The JavaScript promises API will treat anything with a then() method as promise-like (or thenable in promise-speak sigh), so if you use a library that returns a Q promise, that's fine, it'll play nice with the new JavaScript promises. First of all, a Promise is an object. JavaScript promises are one of the most popular ways of writing asynchronous functions that return a single value on completion or failure of the operation.. What is Promise.all()? Promises and then function return values. The new promise resolves when all listed promises are settled, and the array of their results becomes its result. The Promise.all() is a static method (part of Promise API) that executes many promises in parallel, and waits until all of them are settled. The promise is resolved with the given value, or the promise passed as the value if the value was a promise object. Any of the three things can happend: If the value is a promise then promise is returned. Although, as I mentioned, jQuery's Deferreds are a bit … unhelpful. Here the first .then shows 1 and returns new Promise(…) in the line (*).After one second it resolves, and the result (the argument of resolve, here it’s result * 2) is passed on to handler of the second .then.That handler is in the line (**), it shows 2 and does the same thing.. Promises in JavaScript. Putting the keyword async before a function tells the function to return a Promise. Promise.all takes an array of promises (it technically can be any iterable, but is usually an array) and returns a new promise.. Promise resolve() method: Promise.resolve() method in JS returns a Promise object that is resolved with a given value. When we define a promise in JavaScript, it will be resolved when the time comes, or it will get rejected. then (arrayOfResults => {// Do something with all results}); So how do we do that? If you return a Promise then the next chained then function will execute when the Promise that you returned is resolved.. Promise.resolve('foo'). Running JavaScript Promises in parallel is as simple as calling Promise.all() with an array of Promises. This is happening at the bottom of handle(), The handler object carries around both an onResolved callback as well as a reference to resolve().There is more than one copy of resolve() floating around, each promise gets their own copy of this function, and a closure for it to run within. If the code returns something that is not a Promise, then JavaScript automatically wraps it into a resolved promise with that value e.g when it returns an AsyncFunction object: async function oddNumber() { return 7; } Return a Default Value with Promises Using catch By David Walsh on December 23, 2020 Last week I tweeted all of you looking for your best JavaScript Array and Promise tricks , and as always, it didn't disappoint -- I learned quite a bit! 10 The above code shows a normal function which returns some value. Let us take a simple example. function run {const a = 10; return a;} run (); Output. A promise is simply an object that represents a task that will be completed in the future. What value does the second promise resolve to? It can also be the Promise or a thenable to resolve. If the value has a “then” attached to the promise, then the returned promise will follow that “then” to till the final state. Promise. This is also the same for promises in JavaScript. It receives the return value of the first promise. This Promise resolves the value parameter. For instance, the Promise.all below settles after 3 seconds, and then its result is an array [1, 2, 3]: Promise.resolve(value); Parameters. If there is a return statement in the handler function, it returns a fulfilled promise with that return value as the payload. Example. When we try to return the same value from an asynchronous callback function, we simply get nothing. // Parallel return Promise. all ([task1, task2, task3,]). A promise has 2 possible outcomes: it will either be kept when the time comes, or it won’t. The static Promise.resolve() function returns the Promise that is resolved. What then? The return type of Promise function will dictate how future chained then functions behave. But sometimes you need to run then in sequential order. Sometimes you need to run then in sequential order represents a task that be! 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