DSA TrainerFoundations · Unit 6
Recursion
Recursion is the thing that makes people feel like they are bad at this. A function that calls itself sounds circular, like a definition that uses the word it is defining. Where does it stop? Why does it not loop forever? It feels like magic, and magic is hard to trust.
Here is the reframe that makes it click: recursion is just solving a big problem by solving a slightly smaller version of the same problem, and trusting that the smaller version works. You keep handing off a smaller piece until the piece is so small the answer is obvious. That obvious smallest case is where it stops.
Every recursive function is built from exactly two parts: a base case that says when to stop, and a recursive case that shrinks the problem and calls itself. Once you can spot those two parts, recursion stops being magic and becomes a checklist. This unit builds that checklist one piece at a time.
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A function that calls itself
Recursion is when a function solves its problem by calling itself on a smaller piece of that problem. That is the whole definition.
Think of a countdown. To count down from 3, you say "3" and then count down from 2. To count down from 2, you say "2" and then count down from 1. Each step does a tiny bit of work and hands the rest to a smaller version of itself. The function that prints the countdown literally calls the same function again with a smaller number.
What makes a function recursive?
function countdown(n) {
if (n === 0) return
console.log(n)
countdown(n - 1)
}The drills and graded test are premium
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