As of February 7, 2018 Chrome 63+, Firefox 58+, and Opera 50+ support Promise.finally. In Node.js 8.1.4+ (V8 5.8+), the feature is available behind the flag –harmony-promise-finally. The Promise.prototype.finally ECMAScript Proposal is currently in stage 3 of the TC39 process. In the meantime to have promise.finally functionality in all browsers; you can add an additional … Read more
So, basically you’re asking what is the difference between these two (where p is a promise created from some previous code): return p.then(…).catch(…); and return p.catch(…).then(…); There are differences either when p resolves or rejects, but whether those differences matter or not depends upon what the code inside the .then() or .catch() handlers does. What … Read more
2017 update: First, for readers coming today – here is a version that works with Node 7 (4+): function enforceFastProperties(o) { function Sub() {} Sub.prototype = o; var receiver = new Sub(); // create an instance function ic() { return typeof receiver.foo; } // perform access ic(); ic(); return o; eval(“o” + o); // ensure … Read more
What’s the difference? The .then() call will return a promise that will be rejected in case the callback throws an error. This means, when your success logger fails, the error would be passed to the following .catch() callback, but not to the fail callback that goes alongside success. Here’s a control flow diagram: To express … Read more
The old adage goes that you should pick the right tool for the job. ES6 promises provide the basics. If all you ever want or need is the basics, then that should/could work just fine for you. But, there are more tools in the tool bin than just the basics and there are situations where … Read more
How a promise library decides If it has a .then function – that’s the only standard promise libraries use. The Promises/A+ specification has a notion called thenable which is basically “an object with a then method”. Promises will and should assimilate anything with a then method. All of the promise implementation you’ve mentioned do this. … Read more
Promises are not callbacks. A promise represents the future result of an asynchronous operation. Of course, writing them the way you do, you get little benefit. But if you write them the way they are meant to be used, you can write asynchronous code in a way that resembles synchronous code and is much more … Read more
The deferred antipattern (now explicit-construction anti-pattern) coined by Esailija is a common anti-pattern people who are new to promises make, I’ve made it myself when I first used promises. The problem with the above code is that is fails to utilize the fact that promises chain. Promises can chain with .then and you can return … Read more
Break the chain When you need to access the intermediate values in your chain, you should split your chain apart in those single pieces that you need. Instead of attaching one callback and somehow trying to use its parameter multiple times, attach multiple callbacks to the same promise – wherever you need the result value. … Read more
Promises have state, they start as pending and can settle to: fulfilled meaning that the computation completed successfully. rejected meaning that the computation failed. Promise returning functions should never throw, they should return rejections instead. Throwing from a promise returning function will force you to use both a } catch { and a .catch. People … Read more