then on Objects

By  on  

Promises were a revelation in JavaScript development, allowing us to enjoy async processing and avoid callback hell. Many new APIs like Battery API, Cache API, and others use the promise API. One fact you may not know is that you can add a then method to any object to make it Promise-like!

Let's create a generic object with a then method that accepts a resolve function as its argument:

j = { then: resolve => fetch("/").then(resolve) }

With an object featuring a then method, you can call the then method or use await syntax:

j.then(res => console.log(res));
// Response {type: "basic", url: "https://davidwalsh.name/", redirected: false, status: 200, ok: true, …}

// ... or an await...
const response = await j;
// Response {type: "basic", url: "https://davidwalsh.name/", redirected: false, status: 200, ok: true, …}

This technique is interesting and, under the right circumstances, can be employed to represent a logical usage.

Hack or useful? How would you use this perk of then?

Recent Features

  • By
    Responsive and Infinitely Scalable JS Animations

    Back in late 2012 it was not easy to find open source projects using requestAnimationFrame() - this is the hook that allows Javascript code to synchronize with a web browser's native paint loop. Animations using this method can run at 60 fps and deliver fantastic...

  • By
    9 Mind-Blowing Canvas Demos

    The <canvas> element has been a revelation for the visual experts among our ranks.  Canvas provides the means for incredible and efficient animations with the added bonus of no Flash; these developers can flash their awesome JavaScript skills instead.  Here are nine unbelievable canvas demos that...

Incredible Demos

Discussion

  1. Edgar

    Nice, although fetch may be a bad example since you would definitely want to deal with errors. But for promises of which you are sure they always resolve, it’s cool.

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!