index.js

export class TimeoutError extends Error {
	constructor(message) {
		super(message);
		this.name = 'TimeoutError';
	}
}

export default function pTimeout(promise, milliseconds, fallback, options) {
	let timer;
	const cancelablePromise = new Promise((resolve, reject) => {
		if (typeof milliseconds !== 'number' || Math.sign(milliseconds) !== 1) {
			throw new TypeError(`Expected \`milliseconds\` to be a positive number, got \`${milliseconds}\``);
		}

		if (milliseconds === Number.POSITIVE_INFINITY) {
			resolve(promise);
			return;
		}

		options = {
			customTimers: {setTimeout, clearTimeout},
			...options
		};

		timer = options.customTimers.setTimeout.call(undefined, () => {
			if (typeof fallback === 'function') {
				try {
					resolve(fallback());
				} catch (error) {
					reject(error);
				}

				return;
			}

			const message = typeof fallback === 'string' ? fallback : `Promise timed out after ${milliseconds} milliseconds`;
			const timeoutError = fallback instanceof Error ? fallback : new TimeoutError(message);

			if (typeof promise.cancel === 'function') {
				promise.cancel();
			}

			reject(timeoutError);
		}, milliseconds);

		(async () => {
			try {
				resolve(await promise);
			} catch (error) {
				reject(error);
			} finally {
				options.customTimers.clearTimeout.call(undefined, timer);
			}
		})();
	});

	cancelablePromise.clear = () => {
		clearTimeout(timer);
		timer = undefined;
	};

	return cancelablePromise;
}