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JavaScript

JS Basics

JS Variables & Operators

JS Data Types & Conversion

JS Numbers & Math

JS Strings

JS Dates

JS Arrays

JS Control Flow

JS Loops & Iteration

JS Functions

JS Objects

JS Classes & Modules

JS Async Programming

JS Advanced

JS HTML DOM

JS BOM (Browser Object Model)

JS Web APIs

JS AJAX

JS JSON

JS Graphics & Charts

Function Closures

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📘 JavaScript Closures – Functions Remembering Their Scope

A closure is created when a function retains access to its lexical scope, even after the outer function has finished executing.

In simple terms:

A closure is a function that remembers the variables from its outer function, even after that outer function has returned.

🔹 Example 1 – Basic Closure

function outer() {
  let count = 0;
  return function () {
    count++;
    console.log(count);
  };
}

const counter = outer();
counter(); // 1
counter(); // 2

✅ The inner function "remembers" the count variable even after outer() has finished.

🔹 Example 2 – Multiple Closures

function greet(msg) {
  return function (name) {
    console.log(`${msg}, ${name}`);
  };
}

const sayHi = greet("Hi");
sayHi("Ravi"); // Hi, Ravi
sayHi("Neha"); // Hi, Neha

✅ Each call to greet() creates a new closure with its own msg.

Practice Questions

Q1. Write a function createCounter() that returns another function which increases and returns a private counter each time it's called.

Q2. Create a function greet(message) that returns a function which accepts a name and logs message + name.

Q3. Create a function multiplier(factor) that returns a function to multiply any number by that factor.

Q4. Write a closure that stores a secret message and provides a method to read it but not modify it.

Q5. Create a once() function that allows the given function to run only once and ignores subsequent calls.

Q6. Write a closure that maintains a list and provides addItem() and getItems() methods.

Q7. Create a closure that tracks the number of times a button is clicked (simulate clicks using a function).

Q8. Build a function makeAdder(x) that returns another function which adds x to its argument.

Q9. Create a timer() closure that stores the start time and returns the elapsed time when called again.

Q10. Write a closure-based cache() function that stores and returns computed values based on input (e.g., square of number).

Function Closures Quiz

Q1: What is a closure in JavaScript?

A. A function inside another function
B. A function that has access to variables in its outer scope
C. A method that closes the browser tab
D. A function that has no parameters

Q2: Which variables does a closure have access to?

A. Only its local variables
B. Its own variables and parameters
C. Outer function’s variables
D. All of the above

Q3: What is the purpose of using closures?

A. To create global variables
B. To make code slower
C. To encapsulate logic and preserve state
D. To call a function twice

Q4: Which concept allows a function to “remember” the environment it was created in?

A. Hoisting
B. Closure
C. Prototype
D. Callback

Q5: In a closure, what happens to variables declared in the outer function after it returns?

A. They are deleted
B. They are stored in the heap
C. They remain accessible to inner functions
D. They throw an error

Q6: Which of these is a real-life use case of closures?

A. Tracking state in event handlers
B. Creating private variables
C. Function factories
D. All of the above

Q7: Closures are most useful for:

A. Creating global variables
B. Managing private state inside functions
C. Increasing browser performance
D. Making code synchronous
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