javascript - I don’t know if it’s a problem with variable promotion

Question

function Box(age) {
    this.name = 'ss';
    this.age = age;
    this.flag = true;
    return this;
} //定义一个构造函数

var box1 = new Box(10); // new出一个实例

setTimeout(function () {
    box1.flag = false;
    console.log(box1.flag);
}, 5000); //五秒钟之后把 实例box1里面的flag变为false.


var inter = setInterval(function () {
    if (box1) {
        console.log(box1);
        if (!box1.flag) {
            box1 = null;
            var box1 = new Box(20);
        }
    } else {
        console.log('cleared Interval as box1 is null now');
        clearInterval(inter);
    }

}, 1000); //每一秒种先控制台打印出box1, 如果flag为false, 那么就销毁box1，然后再new出一个box1. 

The result is that the direct output box1 is null.

cleared Interval as box1 is null now
false

Is it because var will declare local variables first? As a result, box = null directly after declaration. Then the content in else is output?

Answer 1

Since you have questions, wouldn’t you know the result after removing var?
It is indeed a variable promotion, you might as well output what box1 is in the timer

var inter = setInterval(function () {
    console.log(box1);    
    if (box1) {
        console.log(box1);
        if (!box1.flag) {
            box1 = null;
            var box1 = new Box(20);
        }
    } else {
        console.log('cleared Interval as box1 is null now');
        clearInterval(inter);
    }
 },1000);

The result is that undefined is output first, not null as you think.
is actually equivalent to

var inter = setInterval(function () {
  var box1;
   if (box1) {
        console.log(box1);
        if (!box1.flag) {
            box1 = null;
            box1 = new Box(20);
        }
    } else {
        console.log('cleared Interval as box1 is null now');
        clearInterval(inter);
    }
},1000)

So, the timer was triggered just after reaching the point, and found that box1 was declared, but the actual assignment had not yet started, so the if took the else route