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Record unit tests as you use your application.


TL;DR: Jest snapshots on steroids


npm i -g unit-test-recorder



  1. Make sure you are using git and there are no uncommited changes.
  2. Run unit-test-recorder <your entrypoint>.js. Usually it is index.js or server.js. UTR will require this file as module. Make sure you are not using require.main === module.
  3. It will now crawl through your project and hook itself into all your files. It will use git reset to revert it when testing is done.
  4. As you use your application, it will record activity for all your exported functions.
  5. Press Q or Ctrl + C key on the keyboard to safely stop the recording and your application and start generating the tests.
  6. Run your favourite linter/prettier to clean up the generated test code.

Typescript (beta)

  • Make sure you have typescript installed globally. (npm i -g typescript)
  • unit-test-recorder will look for tsconfig.json in pwd. You can specifiy a differnt file using --typescript-config=my-tsconfig.json. This will make UTR automatically generate tests in typescript.


  • IMPORTANT: Serialization of JSONs in the process of recording may cause slowdown. The APIs may take considerably longer to execute. (2-10x)
  • All local modules are mocked but external modules must be whitelisted. e.g. Create a file whitelist.json with content { "axios": true } and use the flag --whitelist=./whitelist.json to record mocks for functions used by axios.
  • The ./utr_activity directory contains all the snapshots which are used to generate the code for test cases. If UTR finds an existing directory, it will load these snapshots as state. This enables the user to record in multiple sessions. It can be deleted after code has been generated.


Except entrypoint all are optional.

Flag name Description Default
entrypoint (positional) Path to entrypoint of your application None (Required)
--only Run only on these files (relative path, comma separated, supports javascript RegExp) undefined
--except Dont run on these files (relative path, comma separated, supports javascript RegExp) undefined
--whitelist Path to whitelist.json ./whitelist.json
--typescript-config Path to typescript config ./tsconfig.json
--max-tests Maximum number of generated tests per function. Type -1 for infinity. 5
--output-dir The directory in which the tests would be written to. ./
--test-ext Extension for test files (spec/test) test
--size-limit Objects larger than this limit will be moved to a different file 500
--max-stack-depth Properties of a JSON, at a depth higher than this, will not be recorded. 7

Environment variables

Environment variable name Description
UTR_EXPERIMENTAL_ALS Set this flag to use AsyncLocalStorage instead of cls-hooked. (Experimental, requires nodejs v13.10+)


  • Pure functions
  • Dependency injections
  • Mocks
  • Typescript (beta)

Planned features

  • JSX
  • class methods
  • function methods
  • Better typescript support
  • Higher order functions


Detailed explanation can be found here.

Lets take this function as an example

const foo = (a, b) => a + b

UTR uses babel to explicitly instrument it to

const foo = (...p) => recorderWrapper(
    { name:'foo', fileName:'bar' },
    (a, b) => a + b,

The recorderWrapper function is provided by UTR. The function below is a simplified version of its implementation.

const recorderWrapper = (meta, fn, ...p) => {
  const result = fn(...p)
  Recorder.record({ meta, result, params: ...p })
  return result;

This records all calls to this function in a state. Here is a simplified representation.

        { "params": [1, 2], "result": 3 },
        { "params": [2, 3], "result": 5 },

Now using string interpolation, we can generate test cases.

describe('fileName', () => {
  describe('functionName', () => {
    it('test 1', () => {
    it('test 2', () => {


  • UTR is still early in development and may be unstable. Users are recommended to use the --only flag to run UTR on a few files at a time.

Similar software

  1. UIRecorder - Similar idea but for java
  2. redux-test-recorder - Redux middleware which records the states and actions
  3. test-recorder - Frontend only
  4. pythoscope - Python, boilerplate only
  5. Neitsch - Python, MagicMocks