JS closure knowledge points essence

        //1. What is a closure ？  Method is called inside the method 
        //2. The meaning of closures --1. Extend the life cycle of variables  2. Create a private environment 
        // Scope chain   Find the rules ： Nearby principle 
        let a = 1;
        function fn1(){
            let b = 2;
            console.log(a);
        }
        //  console.log(b)
        fn1();

Pictured a Global variable ,b It's a local variable . So you can get it anywhere a Value , And outside the method ( Outside closure ) You can't get b Value , So the output is 1.

        // Bridge internal and external methods 
        function outer(){
            let a1 = 111;
            let a2 = 222;
            return function inner(){
                return a1; 
            }
        }
        function fn5(){
            let getInnerdata = outer();
            console.dir(getInnerdata);
        }
        fn5();
        // Closure will reside in memory => Be careful with closures 

  There are closures in the domain only return One. a1, therefore closure Only a1.

It will always exist dir in , So it will be resident in memory , So be careful with closures

 

 let makeCount = function(){
            let num = 0 ;
            function changeBy(val){
                num+=val;
            }
            return{
                add:function(){
                    changeBy(1);
                },
                reduce:function(){
                    changeBy(-1);
                },
                value:function(){
                    return num;
                }
            }
        }
        let count1 = makeCount();
        let count2 = makeCount();
        count1.add()
        count1.add()
        count2.add()
        console.log(count1.value());
        console.log(count2.value());

The output of the above code is count1 Of value by 2;count2 Of value by 1.

It can be explained in this way , Although they are all in the same way , But they don't interfere with each other . It's like a father with two sons , But the two sons have their own families , Independent relationship . so count1 Two more times ,count2 The addition is a recalculation .