Gift Egbonyi

Posted on Oct 30

Understanding JavaScript Closures

#javascript #closures #webdev #beginners

Hey friends! 👋

I couldn't make it yesterday and I feel sad about that 😔. I lost a relative so I was at her wake. But I'm here today :)

Today is closures, one of those concepts that seems a bit complex but actually very practical once you see it in action.

Closures occur when a function remembers variables from its outer scope, even after that outer function has finished running.

Example 1: Basic Closure

function greet(name) {
  const greeting = 'Hello';
  return function() {
    console.log(`${greeting}, ${name}!`);
  }
}

const sayHello = greet('Alice');
sayHello(); // "Hello, Alice!"

Here’s what’s happening:

greet('Alice') returns a function.
That returned function remembers the variables greeting and name.
Even though greet has finished executing, sayHello() still works.

Example 2: Counter with Closure

Closures are perfect for keeping private variables.

function createCounter() {
  let count = 0;
  return function() {
    count++;
    return count;
  }
}

const counter = createCounter();
console.log(counter()); // 1
console.log(counter()); // 2
console.log(counter()); // 3

count is private. You can’t access it directly from outside.
Each call remembers the previous value.

Example 3: Real-World Use Case

function makeMultiplier(x) {
  return function(y) {
    return x * y;
  }
}

const double = makeMultiplier(2);
const triple = makeMultiplier(3);

console.log(double(5)); // 10
console.log(triple(5)); // 15

Closures let us create flexible and reusable functions.

Here is a Challenge for You

No codepen today but can you answer the question below?

function secretPassword(password) {
  return function(guess) {
    return guess === password ? 'Access granted' : 'Access denied';
  }
}

const checkPassword = secretPassword('1234');
console.log(checkPassword('0000')); // ?
console.log(checkPassword('1234')); // ?

“Why does this work even though secretPassword has finished running?”

🙋🏾‍♀️ Wrap-Up

A closure is a function remembering its scope even after execution.
Useful for private data, partial application, and factory functions.
Always think: “Which variables does this function remember?”

That’s it for today!
What should we talk about next Wednesday? Drop it below 👇

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See you next Wednesday 🚀, on time!

Till then, write clean code and stay curious. 🦋

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