Maintain Returned Values Using Closures in JavaScript and PHP

Use Case: Showing competence in your programming interview

In your programming interview, you may be given an example like this:

var addFive = createBase(5)
addFive(8) // returns 13
addFive(16) // returns 21

And asked to solve it or make sense of it.

The question is how can we access the base value of 5 in the subsequent (and future) addFive() function calls?

The answer is closures.

A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In other words, a closure gives you access to an outer function's scope from an inner function.

Below I'll show you how you can create closures both in JavaScript and PHP to answer the interview question above.

If you'd rather see this post in video format, watch the video below and be sure to subscribe to my YouTube playlist Tyler Answers Interview Questions.

JavaScript Solution

In JavaScript, a closure is just a function that returns a function. Here's how we can make the interview question work:

const createBase = base => {
	return (n) => {
		return n + base
	}
}

const addFive = createBase(5)
addFive(8) // returns 13
addFive(16) // returns 21

Because of the closure, we are able to access the base parameter inside of the returned function after the fact.

PHP Solution

Our PHP solution is nearly identical adding only one necessary keyword: use. Use lets us pass parameters to use in closures.

function createBase($base) {
	return function($n) use ($base) {
		return $n + $base;
	};
};

$addFive = createBase(5);
$addFive(8) // returns 13
$addFive(16) // returns 21

By passing the $base parameter via use we can add it to $n and return the sum.