# Recursion: The Important Lesson on Recursive Functions

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).

This article will use an example to illustrate how a recursive function works.

**Note:**

- 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 reinvocation. - The code written to discontinue the reinvocation 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 recursion 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 (recursion statement): 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 recursion code

In step 2’s code above, notice that the `return`

command 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 + ", " +`

), 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 the `countDown`

recursion function will be:

"2, 1, 0, Recursion Stopped!"

## Conclusion

All in all, a **recursive function** is a function that repeatedly recalls itself until something stops the recall.

## Credit

Featured Image: Menger Maze Pool by Pete Linforth