Although, as I mentioned, jQuery's Deferreds are a bit … unhelpful. This is also the same for promises in JavaScript. Promises and then function return values. When we try to return the same value from an asynchronous callback function, we simply get nothing. 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! 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. A promise has 2 possible outcomes: it will either be kept when the time comes, or it won’t. First of all, a Promise is an object. Let us take a simple example. Promise. Promises in JavaScript. The new promise resolves when all listed promises are settled, and the array of their results becomes its result. 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()? The promise is resolved with the given value, or the promise passed as the value if the value was a promise object. It can also be the Promise or a thenable to resolve. Example. 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.. then (arrayOfResults => {// Do something with all results}); Any of the three things can happend: If the value is a promise then promise is returned. The static Promise.resolve() function returns the Promise that is resolved. If there is a return statement in the handler function, it returns a fulfilled promise with that return value as the payload. Promise.all takes an array of promises (it technically can be any iterable, but is usually an array) and returns a new promise.. 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. When we define a promise in JavaScript, it will be resolved when the time comes, or it will get rejected. Putting the keyword async before a function tells the function to return a Promise. A promise is simply an object that represents a task that will be completed in the future. function run {const a = 10; return a;} run (); Output. If the value has a “then” attached to the promise, then the returned promise will follow that “then” to till the final state. 10 The above code shows a normal function which returns some value. // Parallel return Promise. What then? So how do we do that? But sometimes you need to run then in sequential order. The return type of Promise function will dictate how future chained then functions behave. It receives the return value of the first promise. 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; } all ([task1, task2, task3,]). This Promise resolves the value parameter. What value does the second promise resolve to? Promise.resolve(value); Parameters. 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. 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