Edit We’ve finally fixed this in Node.js 15, it took 5 years but native promise rejections now behave like uncaught exceptions – so it’s fine to just add a process.on(‘uncaughtException’ handler. In Modern Node.js Starting with io.js 1.4 and Node 4.0.0 you can use the process “unhandledRejection” event: process.on(“unhandledRejection”, function(reason, p){ console.log(“Unhandled”, reason, p); // … Read more
NO you can’t get the data synchronously out of a promise like you suggest in your example. The data must be used within a callback function. Alternatively in functional programming style the promise data could be map()ed over. If your are OK using async/await (you should it’s awesome) then you can write code that looks … Read more
Perhaps the most straightforward answer is: P.then(function(data) { return doWork(‘text’, data); }); Or, since this is tagged ecmascript-6, using arrow functions: P.then(data => doWork(‘text’, data)); I find this most readable, and not too much to write.
while (Date.now() – nu < 1000) {} doesn’t simulate a long running task that is syncronous. It is a long running task that is syncronous. Promises are a way to manage asynchronous code, not a way to make code asynchronous, nor a way to simulate multithreading. For that, you need to look at workers. Web … Read more
Promise.resolve is specified to return a resolved promise (mind-blowing, right? 25.4.4.5, 25.4.1.5, 25.4.1.3.). a.then() therefore enqueues a job immediately (25.4.5.3.1, step 8) each time. .then() never returns a fulfilled promise according to this spec (for something interesting, try Promise.resolve().then() in your Chrome console¹). Let’s name the promise a.then(v => Promise.resolve(“A”)) and some of its associated … Read more
The reasoning is that when the callbacks are always asynchronous instead of possibly asynchronous, it gives more consistent and reliable api to use. Consider the following code var pizza; browseStackOverflow().then(function(){ eatPizza(pizza); }); pizza = yesterdaysLeftovers; Now that snippet clearly assumes the onFulfilled will not be called right away and if that wasn’t the case we … Read more
Does there exist a situation where it would be necessary (or just better somehow) to use a deferred? There is no such situation where a deferred is necessary. “Better” is a matter of opinion so I won’t address that here. There’s a reason that the ES6 promise specification does not have a deferred object. You … Read more
As of 2019… …the correct answer is to use async/await with the native fs promises module included in node. Upgrade to Node.js 10 or 11 (already supported by major cloud providers) and do this: const fs = require(‘fs’).promises; // This must run inside a function marked `async`: const file = await fs.readFile(‘filename.txt’, ‘utf8’); await fs.writeFile(‘filename.txt’, … Read more
Intro JavaScript has an asynchronous model. Whenever an asynchronous action is completed you often want to execute some code afterwards. First callbacks were often used to solve this problem. However, when programming code with multiple asynchronous elements a problem arises using callbacks. Because when you nest multiple callbacks inside each other the code becomes hard … Read more
It depends on what you mean by timeout. If you expect the function to stop, then no. If you just want to stop waiting for it, then yes (quick to whip up in ES6): var wait = ms => new Promise(resolve => setTimeout(resolve, ms)); var timeout = (p, ms) => Promise.race([p, wait(ms).then(() => { throw … Read more