Question

我正在学习JavaScript中的变量提升，并发现这种行为很奇怪：

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.
    }
}());

是否正确，在这种情况下，语句使条件成立，以便可以执行？

Answer 1

仅提升提升声明，但不提升任务。您的代码相当于：

var x = 0;

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

    console.log(x); //undefined

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

if语句的条件表达式的计算结果为true，并且已达到x = 1。

Answer 2

顺便说一句，如果你在if语句中声明一个变量，无论if条件是否传递，声明总是被托管。例如：

＆＃13;

console.log(a); //undefined , not respond ReferenceError ,it has been hoisted
if(true){
  var a=1;
}
console.log(b); //same as above
if(false){
  var b=1;
}

＆＃13;

如果声明和变量提升

2 个答案: