as parameter 
function fn(a,b,callBack)
{
    console.log(a+b);
    callBack && callBack(a,b);
}
fn(1,2,function(a,b){
    console.log('this is callback');
});

4、 Closure

1、 Closure definition

Definition ： Closure is A function that has access to a variable in the scope of another function function .（ Closure is a phenomenon ） 

F12 Check whether there are closures

 closure Closure .

2、 The function of closures

stay fn Outside scope visit fn Internal local variables .

Or use higher-order functions .

// Closure ：fn External scopes can access fn Internal local variables .
function fn(){
    var num = 10;
    return function(){
        console.log(num);
    };
}
var f = fn();
f();

The function of closures ： It extends the scope of variables .

5、 Shallow copy and deep copy

Shallow copy ： Copy only one layer , Deeper object level copy only references .

Deep copy ： Copy multiple layers , Every level of data is copied .