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I am working on a quiz for an online course, testing understanding of loops. The problem is:

"Write a function called laugh() that takes one parameter, num that represents the number of “ha”s to return. Such that console.log(laugh(3)); prints "hahaha!"" I solved this using the following loop:

function laugh(num) 
 var string = ""
 for (var i = 0; i < num; i++) 
 string = string + "ha";
 
 return string + "!";

console.log(laugh(3));

But out of curiosity, I increased num and had the loop return i instead of the string:

function laugh(num) 
 var string = "";
 for (var i = 0; i < num; i++) 
 string = string + "ha";
 
 return i;


console.log(laugh(16))

This printed 16. Now I'm confused - shouldn't i only reach 15 since one of the conditions of the loop is that i < num and num is set to 16?

In for loop first is increment done after that condition is check. So when "i" incremented to 15 then it checks 15<16, true so continue with rest of the code.
After that, "i" incremented to 16 and checks 16<16, false so came out from the loop.

So, If you print "i", then it will print current value of "i" which is 16

The value of i starts at 0, then it is checked to see if it is less than 16, and lastly, it is increased by 1. On the last iteration, i is 15, which satisfies the condition of being less than 15, and then it is incremented. Now, i is 16, which is not less than 16, so the for loop finishes with i equal to 16.

See, you start i at 0. Walk through the loop step by step. It will execute for i = 0,1,2,3...15. But now you see, the loop executes for 0. So for any loop of the form (int i = 0; i < num; i++), i reaches num. Convention is to start counting at 0 because that's convention.

The loop exit condition is i < num, so when i >= 16 code string = string + "ha"; is not executing