Practical closure use cases in Javascript - Mini Project: Build & Apply

Practical closure use cases

📖 Scenario: Imagine you are building a simple counter app that keeps track of how many times a button is clicked. You want to keep the count private and only allow it to be changed through specific functions.

🎯 Goal: Build a counter using closures in JavaScript that keeps the count private and provides functions to increment and get the current count.

📋 What You'll Learn

Create a function that returns two functions: one to increment the count and one to get the current count.

Use a closure to keep the count variable private inside the function.

Demonstrate the usage by incrementing the count a few times and printing the current count.

💡 Why This Matters

🌍 Real World

Closures are used in web apps to keep data private and avoid accidental changes from outside code.

💼 Career

Understanding closures helps you write safer, cleaner JavaScript code, a key skill for frontend and backend developers.

1

Create the counter function with a private count variable

Create a function called createCounter that declares a variable count and sets it to 0 inside it.

Javascript

# Your code here
function createCounter() {
    // Your code here
}

Hint

Think of count as a secret number only createCounter can see.

2

Add increment and get functions inside createCounter

Inside createCounter, create two functions: increment that adds 1 to count, and getCount that returns the current count. Return an object with these two functions as properties.

Javascript

function createCounter() {
    let count = 0;
    // Your code here
}

Hint

These inner functions can access count because of closure.

3

Create a counter instance and use increment

Create a variable called counter and set it to the result of calling createCounter(). Then call counter.increment() three times to increase the count.

Javascript

function createCounter() {
    let count = 0;
    function increment() {
        count += 1;
    }
    function getCount() {
        return count;
    }
    return { increment, getCount };
}

// Your code here

Hint

Each call to increment adds one to the private count.

4

Print the current count using getCount

Use console.log to print the result of counter.getCount().

Javascript

function createCounter() {
    let count = 0;
    function increment() {
        count += 1;
    }
    function getCount() {
        return count;
    }
    return { increment, getCount };
}

const counter = createCounter();
counter.increment();
counter.increment();
counter.increment();

// Your code here

Hint

The count should be 3 after three increments.

Practice

1. What is the main benefit of using closures in JavaScript?

easy

A. They make code run faster by optimizing loops.

B. They allow functions to remember variables from their creation context.

C. They automatically free memory when variables are no longer used.

D. They let you write code without using functions.

Solution

1. Step 1: Understand closure concept

   Closures let a function remember variables from the place where it was created, even after that place has finished running.

2. Step 2: Compare options

   Only They allow functions to remember variables from their creation context. correctly describes this key feature. Other options describe unrelated or incorrect ideas.

3. Final Answer:

   They allow functions to remember variables from their creation context. -> Option B

4. Quick Check:

   Closure = Remember variables [OK]

Hint: Closures remember variables where functions are made [OK]

Common Mistakes:

• Thinking closures speed up code automatically
• Confusing closures with garbage collection
• Believing closures remove the need for functions

2. Which of the following is the correct syntax to create a closure that keeps a private counter?

easy

A. function counter() { return count++; let count = 0; }

B. function counter() { var count = 0; count++; return count; }

C. let count = 0; function counter() { count++; return count; }

D. function counter() { let count = 0; return function() { count++; return count; }; }

Solution

1. Step 1: Identify closure pattern

   function counter() { let count = 0; return function() { count++; return count; }; } defines a function that returns another function which accesses the variable 'count' inside its scope, creating a closure.

2. Step 2: Check other options

   function counter() { var count = 0; count++; return count; } increments count but does not return a function, so no closure. let count = 0; function counter() { count++; return count; } uses a global variable, not private. function counter() { return count++; let count = 0; } has syntax error (return before variable declaration).

3. Final Answer:

   function counter() { let count = 0; return function() { count++; return count; }; } -> Option D

4. Quick Check:

   Closure = function inside function with private variable [OK]

Hint: Closure needs inner function accessing outer variable [OK]

Common Mistakes:

• Returning value instead of function for closure
• Using global variables instead of private ones
• Placing return before variable declaration

3. What will be the output of the following code?

function makeAdder(x) {
  return function(y) {
    return x + y;
  };
}
const add5 = makeAdder(5);
console.log(add5(3));

medium

A. 53

B. NaN

C. 8

D. undefined

Solution

1. Step 1: Understand makeAdder function

   makeAdder returns a function that adds its argument y to x, which is remembered via closure.

2. Step 2: Calculate add5(3)

   add5 is makeAdder(5), so x=5. Calling add5(3) returns 5 + 3 = 8.

3. Final Answer:

   8 -> Option C

4. Quick Check:

   Closure adds remembered x to y = 8 [OK]

Hint: Closure remembers x=5, adds y=3 to get 8 [OK]

Common Mistakes:

• Concatenating numbers as strings
• Forgetting closure remembers x
• Expecting undefined or NaN

4. Find the error in this closure example:

function createFuncs() {
  let funcs = [];
  for (var i = 0; i < 3; i++) {
    funcs.push(function() { return i; });
  }
  return funcs;
}
const functions = createFuncs();
console.log(functions[0]());

medium

A. All functions return 3 instead of 0, 1, 2 due to var scoping.

B. Syntax error because of missing semicolons.

C. functions array is empty because push is not called.

D. The code runs correctly and prints 0.

Solution

1. Step 1: Analyze loop variable scope

   Using var for i means one shared variable. All functions capture the same i, which after loop ends is 3.

2. Step 2: Understand closure effect

   Each function returns i, but i is 3 at call time, so all return 3, not 0,1,2.

3. Final Answer:

   All functions return 3 instead of 0, 1, 2 due to var scoping. -> Option A

4. Quick Check:

   var in loop causes shared i = 3 [OK]

Hint: var in loop shares variable; use let for block scope [OK]

Common Mistakes:

• Assuming each function captures separate i
• Thinking semicolons cause error
• Expecting functions array to be empty

5. How can you modify this code to create an array of functions each returning its own index?

function createFuncs() {
  let funcs = [];
  for (var i = 0; i < 3; i++) {
    funcs.push(function() { return i; });
  }
  return funcs;
}
const functions = createFuncs();
console.log(functions[0](), functions[1](), functions[2]());

hard

A. Replace var with let in the for loop declaration.

B. Declare i with var but add a closure with an IIFE inside the loop.

C. Use a global variable instead of i.

D. No change needed; code works as is.

Solution

1. Step 1: Understand problem with var

   var is function-scoped, so all functions share same i, which ends at 3.

2. Step 2: Fix with let for block scope

   Replacing var with let makes i block-scoped, so each iteration has its own i captured by closure.

3. Final Answer:

   Replace var with let in the for loop declaration. -> Option A

4. Quick Check:

   let creates separate i per loop iteration [OK]

Hint: Use let in loops to create separate closure variables [OK]

Common Mistakes:

• Using global variables causing shared state
• Thinking IIFE is only solution
• Believing code works without changes