Promise.all takes an array of promises (it technically can be any iterable, but is usually an array) and returns a new promise.. It receives the return value of the first promise. What value does the second promise resolve to? Promises and then function return values. all ([task1, task2, task3,]). function run {const a = 10; return a;} run (); Output. 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 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. Putting the keyword async before a function tells the function to return a Promise. If you return a Promise then the next chained then function will execute when the Promise that you returned is resolved.. Promise.resolve('foo'). A promise is simply an object that represents a task that will be completed in the future. // Parallel return Promise. 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. First of all, a Promise is an object. The static Promise.resolve() function returns the Promise that is resolved. For instance, the Promise.all below settles after 3 seconds, and then its result is an array [1, 2, 3]: The return type of Promise function will dictate how future chained then functions behave. A promise has 2 possible outcomes: it will either be kept when the time comes, or it won’t. Promise. This Promise resolves the value parameter. The new promise resolves when all listed promises are settled, and the array of their results becomes its result. Although, as I mentioned, jQuery's Deferreds are a bit … unhelpful. Promises in JavaScript. This is also the same for promises in JavaScript. 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. 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; } So how do we do that? then (arrayOfResults => {// Do something with all results}); But sometimes you need to run then in sequential order. It can also be the Promise or a thenable to resolve. Let us take a simple example. 10 The above code shows a normal function which returns some value. The promise is resolved with the given value, or the promise passed as the value if the value was a promise 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()? 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.. What then? Any of the three things can happend: If the value is a promise then promise is returned. When we try to return the same value from an asynchronous callback function, we simply get nothing. Running JavaScript Promises in parallel is as simple as calling Promise.all() with an array of Promises. 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