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
    CSS @supports

    Feature detection via JavaScript is a client side best practice and for all the right reasons, but unfortunately that same functionality hasn't been available within CSS.  What we end up doing is repeating the same properties multiple times with each browser prefix.  Yuck.  Another thing we...

  • By
    Send Text Messages with PHP

    Kids these days, I tell ya.  All they care about is the technology.  The video games.  The bottled water.  Oh, and the texting, always the texting.  Back in my day, all we had was...OK, I had all of these things too.  But I still don't get...

Incredible Demos

  • By
    MooTools & Printing – Creating a Links Table of Contents

    One detail we sometimes forget when considering print for websites is that the user cannot see the URLs of links when the page prints. While showing link URLs isn't always important, some websites could greatly benefit from doing so. This tutorial will show you...

  • By
    Display Images as Grayscale with CSS Filters

    CSS filters aren't yet widely supported but they are indeed impressive and a modern need for web imagery.  CSS filters allow you to modify the display of images in a variety of ways, one of those ways being displaying images as grayscale. Doing so requires the...

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!