Array Properties

This reference covers important Array properties with practical caveats, performance notes, compatibility guidance, and TypeScript tips.

length Legacy

Returns the number of elements in an array. It is writable.

const arr = [1, 2, 3];
console.log(arr.length); // 3

arr.length = 2; // truncate
console.log(arr); // [1, 2]

arr.length = 5; // extend with holes
console.log(arr); // [1, 2, <3 empty items>]

Notes:

• Setting length = 0 clears an array efficiently (O(1) metadata update; engines may free memory lazily).
• Assigning beyond current length creates a sparse array (holes). Prefer dense arrays for performance and predictability.
• Writing length smaller than current size discards elements beyond the new length.

TypeScript:

function truncate<T>(arr: T[], n: number): T[] {
  const copy = arr.slice();
  copy.length = Math.max(0, n);
  return copy; // avoid mutating inputs in reducers/state
}

prototype (Array.prototype) Legacy

Holds the shared methods for all array instances (e.g., map, filter, reduce).

Array.prototype.first = function () {
  return this[0];
};
['a', 'b'].first(); // 'a'

Caution: Avoid modifying built-ins in application code—monkey patching can break dependencies and assumptions in libraries. Prefer standalone helpers or utility modules.

Array.isArray Added in ES5

Safely check if a value is an array.

Array.isArray([]); // true
Array.isArray({}); // false
Array.isArray(new Uint8Array()); // false (typed arrays are not regular Arrays)

TypeScript narrowing example:

function toArray<T>(x: T | T[]): T[] {
  return Array.isArray(x) ? x : [x];
}

Symbol.iterator Added in ES2015

Regular arrays are iterable.

const arr = [1, 2, 3];
for (const v of arr) {
  // iterates values
}

Tip: Use for...of, entries(), or array methods (map, filter) instead of for...in to avoid iterating inherited keys.

toString and join Legacy

[1, 2, 3].toString(); // '1,2,3'
[1, 2, 3].join('-'); // '1-2-3'

Notes:

• join treats holes as empty strings; JSON.stringify([,]) becomes [null] while [,].join(',') becomes "," (empty between commas).
• Prefer join over manual string concatenation in loops for performance and clarity.

constructor Legacy

References the constructor function used to create the instance.

[].constructor === Array; // true

Less‑common but useful symbols

Symbol.isConcatSpreadable Added in ES2015

Controls spread behavior in concat.

const arr = [1, 2];
const arr2 = [3, 4];
arr2[Symbol.isConcatSpreadable] = false;
[].concat(arr, arr2); // [1, 2, [3, 4]]

const like = { 0: 'x', 1: 'y', length: 2, [Symbol.isConcatSpreadable]: true };
[].concat(like); // ['x','y'] (array‑like is spread)

Symbol.species Added in ES2015

Lets subclasses override which constructor is used for derived objects.

class MyArray extends Array {
  static get [Symbol.species]() {
    return Array;
  }
}
const a = new MyArray(1, 2, 3);
const mapped = a.map((x) => x);
mapped instanceof MyArray; // false, uses Array because of Symbol.species

Symbol.unscopables Added in ES2015

Prevents certain props from being introduced into with scopes (legacy edge case).

Array.prototype[Symbol.unscopables];
// e.g., { copyWithin: true, entries: true, fill: true, find: true, ... }

Compatibility notes (ES2023)

If your environment lacks modern immutable methods (toSorted, toReversed, toSpliced, with), provide ponyfills:

export const toSorted = (arr, cmp) => [...arr].sort(cmp);
export const toReversed = (arr) => [...arr].reverse();
export const toSpliced = (arr, start, delCount, ...items) => {
  const copy = arr.slice();
  copy.splice(start, delCount, ...items);
  return copy;
};
export const withAt = (arr, index, value) => {
  const copy = arr.slice();
  copy[index] = value;
  return copy;
};

Targets and notes:

• Node 20+ and evergreen browsers: native support.
• Node 18+/Chrome 90+/legacy evergreen (2020–2022): transpile with Babel + core-js or use the ponyfills above.

Performance checklist

• Prefer dense arrays over sparse ones.
• Avoid delete arr[i]; use toSpliced(i, 1) or splice(i, 1).
• Batch operations (e.g., map once instead of multiple passes) when possible.
• Immutability allocates copies—safer for state, but measure in hot paths.