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Babel Jest Boost

Brings tree-shaking to Jest, speeding up your test runs, using Babel

Overview

babel-jest-boost figures out the original export of your imported specifiers and rewrites your import statements to bypass barrel files and re-exports. This prevents jest from requiring, compiling and executing irrelevant code, speeding up your test runs.

Assume the following structure:

.
├── lib
│   ├── lib.js     # export function libFunc () {}
│   └── index.js   # export * from './lib.js'
└── code.js        # import { libFunc } from './lib';

Only for the testing transpilation babel-jest-boost will convert the import statement in code.js to:

import { libFunc } from '/home/myuser/myproject/lib/lib.js';

It will also replace jest.mock() function calls in exaclty the same manner.

Integration

1. Install the package

npm install -D @gtsopanoglou/babel-jest-boost

2. Update your babel-jest transformer

Option 1: babel-jest-boost/plugin

You may use babel-jest-boost as a regular babel plugin. It needs access to your jest config (moduleNameMapper and modulePaths in particular). To help you do that we export a jestConfig object:

+ const { jestConfig } = require("@gtsopanoglou/babel-jest-boost/config");

 ...

 plugins: [
     [
+       require.resolve('@gtsopanoglou/babel-jest-boost/plugin'),
+       { jestConfig, /* babel-jest-boost options */ }
     ]
 ]

Option 2 (experimental): babel-jest-boost/transformer

This option is not recommended yet because it hasn't been tested thoroughly. Use can the pre-made transformer exported from @gtsopanoglou/babel-jest-boost/transformer which takes care of passing the jestConfig object for you:

- const babelJest = require("babel-jest").default;
+ const babelJestBoost = require("@gtsopanoglou/babel-jest-boost/transformer");

- module.exports = babelJest.createTransformer({ /* babel config */ });
+ module.exports = babelJestBoost.createTransformer({ /* babel config */ }, { /* babel-jest-boost options */ });

3. Run your tests, prevent breakages

Since babel-jest-boost modifies the transpiled code, you will need to clear jest's cache before each run (just for this integration phase) to ensure you see non-cached results:

jest --clearCache && jest # or whatever you testing command is

It is likely that some tests will now break. The breakage may be caused by some implicit dependency in your code that you're not aware of, or some bug within babel-jest-boost. Either way, you are not going to fix them right now. In order to overcome this problem you have two tools: importIgnorePatterns plugin option and the no-boost directive.

Use importIgnorePatterns to match import statements that cause breakages when by-passed. For instance:

import { port } from 'config';

console.log(port)

Use { importIgnorePatterns: ['config'] } to prevent babel-jest-boost from re-writing this import statement.

The no-boost directive prevent the whole file (either test file or source code) from being parsed and re-written by babel-jest-boost. For instance:

// @babel-jest-boost no-boost
import { port } from 'config';

console.log(port)

4. Re-iterate until your tests are green again.

5. Done

Once your tests are green, you are done. You can now keep running your tests are usual without having to clear your cache.

Plugin options

importIgnorePatterns [array\<string>]

Array of strings/regexes, import paths matching these regexes will prevent babel-jest-boost from rewritting them. For instance, assuming the example above:

.
├── lib
│   ├── lib.js     # export function libFunc () {}
│   └── index.js   # export * from './lib.js'
└── code.js        # import { libFunc } from './lib';

In this scenario, importIgnorePatterns will be matched against the only import statement in this tree, import { libFunc } from './lib', so if you wish to exclude imports to ./lib from being re-written, you can use:

{ importIgnorePatterns: ['./lib'] }

This is intended to help you defer refactoring some barrels or modules that are causing trouble or breaking your tests when you integrate this plugin.

ignoreNodeModules [boolean]

Set this flag to true if you want to completely ignore all node_modules imports from being re-written. Default is false.

Plugin directives

no-boost

You can let the plugin know that you do not wish to parse a particular file by adding the following comment anywhere within the file (usually at the top)

// @babel-jest-boost no-boost
import { libFunc } from './lib';

Any import statements within this particular file will not be re-written.

ROADMAP

• 0.1.11 Ensure all different import/export syntaxes are properly treated. ✅
• 0.1.12 Performance increase, tracing prioritization and defensive try/catch ✅
• 0.1.13 Pre-commit lint, format test and github action to test again. ✅
• 0.1.14 Expose ready-made transformer to relief the user from having to pass in the jestConfig object. ✅
• 0.1.15 Performance improvements ✅
• 0.1.16 ignore node_modules config option ✅
• 0.1.17 babel/preset-env compatibility ✅
• 0.1.18 Ensure all different jest.mock calls are properly treated.
• 0.1.19 Expose debugging options to the user (like printing which imports are being rewritten, or the transpiled output of a particular file).
• 0.1.20 Expose a jest reporter to print a high-level overview of what the plugin did within the run (and potientialy report barel file statistics)
• 0.1.21 Performance testing: Fork some open-source codebases, integrate babel-jest-boost and test to measure the performance increase. Do this in the CI/CD pipeline
• 0.1.22 Figure out automatic changelog, version increase, github release, npm publish actions