Let’s look at a simple recursive function that expresses our program at a high level let RUNNING = true const main = async (elem, color = Color ()) => RUNNING ? delay (color, FPS) .then (effect (color => setElemColor (elem, color))) .then (color => main (elem, stepColor (color))) : color We’ve done a little wishful … Read more
I don’t think there are any libraries to do this, but it’s actually quite simple to implement yourself: function sequential(fn) { // limitConcurrency(fn, 1) let q = Promise.resolve(); return function(x) { const p = q.then(() => fn(x)); q = p.reflect(); return p; }; } For multiple concurrent requests it gets a little trickier, but can … Read more
Contrary to both answers in the comments – there is a difference. While Promise.resolve(x); is basically the same as new Promise(function(r){ r(x); }); there is a subtlety. Promise returning functions should generally have the guarantee that they should not throw synchronously since they might throw asynchronously. In order to prevent unexpected results and race conditions … Read more
This will be straightforward if you can use async/await: // Make sure that this code is inside a function declared using // the `async` keyword. let currentProduct; for (let i = 0; i < products.length; i++) { currentProduct = products[i]; // By using await, the code will halt here until // the promise resolves, then … 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
I would recommend using standard JS promises built into the language over an additional library dependency like Bluebird. If you’re using Node 10+, the Node.js docs recommend using util.promisify which returns a Promise<{ stdout, stderr }> object. See an example below: const util = require(‘util’); const exec = util.promisify(require(‘child_process’).exec); async function lsExample() { try { … 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
Promises are just return values you attach callbacks to, instead of passing callbacks into functions. Unless you return all of them, there’s no way for the callbacks to chain, or catch all their errors. Also, return from all the .then‘s the instant you have another promise. This flattens things.
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
This is possible, and even quite easy! Well, if we look at how Angular’s own promises work, we need to get Bluebird to $evalAsync somewhere in order to get the exact same behavior. If we do that, the fact both implementations are Promises/A+ compliant means we can interop between $q code and Bluebird code, meaning … Read more