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