function-in-same-module

Mocking Example: A function used by another function in the same module

In greeting.js we have the following functions:

export function getPlanet () {
    return 'world';
}

export default function getGreeting () {
    return `hello ${getPlanet()}!`;
}

The challenge

We want mock out getPlanet when testing getGreeting per test.

Prior art

This puzzle in particular seems to bamboozle quite a few folks - here's a bunch of stackoverflows/blogs etc discussing approaches to this issue (or similar):

• https://stackoverflow.com/questions/45111198/how-to-mock-functions-in-the-same-module-using-jest/ (full disclosure: that's me asking)
• https://medium.com/@qjli/how-to-mock-specific-module-function-in-jest-715e39a391f4
• jestjs/jest#936
• https://luetkemj.github.io/170421/mocking-modules-in-jest
• (PR to add more!)

Solution

Here's a tl;dr of the solutions usually suggested:

1. Use exports.getPlanet() instead of getPlanet()
2. Use dependency injection
3. Refactoring into a class, or move getPlanet to a seperate file
4. Use babel-plugin-rewire (example)

This repo shows examples for the following approaches:

• #1 (using exports.) (probably avoid this imo)
• #2 (dependency injection)

My personal preference is for dependency injection. exports. seems hacky and ties you to implementation details about how your build handles and compiles ES6 imports/exports.