Lodash compact on Uninitialized Array Slots

When Lodash's _.compact method is executed on an array containing uninitialized slots—commonly known as a sparse array—it strips out those empty slots entirely. Because JavaScript evaluates uninitialized slots as undefined when accessed during iteration, and undefined is a falsey value, _.compact filters them out along with standard falsey elements, returning a dense array containing only the remaining truthy values.

How Lodash Handles Sparse Arrays

An uninitialized slot occurs when an array is initialized with empty slots (such as new Array(3)) or when an element is skipped using array literal syntax (such as [1, , 3]). These slots are not set to undefined; rather, the indices simply do not exist on the array object.

However, _.compact iterates over the target array using a standard index-based loop or iterator. When indexed access occurs on a missing slot (e.g., array[1]), JavaScript returns undefined.

Lodash treats the following values as falsey:

• false
• null
• 0
• "" (empty string)
• undefined
• NaN

Because the uninitialized slot resolves to undefined, it fails Lodash's truthiness check (if (value)) and is excluded from the returned array.

Code Example

const _ = require('lodash');

// Create an array with an uninitialized slot
const sparseArray = [1, , 3];

console.log(sparseArray.length); // 3
console.log(1 in sparseArray);    // false (index 1 is uninitialized)

// Run _.compact
const compactedArray = _.compact(sparseArray);

console.log(compactedArray);        // [1, 3]
console.log(compactedArray.length); // 2
console.log(1 in compactedArray);    // true (index 1 now holds the value 3)

Key Differences from Native Array Methods

Unlike native methods such as Array.prototype.forEach or Array.prototype.map, which automatically skip uninitialized slots without evaluating them, _.compact effectively normalizes them. The resulting array is guaranteed to be a dense array without gaps, containing only truthy elements and with an updated length property that reflects only the retained items.