Scope Chain & Closure

我们先来小试身手,在这之前我们必须有function scope 跟 global varible的基本知识

let a = "hello"

function test1(){
    var a = "Goodbye"
    console.log(a) // Goodbye
    test2() 
}

function test2(){
        console.log(a) //hello
}
test1()

首先我们在全域定义 a = "hello",在function test1里面又定义一次 a = "Goodbye",想当然尔function scope
就会输出Goodbye,并且function test1 callback function test2,但是test2并没有定义a ,那就会变成从全域变数找有没有a的数值,并且找到 a = "hello!!!

接下来让我们把function 改的复杂一点

let a = "Dennis"

function test1(){
    var a = "Jessica"
    console.log(a) //Jessica

    function test2(){
        console.log(a) //Jessica
    }

    function test3(){
        var a = "Jack"
        console.log(a) //Jack
        test2()
    }
    test3()
}


test1()

首先看到test1是一个function scope,定义 a = "Jessica"并且里面有两个function,function test3 call function test2,但function test3定义 a = "Jack",所以function test3会输出"Jack",但是function test2 是function test1里面的所以会抓取function test1的 a,最後输出 "Jessica"