Recursion in JavaScript

The Mirror in the Mirror

Have you ever stood between two mirrors facing each other?

You see yourself. Behind you, another reflection. Behind that, another — getting smaller and smaller until they're too tiny to see, but theoretically going on forever.

Recursion in programming works exactly like this. Except it always stops.

What is recursion?

A recursive function is a function that calls itself. Each time it does, the problem gets smaller. Eventually the problem is so small it solves itself without any more calls — then the answers bubble back up through all the mirrors.

Every recursive function needs exactly two things:

Base case — the condition that stops the recursion. Without this, the function calls itself forever until JavaScript runs out of memory (stack overflow).

Recursive case — the part where the function calls itself with a smaller, simpler version of the problem.

When should you use recursion?

Recursion is most natural when the problem has a recursive structure — when the solution to the whole is built from solutions to smaller versions of the same problem.

It shines in: tree traversal (file systems, the DOM, family trees), sorting algorithms (quicksort, mergesort), mathematical sequences (factorial, Fibonacci), and nested data (parsing JSON, flattening arrays).

For simple iteration? Use a loop. Loops are more readable and more efficient for that job.

The danger: stack overflow

If a recursive function never hits its base case, it keeps calling itself until JavaScript throws a RangeError: Maximum call stack size exceeded. Always write your base case first.

Key takeaways

Recursion is a function calling itself with a smaller version of the problem.

Every recursive function needs a base case — the stopping condition.

Without a base case, recursion runs forever and causes a stack overflow.

Use recursion when the problem naturally breaks into smaller identical sub-problems.

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