如果声明和变量提升 [英] If statement and variable hoisting

问题描述

我正在学习JavaScript中的变量提升，并发现这种行为很奇怪：

I am learning about variable hoisting in JavaScript, and found this behavior weird:

var x = 0;

(function () {
    //Variable declaration from the if statement is hoisted:
    //var x; //undefined

    console.log(x); //undefined

    if (x === undefined) {
        var x = 1; //This statement jumps to the top of the function in form of a variable declaration, and makes the condition become true.
    }
}());

这是否正确，在这种情况下，语句使条件成立，以便可以执行？

Is it correct, that in this case, the statement makes the condition true so it can be executed?

推荐答案

仅提升提升声明，但不提升任务。您的代码相当于：

Hoisting hoists only the declaration, but not the assignment. Your code is equivalent to:

var x = 0;

(function () {
    //Variable declaration from the if statement is hoisted:
    var x;

    console.log(x); //undefined

    if (x === undefined) {
        x = 1;
    }
}());

if语句的条件表达式求值为 true 到达 x = 1 。

The if statement's condition expression evaluates to true and x = 1 is reached.

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