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:

Html code

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