The code you are showing introduces an asynchronous primitive, Promise, which can be passed around and used to access a resource that hasn’t been populated yet. In this case, you want an Image that is fully loaded and has image data that you can use. However, you can’t access the image data until a network … Read more
Using Promise.resolve().then has no advantages over nextTick. It runs on the same queue, but have slightly higher priority, that is, promise handler can prevent next tick callback from ever running, the opposite is not possible. This behaviour is an implementation detail and should not be relied on. Promise.resolve().then is obviously slower (a lot, I think), … Read more
In order to be considered handled, rejected promises should be synchronously chained with then(…, …) (2 arguments) or catch(…). promise.then(() => console.log(‘ok’)) is a separate promise that wasn’t chained with catch(…), so rejected promise will result in unhandled rejection. If I’m in any other page (like stackoverflow.com) it throws the exception This isn’t an exception, … Read more
I can see two possible aspects to your question: The error being thrown in test is in the synchronous (not asynchronous) part of test. Why is it a promise rejection rather than a synchronous exception? The promise from test() is being rejected, why doesn’t catch catch that rejection? #1 – Why is it a rejection? … Read more
Promise.all accepts an Array of Promise objects. You’re getting an Array of undefined because you’re not returning any Promise in your map callback: function addText(queries) { return Promise.all(queries.map(function(query) { // Add `return` here or the `map` returns an Array of `undefined`. return models.queries .findById(query.queryId, { raw: true, attributes: [ “query” ] }) .then(function(queryFetched) { query.text … Read more
Immediately, yes, by specification. From the MDN: The executor function is executed immediately by the Promise implementation, passing resolve and reject functions (the executor is called before the Promise constructor even returns the created object) This is defined in the ECMAScript specification (of course, it’s harder to read…) here (Step 9 as of this edit, showing … Read more
The first issue can easily be bypassed by always providing a function with the right type a -> Promise a. Or by not using then as the bind operation of the monad, but some type-correct ones. Creed is a functionally minded promise library that provides map and chain methods which implements the Fantasy-land spec for … Read more
The relevant specs are here: Promises/A+ point 2.2.4: onFulfilled or onRejected must not be called until the execution context stack contains only platform code. [3.1]. And note 3.1 (emphasis mine): Here “platform code” means engine, environment, and promise implementation code. In practice, this requirement ensures that onFulfilled and onRejected execute asynchronously, after the event loop … Read more
If you don’t care about the results, then it’s quick to whip one up: Promise.eachLimit = async (funcs, limit) => { let rest = funcs.slice(limit); await Promise.all(funcs.slice(0, limit).map(async func => { await func(); while (rest.length) { await rest.shift()(); } })); }; // Demo: var wait = ms => new Promise(resolve => setTimeout(resolve, ms)); async function … Read more
Update When using Node, the file currently must have an .mjs extension to work. Top level awaits can be used in browser modules. When used the script tag must include the type attribute which must be set to module: <script src=”/script.js” type=”module”></script> const start = Date.now() console.log(‘Pre call.’) await delayedCall() console.log(‘Duration:’, Date.now() – start) function … Read more