08 — Arrays and collections

One line: An array is an object with string index keys and a self-adjusting length, and its defaults (string sorting, skipped holes, methods that change it in place) catch everyone once; Map, Set, WeakMap and typed arrays exist for the jobs an array or a plain object does badly.

The chapter starts with what an array is, an object with index keys and a self-adjusting length, and the three surprises that follow: sort comparing strings, holes, and methods that change the array in place beside their ES2023 copying twins. The iteration methods come next, because every later page uses them. The second half covers the collections built for other jobs: Map and Set for keys of any type, WeakMap for keys that must not stay alive, typed arrays for raw bytes, and grouping and set operations.

The pages

Page	Level	In one line	Status
Arrays	101	An array is an object whose index keys are strings and whose length follows the largest index: on an empty array a[5] = "x" makes length 6, a[-1] = "y" adds a property that length ignores, and a.length = 0 deletes the indexed elements only.	stub
sort() compares strings	101	Without a comparator, sort() compares elements as strings, by UTF-16 code units, so [10, 9, 1].sort() is [1, 10, 9]; (a, b) => a - b sorts numbers, and the boolean comparator a > b leaves [3, 1, 2] unsorted in Node.	stub
Holes	201	[1, , 3] has a hole, not an undefined: 1 in arr is false, forEach and map skip index 1 (and map keeps the gap), spread reads it as undefined, and indexOf(undefined) is -1 while includes(undefined) is true.	stub
Mutating and copying methods	101	sort, reverse and splice change the array they are called on, and sort returns that same array, so const b = a.sort() sorts a too; ES2023's toSorted, toReversed, toSpliced and with return a new array, a shallow copy that shares its objects.	stub
map, filter, reduce and friends	101	Every callback gets the element, its index and the whole array, so ["1", "2", "3"].map(parseInt) hands each index to parseInt as a radix and returns [1, NaN, NaN]; reduce with no initial value throws a TypeError on an empty array, where reduce(fn, 0) returns 0.	stub
Map and Set	101	A Map matches keys with SameValueZero: NaN finds NaN and -0 finds 0, but a new {} finds nothing; Map and Set give entries back in insertion order, where a plain object lists integer-like keys such as "1" and "2" first.	stub
WeakMap and WeakSet	301	An object used as a Map key stays alive as long as the Map does; used as a WeakMap key, it is collected once nothing else refers to it, which is why a WeakMap has no size, no keys() and cannot be iterated.	stub
Typed arrays and ArrayBuffer	201	A typed array converts each value to its element type, so new Uint8Array([256, -1, 3.7]) holds 0, 255, 3; it never grows, and views share one ArrayBuffer: a Uint32Array writes it little-endian on x86 and ARM, while DataView reads big-endian by default.	stub
Grouping and set operations	201	Object.groupBy returns an object with a null prototype and string keys, so grouping by an object files every row under "[object Object]" where Map.groupBy keeps the object; the ES2025 Set methods accept any set-like, a Map included, but throw on an array.	stub

Boundaries

Walking any iterable with for...of, generators and lazy iterator helpers is in Control flow and iteration; copying objects and JSON are in Objects; the TypeScript types for arrays and tuples are in Everyday types.