这些块的JS范围如何工作？ [英] How does the JS scope of these blocks work?

问题描述

任何人都可以解释为什么以下产生1,2而另一产生5？它们不应该同时产生5吗？

Can anyone explain why the following produces 1,2 and the other produces 5? Should they not both produce 5?

//produces 1,2
(function () {

    var a = [5];

    function bar() {
        if (!a) {
          var a = [1, 2];
        }
        console.log(a.join());
    }

    bar();

})();

基于阅读一些关于JS闭包的文章，我希望它们都能产生5.看起来不行在任何地方找到一篇文章，可以提供一些有关第一个块产生的原因的见解。

Based on reading some articles about JS closure, I expect them both to produce 5. Can't seem to find an article anywhere that would give some insight as to why the first block produces otherwise.

//produces 5
(function () {

    var a = [5];

    function bar() {
        if (a) {
          console.log(a.join());
        }
        else {
          console.log([1, 2].join())
        }
    }

    bar();

})();

谢谢！

推荐答案

由于javascripts var hoisting ，此代码：

Due to javascripts var hoisting, this code:

(function () {
    var a = [5];
    function bar() {
        if (!a) {
          var a = [1, 2];
        }
        console.log(a.join());
    }
    bar();
})();

相当于此代码：

(function () {
    var a = [5];
    function bar() {
        var a; // a === undefined at this point
        if (!a) {
          a = [1, 2];
        }
        console.log(a.join());
    }
    bar();
})();

所以你可以看到， a 确实会当测试if条件时， falsey （即！= === true）

So you can see, a will indeed be falsey (i.e. !a === true) when the if condition is tested

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