Conditional Promise.all

Have been in a situation where you needed to dynamically shoot out HTTP requests based on some branching logic? Are you using a Promise based HTTP client library like fetch or axios?

Fear not, here is a solution for your needs!

In general the solution is based on coming up with an interface that's not only returning the data when the Promise was resolved, but also give a companion flag that identifies the promise it was initiated from.

	interface DynamicPromiseResult {
	data: unknown;
	promise: string;
	}
	const promises: Array<Promise<DynamicPromiseResult>> = []
	if (true) { // some logic
	const promise1 = fetch('https://jsonplaceholder.typicode.com/posts/1').then(data => ({ data, promise: 'promise1' }));
	promises.push(promise1);
	}
	if (false) { // some logic
	const promise2 = fetch('https://jsonplaceholder.typicode.com/posts/2').then(data => ({ data, promise: 'promise2' }));
	promises.push(promise2);
	}
	const results = await Promise.all(promises)
	results.forEach(result => {
	// pattern matching would be lovely here, but JS/TS does not support it yet
	switch (result.promise) {
	case 'promise1':
	doSomething();
	break;
	case 'promise2':
	doSomething2();
	break;
	default:
	throw new Error('Resulting promise not found');
	}
	});

view raw dynamic.promise.ts hosted with ❤ by GitHub

You can find the idea on this SO post as well.