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Promise Syntax

Promise Syntax

Learn to create Promises in JavaScript with functions, callbacks, and error handling.

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The code examples that we have seen up until now have used Promises that are provided by a third-party library, such as the Promise-based versions of filesystem access available through the Node fs.promise namespace. So, how do we write our own Promises?

Writing Promises

A Promise is an instance of a new Promise class whose constructor requires a function signature that accepts two callback functions, generally named resolve and reject.

Consider the following function definition:

// This function takes two arguments, both of which are functions.
function fnDelayedPromise(
    resolve: () => void, // This function will be called when the promise is resolved.
    reject: () => void   // This function will be called when the promise is rejected.
) {
    // This function will be called after a timeout of 1000ms (1 second).
    function afterTimeout() {
        resolve();
    }
    // Set a timeout of 1000ms and call the afterTimeout function when the timeout expires.
    setTimeout(afterTimeout, 1000);
}
Delay Promise with timeout function

  • We have a function named fnDelayedPromise that accepts two functions as parameters, named resolve and reject, both returning void. Within the body of this function, we define a callback function named afterTimeout on lines 7–9, which will invoke the resolve callback that was passed in as the first argument.

  • It then calls the setTimeout function on line 11, which will cause a one-second delay before ...