Javascript: Why does prefix operator work with modulus but not postfix operator?

Question

I am trying to make a function that increments until it reaches 3 and then starts back from zero (so, called three times it would log 0 then 1 then 2. When using the % operator with the pre and post fix operators, I have confusing results.

Here are my two functions:

var i, j = 0, 0
function run () { 
  console.log(i); 
  i = i++ % 3;
 } // Called three times logs 0, 0, 0

And

function newRun () {
  console.log(j);
  j = ++j % 3;
} // Called three times it logs 0, 1, 2

Why does the prefix operator work and the postfix does not (i.e. in the first function why is i never incremented?

Answer 1

This doesn't have anything to do with the modulo operator. Even

i = i++;

doesn't work - it takes a value, increments it, and then overwrites it with the initially taken value. See also Difference between i++ and ++i in a loop? for how they work.

You probably want to write

i = (i + 1) % 3;