JavaScript—A detailed look at scopes, scope chains, and preparsing

Variables include: global variables, local variables

##In JAvaScript, variables defined in functions are local variables

There is no block-level scope in js ---Variables defined in a pair of brackets. This variable can be used outside the curly brackets.

Scope chain: To use variables, search from the inside out, layer by layer, and you can find it. Directly use

var num=10; //作用域链 级别:0
   var num2=20;
   var str = "abc"
   function f1() {
     var num2=20;
     function f2() {
       var num3=30;
       console.log(num);
     }
     f2();
   }
   f1();

Copy after login

undefined

//变量的提升
    console.log(num);
    var num=100;


//提升之后为：
var num;//变量的声明提前
console.log(num);
    var num=100;

//函数的声明被提前了
    f1();
    function f1() {
      console.log("这个函数,执行了");
    }

f2();
    var f2=function () {
        console.log("小杨好帅哦");
    }

//声明提前后：
var f2;//为一个变量，undefind
f2();//undefind加括号是不被认可的，所以报错
    var f2=function () { 
        console.log("小杨好帅哦");
    }

var f2;
    f2=function () {
      console.log("小杨好帅哦");
    };
    f2();

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