JavaScript

[Abdumajidov2005]

Body

Body (Text):
“Hello developers! 👋
Today I’m playing around with JS arrays and ran into a small puzzle.

let arr = [1, 2, 3, 4];
let result = arr.map(num => {
  if (num % 2 === 0) return num * 2;
});
console.log(result);

Question:

What do you think console.log(result) will output?

Why does it behave that way?

How can we fix it so the result becomes something closer to [1, 2, 3, 4]?

That’s what makes JavaScript interesting — small code, big puzzles! 🧩

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[Thiago-code-lab]

Hello! This is a great puzzle to understand how map works under the hood in JavaScript. Here are the answers to your questions:

1. What will console.log(result) output?
   The output will be:

JavaScript
[undefined, 4, undefined, 8]
2. Why does it behave that way?
The Array.prototype.map() method always creates a new array with the exact same length as the original array. It does this by executing the callback function for every element.

In your code, the if (num % 2 === 0) condition only returns a value for even numbers. When the iteration hits an odd number (like 1 and 3), the condition is not met, and the function reaches the end without a return statement. In JavaScript, when a function doesn't explicitly return anything, it returns undefined by default.

3. How can we fix it?
   It depends on the exact output you are aiming for, but here are the two most common approaches:

Solution A: If you want to keep the original odd numbers ([1, 4, 3, 8])
You just need to ensure the function always returns something. Add a fallback return for when the condition is false:

JavaScript
let result = arr.map(num => {
if (num % 2 === 0) return num * 2;
return num; // Returns the number itself if it's odd
});

// A cleaner way using the ternary operator:
// let result = arr.map(num => num % 2 === 0 ? num * 2 : num);
Solution B: If you only wanted the multiplied even numbers ([4, 8])
Because map cannot change the size of the array, you can't use it to "skip" elements. To achieve that, you should combine filter with map:

JavaScript
let result = arr
.filter(num => num % 2 === 0) // Filters only the evens [2, 4]
.map(num => num * 2); // Multiplies them by two [4, 8]

Happy coding! 🚀

[Lumb3]

console.log(result) will print:

[undefined, 4, undefined, 8]

Why:

• map() always returns an array of the same length.
• Your callback only returns for even numbers.
• For odd numbers, there’s no return, so JavaScript inserts undefined.

Fix (keep odd numbers, double evens):

let arr = [1, 2, 3, 4];
let result = arr.map(num => (num % 2 === 0 ? num * 2 : num));
console.log(result); // [1, 4, 3, 8]

If you wanted only doubled evens ([4, 8]), use filter(...).map(...) instead.