execa/lib/promise.js

// eslint-disable-next-line unicorn/prefer-top-level-await
const nativePromisePrototype = (async () => {})().constructor.prototype;

const descriptors = ['then', 'catch', 'finally'].map(property => [
	property,
	Reflect.getOwnPropertyDescriptor(nativePromisePrototype, property),
]);

// The return value is a mixin of `childProcess` and `Promise`
export const mergePromise = (spawned, promise) => {
	for (const [property, descriptor] of descriptors) {
		// Starting the main `promise` is deferred to avoid consuming streams
		const value = typeof promise === 'function'
			? (...args) => Reflect.apply(descriptor.value, promise(), args)
			: descriptor.value.bind(promise);

		Reflect.defineProperty(spawned, property, {...descriptor, value});
	}
};

// Use promises instead of `child_process` events
export const getSpawnedPromise = spawned => new Promise((resolve, reject) => {
	spawned.on('exit', (exitCode, signal) => {
		resolve({exitCode, signal});
	});

	spawned.on('error', error => {
		reject(error);
	});

	if (spawned.stdin) {
		spawned.stdin.on('error', error => {
			reject(error);
		});
	}
});