since javascript will bound variable scope to its declaration,
or says it will keep the local context.
let’s define a function that will return a function for step add.

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function step_iterator(start, step) {
  return function() {
    var res = start;
    start = start + step;
    return res;
  }
}

then we can get an iterator by call the function

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var numberWithStep2 = step_iterator(0, 2);

and javascript will keep it as a function in numberWithStep2

each time we call the function numberWithStep2

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numberWithStep2();
// will return 0
numberWithStep2();
// will return 2
numberWithStep2();
// will return 4

pretty cool! huh?