Closures in Javascript - Mini Project: Build & Apply

Understanding Closures in JavaScript

📖 Scenario: Imagine you are building a simple counter app. You want to create counters that remember their own counts even after you use them multiple times. This is like having a personal tally for each counter.

🎯 Goal: You will build a function that creates counters using closures. Each counter will keep its own count and increase it when called.

📋 What You'll Learn

Create a function that returns another function

Use a variable inside the outer function to store the count

The inner function should increase and return the count

Create at least one counter using the function

Show the count increasing by calling the counter multiple times

💡 Why This Matters

🌍 Real World

Closures help keep data private and create functions with memory, like counters, timers, or settings in apps.

💼 Career

Understanding closures is important for JavaScript developers to write clean, efficient, and bug-free code that manages state safely.

1

Create the outer function with a count variable

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

Javascript

function createCounter() {
  // Your code here
}

Hint

Remember to use let to declare the count variable inside the function.

2

Return an inner function that increases count

Inside the createCounter function, write a return statement that returns a new function. This inner function should increase count by 1 and return the new value.

Javascript

function createCounter() {
  let count = 0;
  // Return a function that increases and returns count
  // Your code here
}

Hint

The inner function should access and change the count variable from the outer function.

3

Create a counter and call it multiple times

Create a variable called myCounter and set it to the result of calling createCounter(). Then call myCounter() three times, storing each result in variables first, second, and third.

Javascript

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

// Create myCounter and call it three times
// Your code here

Hint

Call createCounter() once and save it to myCounter. Then call myCounter() three times and save each result.

4

Print the results to see the counts

Use console.log to print the values of first, second, and third on separate lines.

Javascript

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

const myCounter = createCounter();
const first = myCounter();
const second = myCounter();
const third = myCounter();

// Print the counts
// Your code here

Hint

Use console.log three times to print each count value on its own line.

Practice

1. What is a closure in JavaScript?

easy

A. A function that has no parameters

B. A function that runs only once

C. A function that returns another function without variables

D. A function that remembers variables from its creation context

Solution

1. Step 1: Understand closure definition

   A closure is a function that keeps access to variables from the place where it was created, even after that place has finished running.

2. Step 2: Match definition to options

   A function that remembers variables from its creation context correctly describes this behavior, while others describe unrelated concepts.

3. Final Answer:

   A function that remembers variables from its creation context -> Option D

4. Quick Check:

   Closure = function remembering variables [OK]

Hint: Closures remember variables from where they were made [OK]

Common Mistakes:

• Thinking closures are functions without parameters
• Confusing closures with immediately invoked functions
• Believing closures only run once

2. Which of the following is the correct syntax to create a closure in JavaScript?

easy

A. function outer() { let x = 5; function inner() { return x; } }

B. function outer() { let x = 5; return x; }

C. function outer() { let x = 5; return function inner() { return x; } }

D. function outer() { return function() { let x = 5; } }

Solution

1. Step 1: Identify closure syntax

   A closure requires an inner function that accesses variables from the outer function and is returned so it can be used later.

2. Step 2: Check options for correct closure

   function outer() { let x = 5; return function inner() { return x; } } returns an inner function that uses outer variable x, forming a closure. Others either don't return the function or don't access outer variables correctly.

3. Final Answer:

   function outer() { let x = 5; return function inner() { return x; } } -> Option C

4. Quick Check:

   Return inner function accessing outer variable [OK]

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

Common Mistakes:

• Not returning the inner function
• Returning only the variable, not a function
• Defining inner function but not using outer variables

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

function makeCounter() {
  let count = 0;
  return function() {
    count += 1;
    return count;
  };
}
const counter = makeCounter();
console.log(counter());
console.log(counter());

medium

A. 1 and 2

B. 0 and 1

C. undefined and undefined

D. 1 and 1

Solution

1. Step 1: Understand closure behavior in makeCounter

   The inner function increments count and returns it. The variable count is remembered by the closure.

2. Step 2: Trace the calls to counter()

   First call: count = 0 + 1 = 1, returns 1. Second call: count = 1 + 1 = 2, returns 2.

3. Final Answer:

   1 and 2 -> Option A

4. Quick Check:

   Closure keeps count incrementing: 1, 2 [OK]

Hint: Closure keeps variable state between calls [OK]

Common Mistakes:

• Thinking count resets each call
• Expecting output 1 and 1
• Confusing closure with global variable

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]());
console.log(functions[1]());

medium

A. All functions return 3 instead of 0, 1, 2

B. Syntax error due to missing semicolons

C. Functions return undefined

D. Functions throw a ReferenceError

Solution

1. Step 1: Identify loop variable scope

   The loop uses var i, which is function-scoped, so all inner functions share the same i.

2. Step 2: Understand closure captures final i value

   After loop ends, i is 3. All functions return 3, not 0, 1, 2 as expected.

3. Final Answer:

   All functions return 3 instead of 0, 1, 2 -> 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 ReferenceError or undefined

5. How can you fix the following code so each function remembers its own value of i?

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

hard

A. Use var but call the function immediately

B. Change var to let in the for loop declaration

C. Return i directly instead of a function

D. Declare i as a global variable

Solution

1. Step 1: Understand block scope with let

   Using let in the for loop creates a new binding of i for each iteration, so closures capture different i values.

2. Step 2: Compare with var behavior

   var is function-scoped, so all closures share the same i. Changing to let fixes this by giving each iteration its own i.

3. Final Answer:

   Change var to let in the for loop declaration -> Option B

4. Quick Check:

   let in loop creates separate i per iteration [OK]

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

Common Mistakes:

• Trying to fix by making i global
• Returning i instead of function
• Using var with immediate call but misunderstanding closures