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Jest Platform

You can cherry pick specific features of Jest and use them as standalone packages. Here's a list of the available packages:


Tool for identifying modified files in a git/hg repository. Exports two functions:

  • getChangedFilesForRoots returns a promise that resolves to an object with the changed files and repos.
  • findRepos returns a promise that resolves to a set of repositories contained in the specified path.


const {getChangedFilesForRoots} = require('jest-changed-files');
// print the set of modified files since last commit in the current repo
getChangedFilesForRoots(['./'], {
lastCommit: true,
}).then(result => console.log(result.changedFiles));

You can read more about jest-changed-files in the readme file.


Tool for visualizing changes in data. Exports a function that compares two values of any type and returns a "pretty-printed" string illustrating the difference between the two arguments.


const {diff} = require('jest-diff');
const a = {a: {b: {c: 5}}};
const b = {a: {b: {c: 6}}};
const result = diff(a, b);
// print diff


Tool for extracting and parsing the comments at the top of a JavaScript file. Exports various functions to manipulate the data inside the comment block.


const {parseWithComments} = require('jest-docblock');
const code = `
* This is a sample
* @flow
console.log('Hello World!');
const parsed = parseWithComments(code);
// prints an object with two attributes: comments and pragmas.

You can read more about jest-docblock in the readme file.


Module that identifies the primitive type of any JavaScript value. Exports a function that returns a string with the type of the value passed as argument.


const {getType} = require('jest-get-type');
const array = [1, 2, 3];
const nullValue = null;
const undefinedValue = undefined;
// prints 'array'
// prints 'null'
// prints 'undefined'


Tool for validating configurations submitted by users. Exports a function that takes two arguments: the user's configuration and an object containing an example configuration and other options. The return value is an object with two attributes:

  • hasDeprecationWarnings, a boolean indicating whether the submitted configuration has deprecation warnings,
  • isValid, a boolean indicating whether the configuration is correct or not.


const {validate} = require('jest-validate');
const configByUser = {
transform: '<rootDir>/node_modules/my-custom-transform',
const result = validate(configByUser, {
comment: ' Documentation:',
exampleConfig: {transform: '<rootDir>/node_modules/babel-jest'},

You can read more about jest-validate in the readme file.


Module used for parallelization of tasks. Exports a class JestWorker that takes the path of Node.js module and lets you call the module's exported methods as if they were class methods, returning a promise that resolves when the specified method finishes its execution in a forked process.


// heavy-task.js
module.exports = {
myHeavyTask: args => {
// long running CPU intensive task.
// main.js
async function main() {
const worker = new Worker(require.resolve('./heavy-task.js'));
// run 2 tasks in parallel with different arguments
const results = await Promise.all([
worker.myHeavyTask({foo: 'bar'}),
worker.myHeavyTask({bar: 'foo'}),

You can read more about jest-worker in the readme file.


Exports a function that converts any JavaScript value into a human-readable string. Supports all built-in JavaScript types out of the box and allows extension for application-specific types via user-defined plugins.


const {format: prettyFormat} = require('pretty-format');
const val = {object: {}};
val.circularReference = val;
val[Symbol('foo')] = 'foo'; = new Map([['prop', 'value']]);
val.array = [-0, Infinity, NaN];

You can read more about pretty-format in the readme file.