When Good AJAX Goes Bad

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When Good AJAX Goes Bad

I’ve prepared a slightly different observable for you in errors.js. This new observable still has all the requests, but they hit a different endpoint on the node server. This new endpoint is programmed to give up and start spitting errors after several dozen requests. Using your knowledge from earlier in the chapter, pass in a function to handle the case where some requests fail to complete.

vanilla/managingAsync/errors-complete.ts

	loadingRequestsBad$
	.pipe(
	scan((prev) => prev + (100 / arrayOfRequests.length), 0)
	)
	.subscribe(percentDone => {
	progressBar.style.width = percentDone + '%';
	progressBar.innerText = Math.round(percentDone) + '%';
	}, err => {
	console.log(err);
	msgArea.innerText = 'Something went wrong, please try again';
	msgArea.style.display = 'block';
	});

Aha! Now when the server falls over, it’s gracefully handled—the user is informed that something’s gone wrong and has some basic remediation instructions. You’ve built an app that can survive even the harshest of conditions without skipping a beat. Users may vent on Twitter, but at least they’ll be venting about the right thing.

In this case, a single subscribe launched 128 AJAX requests. In a browser environment, the number of concurrent AJAX requests to the same domain is limited. In other environments (say, a quick Node script), you may have a long list of tasks and want to limit how many can run at once. The merge constructor and operator include an optional parameter to do just that. Pass in a number as the final argument, and merge will only subscribe to that number of observables at once. Just be careful you don’t have any infinite streams in there!

	// Will only run five requests at a time
	merge(...arrayOfRequests, 5)
	.pipe(
	scan((prev, val) => prev + (100 / arrayOfRequests.length), 0)
	)
	.subscribe(updateProgressbar, handleError);

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When Good AJAX Goes Bad

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When Good AJAX Goes Bad