Skip to content
Swizec Teller - a geek with a hatswizec.com

JavaScript can fetch() now and it's not THAT great

As I was researching that Chrome 52 bug yesterday, my friend Smotko showed me something cool: fetch().

It’s a native JavaScript function for AJAX GET requests. I don’t know what stage it is, but Chrome, Edge, Firefox, and Opera all support it. Safari doesn’t. Nor do any default mobile browsers, only Android Chrome.

You can use this Babel fetch polyfill to bridge the gap. I wonder if that’s good enough to convince our CTO to let me use this. ?

Then again, why? Oh yeah, that strange Chrome 52 bug doesn’t happen and all I’d have to do was change that one API wrapper function that we use for everything … and thus change our whole codebase … and wreak havoc. ?

But The Bug doesn’t happen! Let me show you.

fetch()

Ugh, promises don’t work in the console. That’s a bummer … never did like promises. It’s really just weird syntax sugar for callbacks.

fetch("bla.json").then((response) => {
console.log(response.ok); // prints true
console.log(response.body); // ReadableStream O.o
});

fetch() issues an AJAX request to the server. When it’s done, it triggers the then part of its promise and gives it a response object. This object can do many useful things, one of which is to tell you if it was successful. Another is to give you a ReadableStream.

You probably don’t care about readable streams, so there are functions to parse JSON or get raw text. We’re in the brave new world of promises, so those are promises too.

They work like this:

fetch("bla.json").then((response) => {
response.json().then((json) => console.log(json)); // Object {id: 1, hai: "hello"}
response.text().then((text) => console.log(text)); // nothing
});

The json().then() combination prints a JavaScript object parsed from our data. Which feels roundabout, but the internet tells me that promises are better than The Callback Hell.

This:

fetch(url).then((data) => data.body.json().then((real_data) => doStuff));

Is supposed to be better than:

request.get("bla.json").end((err, res) => console.log(res.body));

¯_(ツ)_

But lets get back to the story: You can’t read the fetch response body twice. See that .text() call? That throws an error saying that oh hey, the body has already been read, so you can’t do it again.

Maybe there’s no good reason you’d want to do that, but what if I do want to?

There’s an even worse caveat, though. Let me show you what happens when your fetch() call returns a 404.

fetch("nope.json")
.catch(() => console.log("something went wrong")) // no error
.then((res) => console.log(res.ok)); // but not okay either

With promises, catch() should give us a chance to catch errors. This keeps our code cleaner because then() can focus on the happy path.

Somebody decided a 404 error is not a good enough reason to reject the promise. You still have to put a lot of error handling into your happy path.

?

But The Bug doesn’t happen! I tried. You can repeat the same request in the then() callback and its then() callback works just fine.

There is an official explanation for how the bug happens in the bug report from 8 days ago. A fix also exists, but it’s not merged yet.

This debacle makes me wish I was braver when first seeing this behavior 3 months ago. Way back when it was Chrome Canary and I thought surely someone would fix it.

What a strange ride it’s been.

Did you enjoy this article?

Published on August 12th, 2016 in Front End, Technical

Learned something new?
Want to become a high value JavaScript expert?

Here's how it works 👇

Leave your email and I'll send you an Interactive Modern JavaScript Cheatsheet 📖right away. After that you'll get thoughtfully written emails every week about React, JavaScript, and your career. Lessons learned over my 20 years in the industry working with companies ranging from tiny startups to Fortune5 behemoths.

Start with an interactive cheatsheet 📖

Then get thoughtful letters 💌 on mindsets, tactics, and technical skills for your career.

"Man, love your simple writing! Yours is the only email I open from marketers and only blog that I give a fuck to read & scroll till the end. And wow always take away lessons with me. Inspiring! And very relatable. 👌"

~ Ashish Kumar

Join over 10,000 engineers just like you already improving their JS careers with my letters, workshops, courses, and talks. ✌️

Have a burning question that you think I can answer? I don't have all of the answers, but I have some! Hit me up on twitter or book a 30min ama for in-depth help.

Ready to Stop copy pasting D3 examples and create data visualizations of your own?  Learn how to build scalable dataviz components your whole team can understand with React for Data Visualization

Curious about Serverless and the modern backend? Check out Serverless Handbook, modern backend for the frontend engineer.

Ready to learn how it all fits together and build a modern webapp from scratch? Learn how to launch a webapp and make your first 💰 on the side with ServerlessReact.Dev

Want to brush up on your modern JavaScript syntax? Check out my interactive cheatsheet: es6cheatsheet.com

By the way, just in case no one has told you it yet today: I love and appreciate you for who you are ❤️