How does this for loop work? for ( ; i < length; i++ )

Question

How does this for loop work? It doesn't make sense to me.

for ( ; i < length; i++ ) {

Answer 1

The loop will simply repeat as long as i is less than length. It simply assumes i is already declared elsewhere.

Actually, all parts within a for loop construct are optional. For example, this is a perfectly valid way to create an endless loop:

​for(;;) window.alert('Are you sick of alerts yet?');​​​​​​​​​

Answer 2

It's a regular for loop that does nothing at all in the initialization step.

This is equivalent to writing:

;
while (i < length) {
    // ...
    i++;
}

except if there's a continue in the ... body, in which case the for loop would execute the i++ before re-evaluating the condition, and the while loop would not.