flat-perf.js

{
	const RUN = 50000;
	const FLAT_LENGTH = 9;
	const a = [
		[0, 1, 2, 3],
		4,
		[5, 6, 7],
		8
	];
	const perf = [];

	const iter = () => {
		for (let n = 0; n <= RUN; n++) {
			const b = [];

			for (const x of a) {
				if (Array.isArray(x)) {
					b.push(...x);
				} else {
					b.push(x);
				}
			}

			if (b.length !== FLAT_LENGTH) {
				throw new Error('Incorrect length: ' + b.length);
			}

			if (n === 0) {
				if (b.some(x => Array.isArray(x))) {
					throw new Error('Array "b" is not flat');
				} else {
					console.time('iter');
				}
			}
		}
		console.timeEnd('iter');
	};
	perf.push(iter);

	const splice = () => {
		for (let n = 0; n <= RUN; n++) {
			const b = a.slice();

			for (let x, i = 0; i < b.length; i++) {
				x = b[i];
				if (Array.isArray(x)) {
					b.splice(i, 1, ...x);
					// i--; // for multi-level nesting
					i += x.length - 1; // for single-level nesting
				}
			}

			if (b.length !== FLAT_LENGTH) {
				throw new Error('Incorrect length: ' + b.length);
			}

			if (n === 0) {
				if (b.some(x => Array.isArray(x))) {
					throw new Error('Array "b" is not flat');
				} else {
					console.time('splice');
				}
			}
		}
		console.timeEnd('splice');
	};
	perf.push(splice);

	const concatSpread = () => {
		for (let n = 0; n <= RUN; n++) {
			const b = [].concat(...a);

			if (b.length !== FLAT_LENGTH) {
				throw new Error('Incorrect length: ' + b.length);
			}

			if (n === 0) {
				if (b.some(x => Array.isArray(x))) {
					throw new Error('Array "b" is not flat');
				} else {
					console.time('concat spread');
				}
			}
		}
		console.timeEnd('concat spread');
	};
	perf.push(concatSpread);

	const concatApply = () => {
		const concat = Array.prototype.concat;
		for (let n = 0; n <= RUN; n++) {
			const b = concat.apply([], a);

			if (b.length !== FLAT_LENGTH) {
				throw new Error('Incorrect length: ' + b.length);
			}

			if (n === 0) {
				if (b.some(x => Array.isArray(x))) {
					throw new Error('Array "b" is not flat');
				} else {
					console.time('concat apply');
				}
			}
		}
		console.timeEnd('concat apply');
	};
	perf.push(concatApply);

	if (Array.prototype.flat) {
		const arrProtoFlat = () => {
			for (let n = 0; n <= RUN; n++) {
				const b = a.flat(1);

				if (b.length !== FLAT_LENGTH) {
					throw new Error('Incorrect length: ' + b.length);
				}

				if (n === 0) {
					if (b.some(x => Array.isArray(x))) {
						throw new Error('Array "b" is not flat');
					} else {
						console.time('Array.prototype.flat');
					}
				}
			}
			console.timeEnd('Array.prototype.flat');
		};
		perf.push(arrProtoFlat);
	}

	const run = function(i) {
		try {
			perf[i]();
		} catch (e) {
			console.error(e.message);
		}

		if (i + 1 < perf.length) {
			setTimeout(run, 0, i + 1);
		}
	};

	perf.length && setTimeout(run, 0, 0);
}