Lodash _.compact Behavior with Sparse Arrays

The Lodash _.compact method removes all falsey values from an array, which directly affects how it handles sparse arrays containing missing index slots (holes). When _.compact encounters an array with unassigned or missing indices, it evaluates those slots as undefined. Because undefined is a falsey value in JavaScript, the method omits the missing slots entirely, returning a newly packed, dense array containing only truthy values.

How Sparse Arrays Work in JavaScript

A sparse array is an array where certain indices have not been assigned a value, creating empty slots. For example:

const sparseArray = [1, , 3]; // Index 1 is an empty slot
console.log(sparseArray.length); // 3
console.log(sparseArray[1]); // undefined

Although the slot at index 1 is not explicitly defined with a value, attempting to read its property returns undefined.

How _.compact Processes Missing Slots

Lodash implements _.compact by iterating over the input array using its total length, checking each index from 0 to length - 1:

const _ = require('lodash');

const input = ['apple', , 'banana', undefined, 0, 'cherry'];
const result = _.compact(input);

console.log(result);
// Output: ['apple', 'banana', 'cherry']

During execution, _.compact evaluates each item:

1. It accesses the element at each index via standard indexed lookup (array[index]).
2. When accessing a missing slot, the operation returns undefined.
3. The method evaluates the truthiness of the value. Since undefined coerces to false, the slot is excluded from the new array.
4. Valid, truthy elements are pushed into a new, dense array.

Comparison with Native Array.prototype.filter

Native JavaScript array methods such as forEach, map, and filter typically skip empty slots without invoking the callback. For instance, using sparseArray.filter(Boolean) also removes empty slots, but it does so because native iterations bypass unassigned indices entirely.

Lodash's _.compact reaches the same end result—a dense array with no holes—because it reads the missing indices directly as undefined and filters them out alongside other falsey values like false, null, 0, "", and NaN.