对于循环值，每次迭代

Question

I'm not sure if these functions do exactly the same thing:

Is:

  for (i=0, l=this.current_columns.length; i<l; i++) {
    if ( this.current_columns[i].text === row[0] ) {
        this.current_columns.splice(i, 1);
        EV.publish('edit_layout.current_columns', {data: [this.current_columns]})
        break;
    }
  }

equal to:

  for (i=0; i<this.current_columns.length; i++) {
    if ( this.current_columns[i].text === row[0] ) {
        this.current_columns.splice(i, 1);
        EV.publish('edit_layout.current_columns', {data: [this.current_columns]})
        break;
    }
  }

In the end outcome?

Answer 1

They actually do the same thing, but it may need some explanation:

The difference in the loop itself is that the length of the array is stored in a variable in the first version.

Storing the length in a variable means that the loop will run to the original length of the array, if the array changes inside the loop, that would be a problem.

The array happens to change in the loop, but as you also have a break; after that, it's not a problem. The loop ends, so it doesn't matter that the length in the variable is no longer correct.

Answer 2

This part:

for (i=0, l=this.current_columns.length; i<l; i++) {

the first part: i=0, l=this.current_columns.length is executed once, whereas i<l is executed at each iteration.

It is thus generally better to precalculate the boundary of a loop. Be careful though, because sometimes your loop modifies the length of the array it traverses, you have then a choice: whether you calculate at each iteration the length of the array or you modify i and l in consequence (if you remove an element l = l - 1 , and i = i - 1 )