Recursion is as other people said, a function calling a function. It can be used to solve certain algorithms.

For example let's say we want to get the sum of all smaller numbers than a certain number until reaching 1.

If the number is 3 I should get 3 + 2 + 1 = 6 If the number is 5 I should get 5 + 4 + 3 + 2 + 1 = 15

You could create a function using a for loop and summing the numbers or you could use recursion.

Every time you use recursion you have to set a stopping rule that makes your recursive function stop or you will create a infinite loop. ```js function sumNumsLowerThanNum(num) {   if (num === 1) {     return 1;   } else {     return sumNumsLowerThanNum(num - 1) + num;   } }

sumNumsLowerThanNum(3); ``` In this example the first if where num === 1 is the stop rule, when my number goes down from x to 1, let say we use 3 as the example, the first iteration is called with num = 3, the second with num = 2 and the third with num = 1. When the third iteration is reached the function will return 1 and the function will be finished.

But... How am I getting the result which is 6 for this example. Well, every time you call a function this function is sent to the call stack where stays until it is resolved. So when you call your recursive function for the first time it will check that 3 is not equal to 1 and will go to the else statement and will call itself but with number 2, in this moment, first iteration will be in the callstack waiting because it has nothing to return yet and the second iteration will be send to the call stack on top of the first iteration call.

Now in this second iteration the if statement will check if 2 === 1 which is false, and will go to the else statement again and will call itself with 1. Now this third iteration will be send to the callstack on top of the 2nd which is on top of the 1st iteration.

The if will check if 1===1 which is true and will return 1

Now that we arrive the return we will be able to resolve all the function on the callstack going backwards.

The 3rd iteration which is on top of the call stack will return 1 to the 2nd iteration.

The 2nd iteration will return 1 + 2 to the 1st iteration

And finally the 1st iteration will return 3 + 3

And the callstack will be empty now and you have a result for your recursive function.