Is there anyway to extend a jest configuration file? - jestjs

Question

Within a node app where I'm using Jest to test client side code (testEnvironment: 'jsdom') and server side code (testEnvironment:'node') as well collecting code coverage for both client and server side.
Currently I'm using 4 Jest config files with lots of redundant configuration to accomplish this.
client
{
"bail": true,
"verbose": true,
"notify": true,
"scriptPreprocessor": "./node_modules/babel-jest",
"testPathIgnorePatterns": [
"./node_modules",
"./coverage",
"./dist",
"./build"
],
"testRegex": "\\.test\\.js"
}
client coverage
{
"bail": true,
"verbose": true,
"notify": true,
"scriptPreprocessor": "./node_modules/babel-jest",
"testPathIgnorePatterns": [
"./node_modules",
"./coverage",
"./dist",
"./build"
],
"testRegex": "\\.test\\.js",
"collectCoverageFrom": ["**/*.js", "!**/node_modules/**"],
"collectCoverage": true,
"coverageDirectory": "./coverage",
"coveragePathIgnorePatterns": [
"./node_modules",
"./coverage",
"./dist",
"./build",
"./test"
],
"coverageThreshold": {
"global": {
"branches": 100,
"functions": 100,
"lines": 100,
"statements": 100
}
}
}
server
{
"bail": true,
"verbose": true,
"notify": true,
"scriptPreprocessor": "./node_modules/babel-jest",
"testPathIgnorePatterns": [
"./node_modules",
"./coverage",
"./dist",
"./build"
],
"testRegex": "\\.test\\.js",
"testEnvironment": "node"
}
server coverage
{
"bail": true,
"verbose": true,
"notify": true,
"scriptPreprocessor": "./node_modules/babel-jest",
"testPathIgnorePatterns": [
"./node_modules",
"./coverage",
"./dist",
"./build"
],
"testRegex": "\\.test\\.js",
"testEnvironment": "node",
"collectCoverageFrom": ["**/*.js", "!**/node_modules/**"],
"collectCoverage": true,
"coverageDirectory": "./coverage",
"coveragePathIgnorePatterns": [
"./node_modules",
"./coverage",
"./dist",
"./build",
"./test"
],
"coverageThreshold": {
"global": {
"branches": 100,
"functions": 100,
"lines": 100,
"statements": 100
}
}
}
How can I achieve this without repeating my configuration 4 times? I've looked at the preset configuration option. Using that I have to create a separate package for each configuration. Is that the recommended way?

Answer 1

Yes, you could define shared jest.config.js
and reuse it in your specific configs:
It's nice to use <rootDir> in all paths in your shared config, so those could be reused too.
client/jest.config.js
const sharedConfig = require('../jest.config.js');
module.exports = {
...sharedConfig,
'rootDir': './',
}
server/jest.config.js
const sharedConfig = require('../jest.config.js');
module.exports = {
...sharedConfig,
'rootDir': './',
"testEnvironment": "node"
}
You could also reuse jest defaults if needed: Jest Documentation - Configuring Jest

Answer 2

Yes, since Jest v20 you can define config as a JS file and use it to share common parts of the similar configs. Docs on configuring Jest.
By default Jest looks up for:
jest.config.js
"jest" entry in package.json
...and treats the parent directory as a rootDir.
Also be sure to check out the projects option, which makes it easier to run Jest inside monorepos (e.g. client + server code in one codebase). See this answer for reference: Testing two environments with jest