Cover image for Make your Angular tests 1000% faster by switching from Karma to Jest

Make your Angular tests 1000% faster by switching from Karma to Jest

dylanwatsonsoftware profile image Dylan Watson Updated on ・7 min read

It sounds sensationalist but it's true.

One of the projects I'm working on has an Angular 8 frontend with over 1000 unit/component tests. These used to all run in Karma and take around 15mins but now they take about 1 min.

But why?

What fast tests not good enough for you?
Some other things I've been loving:

  • Nice error messages
  • Easy debugging in VS Code (finally!)
  • Really nice auto run and error message plugins for VS code
  • Ability to write to disk (Maybe not that useful but I found it handy for some tests)

But how?

Well, let me tell ye a story.

Actually scrap that, you're reading this because you want to convert to jest, maybe you've tried it before and failed, maybe you just want to give it a go - either way let's dig into it.

The approach

If you have a decent sized project, (as with anything in Software) the best way to do it is incrementally.

As we have over 1000 tests, we knew it would take a while to convert them and couldn't do the "big bang" approach as we have about 5 different teams working on the app at any one time - we knew we'd need to run karma and jest side-by-side for a period of time. For us, this ended up being nearly a week but it could have taken way longer!

We naturally are following best software dev practices, so at the end of each step we should be able to create a pull request, run our build, tests and merge to master safely.

Just remember, this is a marathon not a sprint (pardon the pun). As soon as you get a test suite/file passing, commit it. Don't commit broken tests (sounds obvious but you can forget this in the heat of a conversion like this). And don't forget to enlist the help of your fellow developers. This will affect them too so they will want to help out - let them!

With this in mind, our basic approach was this:

  • Install jest
  • Get the first test running with Jest (perhaps a brand new test)
  • Migrate an old test suite/file, using what we've learnt
  • Write a script to migrate an old suite (based on the manual process we just went though)
  • Migrate the next test suite using the script, adding anything to the script that is missing
  • Rinse & Repeat until all the tests are migrated.

Remember, as soon as a test is green -> commit it!
(Jests --onlyChanged flag is very handy here)

Getting started

We start by setting up the jest basics.

Install it:

npm install --save-dev jest @types/jest jest-preset-angular glob @angular-builders/jest

Create a jest.config (for Angular) in the project folder:

var preset = require("jest-preset-angular/jest-preset");
module.exports = {
  preset: "jest-preset-angular",
  setupFilesAfterEnv: [
  testMatch: ["**/*.test.ts"],
  globals: {
    "ts-jest": {
      tsConfig: "src/tsconfig.test.json",
      isolatedModules: true

Create a tsconfig.test.json for jest:
This should be very similar to your main tsconfig, but with jest types added.

  "compilerOptions": {
    "baseUrl": "",
    "declaration": false,
    "downlevelIteration": true,
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "mapRoot": "./",
    "module": "commonjs",
    "moduleResolution": "node",
    "outDir": "../dist/out-tsc",
    "sourceMap": true,
    "target": "es5",
    "lib": ["es2015", "dom"],
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "typeRoots": ["../node_modules/@types"]

If you use jasmine.createSpy or jasmine.createSpyObj, to aid in the migration, you may need a create-spy.ts:

export const createSpyObj = (
): { [key: string]: jest.Mock<any> } => {
  let obj: any = {};

  for (let i = 0; i < methodNames.length; i++) {
    obj[methodNames[i]] = jest.fn();

  return obj;

export const createSpy = (
) => {
  return jest.fn();

Import this where ever you have broken tests (after running the migration script) relating to creatSpy or createSpyObj.

In order to get jest to actually run, you'll need to create a new test config for karma in your angular.json and replace the existing one with jest:

        "test": {
          "builder": "@angular-builders/jest:run",
          "options": {
            "tsConfig": "<rootDir>/src/tsconfig.test.json"

If you simply replace karma with jest, you will not be able to run karma and jest tests side-by-side!

Instead, create a new project config inside your angular.json that resembles something like this:

    "karma": {
      "root": "",
      "sourceRoot": "src",
      "projectType": "application",
      "architect": {
        "test": {
          "builder": "@angular-devkit/build-angular:karma",
          "options": {
            "main": "src/test.ts",
            "tsConfig": "src/tsconfig.json",
            "karmaConfig": "./karma.conf.js",
            "polyfills": "src/polyfills.ts",
            "scripts": [

            "styles": [],
            "assets": []

Then add another script to your package.json
"test-karma": "ng test karma"

From now on, jest will run your jest tests and npm run test-karma will run the leftover karma tests.

Visualising Progress

Since this is a big job, we want to see some progress and get others involved, so having a script that outputs the percentage of tests that have been converted is also a really good morale boost.

Here is the script we used. We simply ran it at the end of our builds.

