Recursion in JavaScript
In JavaScript, recursion is when you call a function from within itself until or unless a condition is a met. Today, we’re going to look at how it works.
Let’s dig in!
Count up by one
Let’s say we have a number, and we want to increase its value by 1. We have a helper function that takes a number, and returns that number plus 1.
/**
* Add one to a number
* @param {Number} num The number to increase
* @return {Number} The new number
*/
function upByOne (num) {
return num + 1;
}
To use it, you can do something like this.
// returns 42
let more = upByOne(41);
This is an absurd example, of course. It would be faster and easier to just add 1 to the number.
But it will help illustrate how recursion works, so let’s roll with it.
Adding recursion
Now, let’s say you wanted whatever number was returned to have a value of 10 or higher. If you passed in 7, instead of getting 8 back, you want to get back 10.
We can use recursion for that!
Inside our upByOne() function, we’ll add 1 to num. Then, we’ll check if it’s value is less than 10. If so, we’ll pass it back into upByOne() and return the result. Otherwise, we’ll return the new number.
/**
* Add one to a number
* @param {Number} num The number to increase
* @return {Number} The new number
*/
function upByOne (num) {
let bigger = num + 1;
if (bigger < 10) {
return upByOne(bigger);
}
return bigger;
}
If you passed in 7, for example, bigger would have a value of 8. The if statement would return true, and 8 would get passed into upByOne().
The number would get increased to 9, the check would run again, and the process would repeat, until bigger had a value of 10.
// returns 10
let minOfTen = upByOne(7);