需要帮助来了解javascript反向方法的重写解决方案的部分逻辑吗？

Question

So I was trying to understand this solution of rewriting the Array reverse method from scratch:

Array.prototype.reverse = function() {
  for(var i = 0, j = this.length-1; i < j; i++, j--) {
    var tmp = this[i];
    this[i] = this[j];
    this[j] = tmp;
  }
  return this;
 };

So I understand j starts as the amount of array elements - 1, and i is initially 0. And then I believe they are both incremented or decremented by 1 until i is no longer less than j..What I don't understand is the following lines:

    var tmp = this[i];
    this[i] = this[j];
    this[j] = tmp;

Could anyone anyone what is going on in these lines?

Answer 1

Let's work through it with an example:

// assume `this` = ["a", "b", "c"]
// assume i = 0 and j = 2

var tmp = this[i];
// tmp:  "a"
// this: ["a", "b", "c"]

this[i] = this[j];
// tmp:  "a"
// this: ["c", "b", "c"]

this[j] = tmp;
// tmp:  "a"
// this: ["c", "b", "a"]

Notice that step two is [c,b,c] and the [a] is totally gone. This is why we save it to tmp, so that we can put it back in step three.