Build a Counter with a Closure in JavaScript (Exercise)

The challenge

Write makeCounter(start, step) that returns a function. Each time that function is called, it returns the next number: first start + step, then start + 2 × step, and so on. Each counter must be independent.

Example

const byTwo = makeCounter(0, 2);
byTwo();  // 2
byTwo();  // 4

Try it

Write your answer below, then press Check my answer. Your code runs right here in your browser, and the tests tell you what works.

Your solution

It prints

What the tests check

Counting by 2 from 0 gives 2, then 4: (() => { const c = makeCounter(0, 2); c(); return c(); })() gives 4
Counting down from 10 gives 9: (() => { const c = makeCounter(10, -1); return c(); })() gives 9
Counters don’t share: (() => { const a = makeCounter(0, 1); const b = makeCounter(0, 1); a(); a(); return b(); })() gives 1

Hints

Stuck? Open one hint at a time. Try again after each one.

Hint 1

Keep a let current = start; inside makeCounter, and return a function that changes it.

Hint 2

Each call to makeCounter creates a fresh current, so counters are independent.

A solution

There’s always more than one right way. Read it, make sure you understand it, then try writing it yourself without looking.

Show one possible solution

Solution

function makeCounter(start, step) {
  let current = start;
  return () => {
    current += step;
    return current;
  };
}

Learn what you need

This exercise uses what Grade 9: Scope, Hoisting & Closures teaches. Read those notes first if any of it is new.

Build a counter with a closure