Javascript Interview Questiosn

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🤔 JavaScript Interview Question: var vs let with setTimeout

Can you guess the output before running the code? 👇

for (var i = 0; i < 3; i++) {
    setTimeout(function () {
        console.log(i);
    }, 1000);
}

for (let j = 0; j < 3; j++) {
    setTimeout(function () {
        console.log(j);
    }, 1000);
}

❓What will be printed after 1 second?

Option A

0
1
2
0
1
2

Option B

3
3
3
0
1
2

Option C

0
0
0
3
3
3

👇 Comment your answer before reading the explanation!

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✅ Correct Answer: Option B

3
3
3
0
1
2

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Why does var print 3 3 3?

for (var i = 0; i < 3; i++) {
    setTimeout(() => {
        console.log(i);
    }, 1000);
}

var is function-scoped, not block-scoped.

There is only one variable i shared across all iterations.

By the time setTimeout() executes (after 1 second), the loop has already finished.

i = 3

So every callback prints:

3
3
3

Visual Representation

Loop Starts

Iteration 1 ─┐
Iteration 2 ─┼──► Same variable: i
Iteration 3 ─┘

Loop Ends
i = 3

After 1 second

Callback 1 → 3
Callback 2 → 3
Callback 3 → 3

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Why does let print 0 1 2?

for (let j = 0; j < 3; j++) {
    setTimeout(() => {
        console.log(j);
    }, 1000);
}

let is block-scoped.

JavaScript creates a new copy of j for every iteration.

Iteration 1 → j = 0
Iteration 2 → j = 1
Iteration 3 → j = 2

When the callbacks execute later, each one remembers its own value.

0
1
2

Visual Representation

Iteration 1 → j = 0
           ↓
       Callback remembers 0

Iteration 2 → j = 1
           ↓
       Callback remembers 1

Iteration 3 → j = 2
           ↓
       Callback remembers 2

After 1 second

0
1
2

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💡 Interview Follow-up Question

How can you make var print 0 1 2?

Answer:

for (var i = 0; i < 3; i++) {
    ((x) => {
        setTimeout(() => {
            console.log(x);
        }, 1000);
    })(i);
}

or simply use let, which is the modern and recommended approach.

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🎯 Key Takeaways

✅ var → Function Scope
✅ let → Block Scope
✅ setTimeout() is asynchronous and executes after the loop completes.
✅ Closures capture variables differently depending on how they're declared.

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📌 Save this post if you're preparing for JavaScript, Angular, React, or Node.js interviews.

💬 Bonus Interview Question:

Can you explain what a Closure is and why it affects this example?

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