What is "undefined x 1" in JavaScript?

The "undefined x 1" notation is not a JavaScript feature but Chrome's way of displaying sparse arrays with uninitialized elements. Instead of showing every undefined value, Chrome uses compact notation for better readability.

What Creates "undefined x n"

When you create an array with the Array() constructor or have gaps in array indexes, Chrome displays uninitialized slots as "undefined x count":

console.log(Array(5));

[undefined × 5]

Sparse Arrays Example

Arrays with missing indexes also show this notation:

let arr = [1, , , 4];  // Missing elements at index 1 and 2
console.log(arr);
console.log(arr.length);

[1, undefined × 2, 4]
4

Difference from Explicit undefined

There's a difference between uninitialized slots and explicitly set undefined values:

let sparse = Array(3);           // Uninitialized slots
let explicit = [undefined, undefined, undefined]; // Explicit undefined

console.log("Sparse array:");
console.log(sparse);
console.log("Explicit undefined:");
console.log(explicit);

Sparse array:
[undefined × 3]
Explicit undefined:
[undefined, undefined, undefined]

Behavior with Array Methods

Uninitialized slots behave differently with array methods:

let sparse = Array(3);
let explicit = [undefined, undefined, undefined];

console.log("Sparse forEach count:");
let sparseCount = 0;
sparse.forEach(() => sparseCount++);
console.log(sparseCount);

console.log("Explicit forEach count:");
let explicitCount = 0;
explicit.forEach(() => explicitCount++);
console.log(explicitCount);

Sparse forEach count:
0
Explicit forEach count:
3

Conclusion

Chrome's "undefined x n" notation improves readability for sparse arrays. Remember that uninitialized slots behave differently from explicit undefined values in array operations.