Counter

Counter

What is this problem about?

The Counter interview question is a fundamental JavaScript/TypeScript problem. You are asked to write a function that takes an integer $n$ and returns a "counter" function. This returned function, when called for the first time, should return $n$. For every subsequent call, it should return the previous value plus one ($n+1$, then $n+2$, and so on).

Why is this asked in interviews?

Tech giants like Google and Meta use the Counter coding problem to test a candidate's understanding of Closures. Closure is a core concept in functional programming where an inner function has access to the variables of its outer function even after the outer function has finished execution. This problem is a quick way to verify if a candidate knows how to maintain state without global variables.

Algorithmic pattern used

This problem uses the Closure pattern.

1. Define a parent function that accepts an initial value.
2. Inside the parent, define a local variable to store the "current" state.
3. Return a child function that increments this local variable and returns the old (or new) value.

Example explanation

Initialize createCounter(10).

• Call 1: Returns 10. Internally, the state moves to 11.
• Call 2: Returns 11. Internally, moves to 12.
• Call 3: Returns 12. The counter "remembers" its progress because the returned function maintains a reference to the environment in which it was created.

Common mistakes candidates make

• Global Variables: Using a global variable to track the count, which would lead to bugs if multiple counters were created.
• Increment Logic: Returning $n+1$ on the very first call instead of $n$.
• Syntactic confusion: Misunderstanding the requirement to return a function rather than a number.

Interview preparation tip

Closures are the backbone of many design patterns in JavaScript (like the Module pattern). Be prepared to explain exactly where the state is being stored in memory.