NOTES.md

Explanation: 2620. Counter

Key Topics

Closures, Factory Functions

Problem In My Own Words

Create a function that returns a counter function that increments the given initial value every time that counter function is called. Every time the counter function is called, the number should be one more than its previous value and shouldn't reset to its initial value.

Final Solution

• Time Complexity: O(1) for each counter function call

function createCounter(n) {
	let count = n;
	return () => count++;
}

Process

How closure works here:

The outer function createCounter(n) takes the initial value of n and initializes the count variable to n. This allows me to create a starting point for the returned (inner) function to reference when the number needs to be incremented. The inner function would have access to the count variable through closure. The inner function returns the current value of count and increments the count for the next counter() call.

The use of let when assigning the count variable is imperative for the inner function to work because it allows the inner function to reassign the value.

In order to use and test the createCounter(n) function:

1. We can create a counter by calling the createCounter(10) function - with 10 as the initial value.
2. The returned function that does the count incrementing is stored in the const counter variable.
3. Each counter() call will return the current value and then increment it.

Difference between pre-increment and post-increment

• count++ returns the value and then, increments the value
• ++count increments the value, and then returns the value

Test Cases & Findings

const counter = createCounter(10);
counter(); // 10 - initial value
counter(); // 11 - incremented once
counter(); // 12 - incremented twice

/* Note: Additional counters can be created and they would have their own count variable */
const secondCounter = createCounter(151);
secoundCunter(); // 151 - inital value
secondCcounter(); // 152 - incremented once
secondCcounter(); // 153 - incremented twice

counter(); // 13 - incremented thrice

What I Learned

• How closures maintain state
• Difference between post-increment count++ and pre-increment (++count)
• How factory functions work
• Function scope

Resources

• MDN: Factory Functions
• Digital Ocean: Understanding Closures