Recursion is simply when a function calls itself.

Lets jump right in and take a look at probably the most famous recursion example. This example returns the factorial of a supplied integer:

 function factorial(x) {
  if (x < 0) return;
  if (x === 0) return 1;
  return x * factorial(x - 1);
}

factorial(3);
// 6

Woah. It’s Okay if that makes no sense to you.The important part is happening on line 4:return x * factorial(x — 1);. As you can see the function is actually calling itself again (factorial(x-1)) but with a parameter that is one less than when it was called the first time. This makes it a recursive function.

To get the factorial of a number you multiply that number by itself minus one until you reach the number one.

Example 1: The factorial of 4 is 4 * 3 * 2 * 1 or 24.

"Example 2: The factorial of 2 is just 2 * 1 or "2.

Awesome now that our High School Math lesson is over we can return to the good stuff!

All recursive functions should have three key features:

Simply put: if(something bad happend)[ STOP }; The Termination Condition is our recursion fail-safe. Think of it like your emergency brake. It’s put there in case of bad input to prevent the recursion from ever running. In our factorial example if(x < 0) return;is our termination condition. It’s not possible to factorial a negative number and thus we don’t even want to run our recursion if a negative number is input.

Simply put: if(this happens) [ Yay! We're done }; The Base Case is similar to our termination condition in that it also stops our recursion. But remember the termination condition is a catch-all for bad data. Whereas the base case is the goal of our recursive function. Base cases are usually within an if statement. In the factorial example if(x === 0) return 1; is our base case. We know that once we’ve gotten x down to zero we’ve succeeded in determining our factorial!

Simply put: Our function calling itself. In the factorial examplereturn x * factorial(x — 1); is where the recursion actually happens. We’re returning the value of the number x multiplied by the value of whateverfactorial(x-1)evaluates to.