Javascript closures. What, why and how

What is a closure and why would I want to use one?

A JavaScript closure is a programming construct that allows a function to be permanently associated with the scope within which it was declared, the effect of this it to facilitate the creation of functions with access to long lived private variables.

What is the use case?

Consider the following basic addition function which allows numbers to added to a count but not subtracted. This implementation works but the obvious issue is that the count variable has global scope and can be trivially reset by any function.

let count = 0;

function add(value) {
  count = count + value;
}

add(2);
console.log(count);
add(3);
console.log(count);
2
5

We can attempt to resolve this by moving the count variable inside the add function.

function add(value) {
  let count = 0;

  count = count + value;
  return count;
}

console.log(add(2));
console.log(add(3));

However this does not work as the count value is now reset every time the function is called.

2
3

Implementing a closure

We can solve both of these issues by creating a function that returns a count variable along with a set of functions that control access to the count.

function additionFactory() {
  let count = 0;

  return {
    add(value) {
      count = count + value;
    },

    reset() {
      count = 0;
    },

    value() {
      return count;
    },
  };
}

const counter = additionFactory();

counter.add(2);
console.log(counter.value());
counter.add(3);
console.log(counter.value());
2
5

When the additionFactory function is called the in-scope local variables such as count come along for the ride, allowing the count to manipulated using only the defined add and reset functions, effectively creating private variables.

Further reading

Learn more about using closures to simulate private methods and how to use private classes methods

ES2022 also includes support for private class fields

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