Understanding “Closure Capturing the Wrong Value in Loops” in JavaScript

Step 1: What is a Closure?

A closure in JavaScript is a function that remembers the variables from its parent scope, even after the parent function has finished executing.

Example of a Closure:

function outer() {
    let message = "Hello, World!";
    return function inner() {
        console.log(message); // ✅ Inner function remembers 'message' even after outer() finishes
    };
}

const greet = outer();
greet(); // ✅ "Hello, World!"

• inner() remembers the value of message even after outer() has finished executing.
• This is because JavaScript preserves the variable’s reference, not just its value.

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Step 2: How Closures Cause Issues in Loops

When using closures inside a loop, all functions inside the loop reference the same variable. This causes unexpected behavior when dealing with asynchronous operations.

Problem: Closure Captures the Wrong Value

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

Expected Output:

0
1
2

Actual Output (Incorrect):

3
3
3

Why does this happen?

• var i is function-scoped, so all functions in setTimeout() share the same i.
• By the time setTimeout() executes (after 1 second), the loop has already completed.
• i has reached 3, so all closures reference i = 3, not their expected loop value.

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Step 3: How to Fix the Issue

Solution 1: Use let Instead of var (Block Scope)

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

✅ Correct Output:

0
1
2

Why Does This Work?

• let i is block-scoped, meaning each iteration creates a new copy of i.
• Each closure captures its own unique i value.

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Solution 2: Use an Immediately Invoked Function Expression (IIFE)

If you must use var, you can create a separate function scope using an IIFE (Immediately Invoked Function Expression).

for (var i = 0; i < 3; i++) {
    (function(i) { // Create a separate scope for each iteration
        setTimeout(function() {
            console.log(i);
        }, 1000);
    })(i); // Pass the current value of i
}

✅ Correct Output:

0
1
2

Why Does This Work?

• The IIFE immediately executes, capturing the current i in a new function scope.
• setTimeout inside the IIFE gets its own copy of i.

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Solution 3: Use setTimeout‘s Third Parameter

A lesser-known trick is to pass i as a third argument to setTimeout.

for (var i = 0; i < 3; i++) {
    setTimeout(function(i) {
        console.log(i);
    }, 1000, i); // Pass i as an argument to setTimeout
}

✅ Correct Output:

0
1
2

Why Does This Work?

• The third parameter of setTimeout allows passing additional arguments.
• This ensures each closure gets the correct i value.

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Step 4: Summary of Fixes

Solution	Works with var?	Works with let?	Why It Works?
Use let	❌ No	✅ Yes	Each iteration creates a new scope
Use IIFE	✅ Yes	✅ Yes	Each loop iteration has a unique scope
Use setTimeout‘s third argument	✅ Yes	✅ Yes	Passes a unique value for each closure

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Step 5: Best Practices

✅ Use let instead of var whenever possible.
✅ If you must use var, wrap the logic inside an IIFE to create separate scopes.
✅ Use setTimeout’s third argument as a shortcut if applicable.

By following these best practices, you can avoid closure capturing issues in loops and write more predictable, bug-free JavaScript!

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