Draft polyfill of my tc39 proposal: https://es.discourse.group/t/strictarray/1791

StrictArray.js

'use strict'
const StrictArray = (() => {
	const
		MAX_LEN = 2**32 - 1,
		MAX_IDX = MAX_LEN - 1

	const is_neg_zero = x => x === 0 && 1 / x < 0
	
	const is_int_num = x =>
		typeof x == 'number' && x % 1 == 0
	
	const is_len = x =>
		typeof x == 'number' &&
		x >= 0 && x <= MAX_LEN &&
		!is_neg_zero(x) &&
		is_int_num(x)
	
	const is_index = x => is_len(x) && x < MAX_LEN

	const
		Array_from = Array.from,
		arr_from_len = (n, filler) => Array_from({length: n}, () => filler)

	const
		ERR_STR = 'Index is invalid or out-of-bounds',
		ERR_LEN = 'Invalid length'

	const lib = class StrictArray {
		#main
		constructor(...args) {
			this.#main = args
		}
		static withLen(n, filler) {
			if (!is_len(n))
				throw new RangeError(ERR_LEN)

			return new StrictArray(...arr_from_len(n, filler))
		}

		#is_own_index(i) {
			return is_index(i) && i < this.#main.length
		}

		get length() {
			return this.#main.length
		}
		/* // is this a bad idea?
		set length(n) {
			if (!is_len(n))
				throw new RangeError(ERR_LEN)
			return this.#main.length = n
		}
		*/

		get(i) {
			if (this.#is_own_index(i))
				return this.#main[i]
			throw new RangeError(ERR_STR)
		}
		set(i, v) {
			if (this.#is_own_index(i))
				return this.#main[i] = v
			throw new RangeError(ERR_STR)
		}
		
		at(i) {
			if (!is_int_num(i))
				throw new RangeError(ERR_STR)

			if (i < 0) i += this.#main.length

			return this.get(i)
		}
		setAt(i, v) {
			if (!is_int_num(i))
				throw new RangeError(ERR_STR)

			if (i < 0) i += this.#main.length

			return this.set(i, v)
		}

		_get_main() {return this.#main}
	}

	const get_main = lib.prototype._get_main
	delete lib.prototype._get_main;

	for (const k of Object.getOwnPropertyNames(Array.prototype)) {
		if (k in lib.prototype)
			continue
		lib.prototype[k] = function(...args) {
			return [][k].call(get_main.call(this), ...args)
		}
	}

	return lib
})()