Did you know about sparse array in Javascript?

I stumbled upon something called a sparse array in the MDN web docs. It's one of those things that makes JavaScript "interesting" 😂.

Sparse arrays are arrays with empty slots. However, these empty slots are not the same as slots filled with undefined. There are several ways to create sparse arrays:

// Using the Array constructor:
const a = Array(5) // [ <5 empty items> ]

// Using consecutive commas in an array literal:
const b = [1, 2, , , 5] // [ 1, 2, <2 empty items>, 5 ]

// Directly setting a slot with an index greater than the array's length:
const c = [1, 2]
c[4] = 5 // [ 1, 2, <2 empty items>, 5 ]

// Elongating an array by directly setting its length:
const d = [1, 2]
d.length = 5 // [ 1, 2, <3 empty items> ]

// Deleting an element:
const e = [1, 2, 3, 4, 5]
delete e[2] // [ 1, 2, <1 empty item>, 4, 5 ]

Consider the following sparse array: const arr = [1, 2, , , 5]. These are important characteristics of a sparse array:

1. Accessing the empty slots returns undefined. For example, console.log(arr[2]); // undefined.
2. A new array created by spreading a sparse array will have undefined values in the empty slots. For example, const another = [...arr]; // "another" is [1, 2, undefined, undefined, 5].
3. Array iteration methods like map, filter, some, and Object.keys skip the empty slots. For instance:

const mapped = arr.map((i) => i + 1) // [ 2, 3, <2 empty items>, 6 ]
arr.forEach((i) => console.log(i)) // 1 2 5
const filtered = arr.filter(() => true) // [ 1, 2, 5 ]
const hasFalsy = arr.some((k) => !k) // false

// Property enumeration
const keys = Object.keys(arr) // [ '0', '1', '4' ]

I recommend reading more about how array methods handle empty slots in sparse arrays in the Array methods and empty slots section of the MDN web docs.

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