Create a file and name it something painfully obvious, like check-progress.js:

var glob = require("glob")

Reset = "\x1b[0m"
FgRed = "\x1b[31m"
FgGreen = "\x1b[32m"
FgYellow = "\x1b[33m"
FgWhite = "\x1b[37m"

let specs = glob.sync("src/**/*.spec.ts");
let tests = glob.sync("src/**/*.test.ts");

console.log(FgYellow, `${specs.join('\n')}`, Reset)

if (specs.length) {
  console.log(FgRed, specs.length + " slow karma tests")
} else {
  console.log(FgGreen, 'Wooooooooooooooooooooo! Jest conversion complete!')
console.log(FgWhite, tests.length + " fast jest tests")
console.log(FgGreen, (tests.length * 100 / (tests.length + specs.length)).toFixed(2) + "% complete in switching tests to jest", Reset)

Then just run node check-progress.js


If you are using VS Code, you may find the plugins Jest and Jest Runner very handy for running and also debugging your tests (Finally!).

The actual migration

With all our setup out of the way, we should be able to start incrementally converting tests.
There are tools out there like jest-codemods that are meant to do the conversion for you but we didn't have any luck with this, so we built our own. Below is the simple script we used. When we found a case or type of test it couldn't handle, we simply added to the script. You will likely need to continue that pattern for your tests, but this might be a good start.

Note that since we want to run karma specs alongside jest tests (until we've finished converting all the tests), we have chosen the convention of spec.ts for karma tests and test.ts for jest tests. The script below will, after conversion, rename the spec to *.test.ts so your git diff will likely show a bunch of deleted files (the spec.ts files). For this reason it's probably best to just run this on a single test file to start with.

Create a file called convert-to-jest.js:

var fs = require('fs')
var filename = process.argv[2]

if (!filename) {
  let specs = require('glob').sync("src/**/*.spec.ts");
  for (spec of specs) {
    if (!spec.includes('pact')) {
} else {

function convertToJest(filename) {
  if (!filename.startsWith('C:')) {
    filename = './' + filename

  fs.readFile(filename, 'utf8', function (err, data) {
    if (err) {
      return console.log(err);
    var result = data;
    result = result.replace(' } from \'@ngneat/spectator\';', ', SpyObject } from \'@ngneat/spectator/jest\';');
    result = result.replace('} from \'@ngneat/spectator\';', ', SpyObject } from \'@ngneat/spectator/jest\';');
    result = result.replace(/SpyObj</g, 'SpyObject<');
    result = result.replace(/\.and\.returnValue/g, '.mockReturnValue');
    result = result.replace(/\.spec\'/g, '.test');
    result = result.replace(/jasmine\.SpyObj/g, 'SpyObj');
    result = result.replace(/spyOn/g, 'jest.spyOn');
    result = result.replace(/spyOnProperty/g, 'spyOn');
    result = result.replace(/expect\((.*)\.calls\.first\(\)\.args\)\.toEqual\(\[(.*)\]\);/g, 'expect($1).toHaveBeenCalledWith($2);')
    result = result.replace(/expect\((.*)\.calls\.any\(\)\)\.toBe\((.*)\);/g, 'expect($1).toHaveBeenCalledWith($2);');
    result = result.replace(/expect\((.*)\.calls\.mostRecent\(\)(\.args\[.*\])?\)\.toEqual\((.*)\);/g, 'expect($1).toHaveBeenCalledWith($2);');
    result = result.replace(/expect\((.*)\.calls\.count\(\)\)\.toBe\((.*)\);/g, 'expect($1).toHaveBeenCalledTimes($2);');
    result = result.replace(/expect\((.*)\.calls\.count\(\)\)\.toEqual\((.*)\);/g, 'expect($1).toHaveBeenCalledTimes($2);');
    result = result.replace(/\.calls\.first\(\).args/g, '.mock.calls[0].args');
    result = result.replace(/and.callFake/g, 'mockImplementation');
    // result = result.replace(/createService\(/g, 'createServiceFactory(');
    // result = result.replace(/createService,/g, 'createServiceFactory,');

    if (result.includes('createSpyObj')) {
      result = result.replace(/jasmine\.createSpyObj/g, 'createSpyObj');
      result = result.replace(/createSpyObject/g, 'createSpyObj');

      var numberOfSlashesinFilename = (filename.replace('./src/app/', '').match(/\//g) || []).length;
      var prefix = "./"
      for (var i = 0; i < numberOfSlashesinFilename; i++) {
        prefix += "../"

      result = 'import { createSpyObj } from \'' + prefix + 'shared/testing/SpyObj\';\r\n' + result;

    result = result.replace('import SpyObj = SpyObj;', '');
    result = result.replace('import Spy = jasmine.Spy;', '');
    result = result.replace('import createSpyObj = createSpyObj;', '');
    result = result.replace(/ Spy;/g, ' jest.SpyInstance;');
    result = result.replace(/jasmine\.Spy;/g, 'jest.SpyInstance;');

