Implement a make counter function

Mar 27, 2024

The makeCounter problem is the classic interview question when it comes to showcasing closures ability to do data hiding and encapsulation. It’s also the best example, using which we can explain closures.

Variation: Basic implementation

1. Basic Implementation

Implement a makeCounter function which returns a function, which when invoked increment the initial value by a step value provided.

function makeCounter(initialValue = 0, step = 1) {
  let count = initialValue;

  function counterClosure() {
    let result = count;
    // assuming we need to return initial value in first invocation
    count += step;
    return result;

    /** Note:
     * In cases when we don't need to return the intial value in first invocation
     * we can simply alter the logic to be
     * count+=step;
     * return count;
     */
  }

  return counterClosure;
}

// Usage
const counter = makeCounter(0, 1);

console.log(counter()); // yields 0
console.log(counter()); // yields 1
console.log(counter()); // yield 2

Variation: Increment, Decrement, Get, Set and Reset

2. Counter with state utilities implementation

Implement a makeCounter utility function which ability to increment, decrement, set, reset and get values of the counter.

function makeCounter(initialValue = 0, step = 1) {
  let count = initialValue;

  const counterUtilities = {
    get() {
      return count;
    },
    set(newValue) {
      count = newValue;
      return this.get();
    },
    reset() {
      count = initialValue;
      return this.get();
    },
    increment() {
      count = count + step;
      return this.get();
    },
    decrement() {
      count = count - step;
      return this.get();
    }
  };

  return counterUtilities;
}

// Usage
const counter = makeCounter(0, 1);

console.log(counter.get()); // yields 0
console.log(counter.increment()); // yields 1
console.log(counter.increment()); // yields 2
console.log(counter.increment()); // yields 3
console.log(counter.decrement()); // yields 2
console.log(counter.get()); // yields 2
console.log(counter.set(5)); // yields 5
console.log(counter.get()); // yields 5
console.log(counter.reset()); // yields 0

Note: There can also be a requirement to implement a min and max limit values on the counter. This is very straight forward, where before changing the counter values, we can simply run a check and limit the value in case it overflows or underflows the max and min values.

Implement a make counter function

1. Basic Implementation

2. Counter with state utilities implementation

Further Reading