Practical closure use cases in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Closure and private variables

What is the output of this code when calling counter.increment() three times and then counter.get()?

Javascript

function createCounter() {
  let count = 0;
  return {
    increment() {
      count++;
    },
    get() {
      return count;
    }
  };
}
const counter = createCounter();
counter.increment();
counter.increment();
counter.increment();
console.log(counter.get());

A 0

B TypeError

C undefined

D 3

💡 Hint

Think about how the variable count is kept inside the function and accessed only through returned methods.

The variable count is private inside createCounter. Each call to increment() increases it by 1. After three increments, get() returns 3.

❓ Predict Output

intermediate

Closure capturing loop variable

What will be logged by the following code?

Javascript

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

A 3 3 3

B 0 1 2

C undefined undefined undefined

D 0 0 0

💡 Hint

Remember that var is function-scoped, not block-scoped.

The variable i is shared in the loop because it is declared with var. When the functions run, i is 3, so all return 3.

❓ Predict Output

advanced

Fixing closure in loop with IIFE

What is the output of this fixed code?

Javascript

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

A undefined undefined undefined

B 3 3 3

C 0 1 2

D TypeError

💡 Hint

The IIFE captures the current value of i as j for each function.

The IIFE creates a new scope for each iteration, capturing the current i value as j. Each function returns its own j value.

🧠 Conceptual

advanced

Why use closures for data privacy?

Which of the following best explains why closures are used to create private data in JavaScript?

A Closures allow functions to access variables from their outer scope even after the outer function has finished executing.

B Closures prevent any function from accessing variables outside its own scope.

C Closures automatically encrypt variables to keep them private.

D Closures make variables global so all functions can share them.

💡 Hint

Think about how inner functions remember variables from where they were created.

Closures keep the variables alive in the outer function's scope, allowing inner functions to access them even after the outer function returns, enabling data privacy.

❓ Predict Output

expert

Closure with asynchronous callbacks

What will be the output of this code snippet?

Javascript

function delayedLogger() {
  for (let i = 0; i < 3; i++) {
    setTimeout(() => {
      console.log(i);
    }, 100);
  }
}
delayedLogger();

A undefined undefined undefined

B 0 1 2

C 3 3 3

D 0 0 0

💡 Hint

Consider how let differs from var in loops and closures.

Using let creates a new binding for each loop iteration, so each callback logs its own i value.

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