Swizec Teller - a geek with a hatswizec.com

    Mocking and testing fetch requests with Jest

    Some days your code flows, your fingers fly, and you're god amongst nerds. Other days you're testing fetch() requests.

    So I'm writing this to save you some time. The definitive how to mock and test fetch requests guide for 2020.

    First, a little background

    Why even test requests?

    Click through for source
    Click through for source

    Integration testing. Also known as the only useful testing. ๐Ÿ˜›

    Semantics aside, most bugs happen on the edges. The touch points between systems. Interfaces between modules.

    What bigger in-between point than calling an API? Lots can go wrong. Server can fail, network down, strange response codes, or just corruption on the wire.

    Take for example this isAuthenticated method from a library I built at work.

    Click through for source
    Click through for source

    Is the user authenticated? Check local cookies. That's always gonna work.

    And if cookies look correct, you gotta ask the server if they're actually correct. User might have been banned, deleted, access revoked, or just changed their password.

    That API request is the most likely failure point.

    Wanna really test this method? Gonna have to test the fetch.

    Why mock fetch requests?

    A pure unit testing approach says you should never mock requests to other systems.

    You assume the request works and test that your function makes the request. Then you test the function that's called after the fetch.

    And you enter the unit testing fallacy.

    Besides, structuring your code to allow a pure unit testing approach makes it harder to read, understand, and tedious to work with. You'd need something like this:

    Click through for source
    Click through for source

    Yeah that's not gonna get out of hand ๐Ÿ™„

    Keep your code simple and mock the request instead. Hook into the fetch() API and manipulate what it returns. That is the way to testing bliss my friend.

    How to mock fetch requests in Jest

    After more hours of trial and error than I dare admit, here's what I found: You'll need to mock the fetch method itself, write a mock for each request, and use a fetch mocking library.


    The library that worked best for me was fetch-mock.

    Click through for source
    Click through for source


    You tell Jest to use a mock library like this:

    Click through for source
    Click through for source

    Jest imports this file instead of isomorphic-fetch when running your code. Same approach works to replace any other library.

    Put a file of <library name> in src/__mocks__ and that file becomes said library. In this case we're replacing the isomorphic-fetch library with a fetch-mock sandbox.

    So when your code says

    Click through for source
    Click through for source

    It's actually getting the sandbox ๐Ÿค˜

    You'll need to import a fetch to support mocking โ€“ can't rely on the global window.fetch I'm afraid. You can't use that in test environments anyway since it doesn't exist.


    Using TypeScript in tests, you'll have to jump through another hoop: TypeScript doesn't understand that isomorphic-fetch is mocked and gets confused.

    You have to tell the type system that "Hey, this is actually fetch-mock. All is well."

    Click through for source
    Click through for source

    Convoluted but how else is TypeScript supposed to know isomorphic-fetch is actually fetch-mock ...

    PS: I'm assuming Jest because it's become the industry standard for JavaScript testing in the past few years.

    And now it works โœŒ๏ธ

    All you gotta do now is mock a request and write your test.

    Click through for source
    Click through for source

    Each of those tests is saying "When you fetch() this URL, return this object. No network calls are made, no servers harmed, no ambiguity about what happens. Just solid tests doing exactly what you want.

    And now you know your code works. Bliss ๐Ÿ˜Œ

    Happy Friday,

    Did you enjoy this article?

    Published on February 28th, 2020 in Front End, Technical

    Learned something new?
    Want to become an expert?

    Here's how it works ๐Ÿ‘‡

    Leave your email and I'll send you thoughtfully written emails every week about React, JavaScript, and your career. Lessons learned over 20 years in the industry working with companies ranging from tiny startups to Fortune5 behemoths.

    Join Swizec's Newsletter

    And get thoughtful letters ๐Ÿ’Œ on mindsets, tactics, and technical skills for your career. Real lessons from building production software. No bullshit.

    "Man, love your simple writing! Yours is the only newsletter I open 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. ๐Ÿ‘Œ"

    ~ Ashish Kumar

    Join over 14,000 engineers just like you already improving their careers with my letters, workshops, courses, and talks. โœŒ๏ธ

    Have a burning question that you think I can answer?ย 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? ย 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 ๐Ÿ’ฐ on the side with ServerlessReact.Dev

    Want to brush up on your modern JavaScript syntax?ย 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 areย โค๏ธ

    Created bySwizecwith โค๏ธ