var const let作用域

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1.与第二个情况对比

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const fn2 = () => {
    var a1 = 88;
    const fn1 = () => {
    	console.log('1~~~~', a1)
        let a1 = 999;
    }
    fn1();
    console.log('2~~~~',a1)
}
fn2();
// 第4行 VM270:4 Uncaught ReferenceError: a1 is not defined

总结：函数会从自身的活动对象开始，一层层向上寻找自己所需的变量，该函数在寻找a1时发现，自己的作用域里声明了let,它就不会再往上找，而let在声明之前是不可用的，所以就会报错

2.与第一个情况对比

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const fn2 = () => {
    var a1 = 88;
    const fn1 = () => {
    	console.log('1~~~~', a1)
        a1 = 999;
    }
    fn1();
    console.log('2~~~~',a1)
}
fn2();
// 1~~~~ 88
// 2~~~~ 999

3.执行let a1的时候不会报错

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const fn2 = () => {
    var a1 = 88;
    const fn1 = () => {
     let a1 = 999;
    }
    fn1();
    console.log('2~~~~',a1)
}
fn2();
// 2~~~~ 88

4.与1对比，执行let a1的时候报错

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const fn2 = () => {
    var a1 = 88;
    const fn1 = () => {
     var a1 = 777;
     let a1 = 999;
    }
    fn1();
    console.log('2~~~~',a1)
}
fn2();
// 第5行 Uncaught SyntaxError: Identifier 'a1' has already been
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