**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!"
```

NoteIn 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.

Thanks for reading!

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