Question

I think it should alert 4 times, but why it just alerted just 2 times. Is there anyone can make an explanation for me?

   var arr = new Array;

   arr.push("1");
   arr.push("2");
   arr.push("3");
   arr.push("4");

   for(var i=0;i<arr.length;i++){
     alert(arr.pop());
   }

Answer 1

Change your for loop to a while loop:

while(arr.length) {
    alert(arr.pop());   
}

The problem is that you remove elements from the array, so the length decreases, ending your for loop early. After 2 iterations of your for loop, arr.length is 2, and i is also 2, so the loop ends.

Here’s a working example of the above.