/ / What Is Recursion? How Does It Work?

# What Is Recursion? How Does It Work? Recursion is a method by which a problem gets solved through iteration.

In other words, a recursive function is a function that repetitively invokes itself infinitely (or until something stops it).

## Important stuff to know about recursive function

Keep these two essential pieces of info in mind whenever you choose to use recursive functions.

### Info 1: Recursion is not an IIFE

A recursive function is different from an Immediately Invoking function Expression (IIFE).

An IIFE automatically invokes itself once.

However, a recursive function automatically invokes itself repeatedly for an unlimited amount of time or until something stops its re-invocation.

### Info 2: A recursive function needs a base case

The code written to discontinue the re-invocation of a recursive function is called a base case.

It is always important to define a base case when creating a recursive function — so that the function will not run endlessly, thereby crashing the browser.

## Example of a recursive function

Below is a JavaScript code that returns a concatenation of all the values returned through the `countDown()` function’s recursive invocation.

```// Create a recursive function:
function countDown(num) {
// Define the base case of this recursive function:
if (num < 0) {
return "Recursion Stopped!";
}

// Define the recursive case:
return num + ", " + countDown(num - 1);
}

// Invoke the countDown() recursive function:
countDown(2);

// The invocation above will return:
"2, 1, 0, Recursion Stopped!"```

Note

In the recursive algorithm above, the `countDown(num - 1)` code makes the whole function a recursion because it is the code that makes `countDown()` recall itself repeatedly.

## A look at the events behind the scenes

When we invoked the `countDown` function and passed in the value `2` (that is, `countDown(2)`), the algorithm started running as follows:

### Step 1: Check if 2 is less than 0

The computer checked if the value `2` — that we passed to the `num` parameter of the `countDown` function — is less than `0`.

Since `2` is not less than `0`, the computer didn’t execute the `if` statement’s code. Instead, it skipped to the next code after the `if` statement — which is the recursion code.

### Step 2: Execute the return statement

After skipping the `if` statement, the computer executed the `return num + " " + countDown(num - 1)` code — but substituted the `num` parameter with the parameter’s value (that is, `2`) like so:

```return num + ", " + countDown(num - 1);
return 2 + ", " + countDown(2 - 1);
return 2 + ", " + countDown(1);```

### Step 3: Execute only the recursive statement

In step 2’s code above, notice that the `return` command can’t return any value because the `return` statement includes a recursive code (`countDown(1)`) recalling the `countDown` function.

Therefore, while retaining the other parts of the `return` statement (that is, `2 + ", " +`), the computer will execute only the recursion code (`countDown(1)`).

In other words, the `countDown(1)` code will automatically invoke the `countDown` function while passing in the value `1`. Then, the algorithm will start running again by checking if `1` is less than `0`.

Since `1` is not less than `0`, the computer skipped to the recursion code like so:

```return 2 + ", " + num + ", " + countDown(num - 1);
return 2 + ", " + 1 + ", " + countDown(1 - 1);
return 2 + ", " + 1 + ", " + countDown(0);```

### Step 4: Invoke only the recursion code

Again, notice that the `return` command (in step 3) cannot return any value because the `return` statement includes a recursion code (`countDown(0)`) that recalls the `countDown` function.

Therefore, while retaining the other parts of the `return` statement (that is, `2 + ", " + 1 + ", " +`), the computer will execute only the recursion code (`countDown(0)`). So, the `countDown(0)` code will automatically invoke the `countDown` function while passing in the value `0`.

Then, the function will start running again by checking if `0` is less than `0`.

Since `0` is not less than `0`, the computer skipped to the recursion code like so:

```return 2 + ", " + 1 + ", " + num + ", " + countDown(num - 1);
return 2 + ", " + 1 + ", " + 0 + ", " + countDown(0 - 1);
return 2 + ", " + 1 + ", " + 0 + ", " + countDown(-1);```

### Step 5: Execute only the recursion code

Yet again, the `return` command (in step 4) can’t return any value because the `return` statement includes a recursion code (`countDown(-1)`) recalling the `countDown` function.

Therefore, while retaining the other parts of the `return` statement (that is, `2 + ", " + 1 + ", " + 0 + ", " +`), the computer will execute only the recursion code (`countDown(-1)`). So, the `countDown(-1)` code will automatically invoke the `countDown` function while passing in the value `-1`.

Then, the function will start running again by checking if `-1` is less than `0`.

At this point, `-1` is less than `0`. Therefore, the computer will execute the code of the `if` statement by returning the value `“Recursion Stopped!”` like so:

`return 2 + ", " + 1 + ", " + 0 + ", " + "Recursion Stopped!";`

At last, the `return` statement now has values it can validly concatenate and return. Therefore, the returned value from `countDown` will be:

`"2, 1, 0, Recursion Stopped!"`

## Wrapping it up

In this article, we learned that a recursive function is a function that repeatedly recalls itself until something stops the recall.