js closures and loops

function box(){
  var arr = [];
  for(var i=0;i<5;i++){
    arr[i]=function(){
      return i;
    }
  }
  return arr;
}
var b = box();
console.log(b.length);
for(var i=0;i<b.length;i++){
  console.log(b[i]())
}

The above code will print out 5 5

Because b[i]() calls an anonymous function, but the anonymous function does not execute itself, so by the time it is called, box() has already been executed. . . .

Change the following:

function box(){
  var arr = [];
  for(var i=0;i<5;i++){
    arr[i]=(
      function(num){
        console.log("ccc="+num)
        return num;
      }
    )(i)
  }
  return arr;
}
var b = box();
console.log(b.length);
for(var i=0;i<b.length;i++){
  console.log(b[i])
}

Execution result:

num=0
num=1
num=2
num=3
num=4
5
0
1
2
3
4

After modification, the anonymous function is allowed to execute itself, resulting in the final b[i] returning an array instead of a function