Recursion: A Useful Look at How Recursive Functions Work

Recursion is the 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 outputs a concatenation of all the values returned through the

countDown()

// 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)

countDown()

A look at the events behind the scenes

When we invoked the

countDown

2

countDown(2)

Step 1: Check if

2

0

The computer checked if the value

2

num

countDown

0

Since

2

0

if

if

Step 2: Execute the

return

After skipping the

if

return num + " " + countDown(num - 1)

num

2

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

return

countDown(1)

countDown

Therefore, while retaining the other parts of the

return

2 + ", " +

countDown(1)

In other words, the

countDown(1)

countDown

1

1

0

Since

1

0

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

return

countDown(0)

countDown

Therefore, while retaining the other parts of the

return

2 + ", " + 1 + ", " +

countDown(0)

countDown(0)

countDown

0

Then, the function will start running again by checking if

0

0

Since

0

0

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

return

countDown(-1)

countDown

Therefore, while retaining the other parts of the

return

2 + ", " + 1 + ", " + 0 + ", " +

countDown(-1)

countDown(-1)

countDown

-1

Then, the function will start running again by checking if

-1

0

At this point,

-1

0

if

“Recursion Stopped!”

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

At last, the

return

countDown

"2, 1, 0, Recursion Stopped!"

Conclusion

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

Previously published at - CodeSweetly