Testing React with Enzyme, Jest and Fetch

I have been having fun recently working on a react app. Typically I use TDD.

This can be difficult when you have code that goes:

fetch.then(res => json)
.then(res => this.setState(res.aValue)
 )

Testing this gets awkward. Changing the signature is too invasive. You can easily mock the fetch but get a race condition reading the state.

This is the solution that I found works. By abusing the javascript sequencing a bit – do what you need to then check this:

setTimeout(() => { assert thing.state}, 0)