initialExpression

I just want to confirm what you mean by initialExpression. for example consider this loop.

for (var i = 1; i <= 10; i += 1) { //do something }

for defines the for loop. var i = 1 creates a variable named i and initializes it's value to 1. We use this as a counter as the loop executes. i <= 10 is the condition for the loop to keep running. In this case it will only keep running if our counter, i, is less than or equal to 10. i += 1 increments the counter each time it runs. This way the loop will eventually stop running.

Is this what you were needing more help with? If so I hope my response helped to clarify things. If you need additional help let me know.