Closures in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a closure with loop variable

What is the output of the following code snippet?

Javascript

function createFunctions() {
  const funcs = [];
  for (let 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 0 0 0

B 3 3 3

C undefined undefined undefined

D 0 1 2

💡 Hint

Think about how the let keyword affects the loop variable scope inside closures.

Using let in the loop creates a new binding for each iteration, so each function remembers its own i value.

❓ Predict Output

intermediate

Closure capturing variable after loop with var

What will be printed by this 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 undefined undefined undefined

C 0 0 0

D 0 1 2

💡 Hint

Consider how var differs from let in loops and closures.

Using var declares i in function scope, so all functions share the same i which ends at 3 after the loop.

🧠 Conceptual

advanced

Why closures keep variables alive

Why do closures in JavaScript keep variables alive even after the outer function has finished?

A Because closures create a copy of the variables and store them separately.

B Because closures keep a reference to the variables in the outer function's scope, preventing garbage collection.

C Because JavaScript automatically duplicates all variables used inside functions.

D Because closures run the outer function again every time they are called.

💡 Hint

Think about how memory and references work in JavaScript functions.

Closures keep references to variables in their outer scope, so those variables stay in memory as long as the closure exists.

❓ Predict Output

advanced

Output of nested closures with parameters

What is the output of this code?

Javascript

function outer(x) {
  return function inner(y) {
    return function innermost(z) {
      return x + y + z;
    };
  };
}
const result = outer(1)(2)(3);
console.log(result);

A undefined

B 123

C 6

D NaN

💡 Hint

Add the numbers passed through each function call.

Each function returns another function that remembers the parameters passed, so the final sum is 1 + 2 + 3 = 6.

❓ Predict Output

expert

Closure with asynchronous behavior and loop

What will this code print to the console?

Javascript

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

A 3 3 3

B 0 1 2

C undefined undefined undefined

D 0 0 0

💡 Hint

Consider how var and asynchronous callbacks interact.

The loop finishes before the timeouts run, so i is 3 in all callbacks, printing 3 three times.