Weird looking Javascript for loop

Question

I have never seen a JavaScript loop such as this for( ; i-- ; ), used in the code:

uid: function (len) {     var str = '';     var src = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';     var src_len = src.length;     var i = len;      for (; i--;) {         str += src.charAt(this.ran_no(0, src_len - 1));     }      return str; } 

I understand the behavior, but I would like it if somebody could share some insights about this type of for loop.

Answer 1

This is a syntax of the for-loop construction:

for ([initialization]; [condition]; [final-expression])
     statement

In your case for (; i--;) {:

• no variables are initialized, because var i = len; inintialized earlier, so it's not needed.
• condition part will be truthy until i becomes 0 then loop will terminate. i-- is executed on before each iteration, and due to -- operator it will eventually become 0, so it's falsy, and loop will stop.
• since i is decremented in condition part of the loop, final-expression is not needed too. Another way to put it: since i is not used inside the loop, it does not matter whether we decrement it before each loop iteration or after each loop iteration.

That being said, it's better to avoid writing loops like above, as it's pretty confusing and hard to read. Prefer traditional for-loops notation.