    if (!result.includes('@ngneat/spectator') && result.includes('SpyObject')) {
      result = 'import { SpyObject } from \'@ngneat/spectator/jest\';\r\n' + result;
    if (result.includes('MatDialog') && !result.includes('@angular/material/dialog')) {
      result = result.replace(/import \{(.*)MatDialog, (.*)\}/g, 'import {$1$2}');
      result = result.replace(/import \{(.*)MatDialogModule, (.*)\}/g, 'import {$1$2}');
      result = result.replace(/import \{(.*)MatDialogModule(.*)\}/g, 'import {$1$2}');
      result = result.replace(/import \{(.*)MAT_DIALOG_DATA, (.*)\}/g, 'import {$1$2}');
      result = result.replace(/import \{(.*)MatDialogRef, (.*)\}/g, 'import {$1$2}');
      result = 'import { MatDialog, MatDialogModule, MAT_DIALOG_DATA, MatDialogRef } from \'@angular/material/dialog\';\r\n' + result;

    if (result.includes('withArgs')) {
      result = result.replace(/(.*)\.withArgs\((.*)\)\.mockReturnValue\((.*)\)/g, `$1.mockImplementation(flag => {
        switch (flag) {
          case $2:
            return $3;

    result = result.replace(/jest\.jest/g, 'jest');

    let newFile = filename.replace('.spec.ts', '.test.ts');
    fs.writeFile(newFile, result, 'utf8', function (err) {
      if (err)
        return console.log(err);
      console.log('Successfully wrote ' + newFile);
      if (newFile != filename) {

You'll just need to run:
node convert-to-jest.js <optional path to specific test>

The interesting bit

Now we get to the interesting bit - running the test.
Assuming you've setup your angular.json for jest correctly, you should be able to just run ng test.

I call this "the interesting bit" because I can't really give you much more guidance if it doesn't work. You'll need to figure out why your tests aren't working, for yourself. Of course, if you get lucky and they just work, it's time to convert the next test!

You may also find that if you bulk convert all the tests, there may be some that "just work". If this is the case, you can simply commit these and move on with the rest. You'll also find one command very handy:
ng test --onlyChanged
This is git aware and only runs tests that have changes sitting uncommitted in your git repo. You will find this very handy if you try to bulk convert your tests.

Also since jest outputs a lot of error info, when there are failures, you may want to additionally add:
ng test --onlyChanged --bail
This means that jest will stop on the first test failure, allowing you to focus on that.

Armed with these simple techniques alone, you should be able to convert a bulk of your tests quite quickly.

Results (Check my maths)

Our builds used to take about 15mins to run 1200 tests. After converting to jest our tests now take about 1.5mins. Thats a change from 80 test/min up to 800 test/min - 1000% faster! Ok technically I could just say 10x faster but bigger numbers are better right?

Posted on by:

dylanwatsonsoftware profile

Dylan Watson


Just another developer trying to get his tests fast, his features to add value and his bloody code to compile!


markdown guide

Hello, I do not doubt about the benefits, but I have one question when I read at the results. If this is so fast why is it not the default test runner packaged with Angular ? Are there any hidden tradeoffs when using jest instead of Karma ?


Hmm definitely a fair question. I'd say that the tradeoffs I've noticed relate to the fact that you are not running in a real browser (it uses jsdom).
This means that:

  1. there is a risk that jsdom differs from your targeted browser
  2. Debugging is much less visual, though there are tools to nicely print out the html.

From my perspective, despite this, the trade-off has been worth it so far for my team.

  • Web Components support
  • no import/export support

although these things have been around a while, for example Web Components support in jsdom



You're awesome. I just used your guide to switch from Karma to Jest. I used Jest for the first time in another Angular project and loved how easy it was to create and manage tests. I couldn't wait to swap our karma tests out for it. I just updated angular in one of our projects then used this guide to switch over. Very clear and concise. Thanks a bunch!


You're welcome! Glad it could be of use!


Nice one! Well explained and would be a massive time saver for anyone else in the same situation!

Checking your maths... you start at 100% and the final speed is 1000% which makes it 900% faster, right? But still pretty fast. I guess. 😜


What am i doing wrong? :-) Angular 9 app + 400 tests approx, 8 core CPU.
Karma runs in ~1m 20sec
Jest runs in ~40sec.

That makes Jest two times faster but:

  1. It runs multiple node instances and takes all my CPU power. Literally I can't run anything else. The reason i think it's forced to compile multiple times because of many node instances.

  2. I can't run some of my tests in Jest because they depend on WebRTC and i can hardly mock it. There's a small number of failed tests in Jest that i need to review as well.

That doesn't look like an ultimate solution for everybody.


Hmm yep I think thats definitely fair. Due to the fact that Jest runs jsdom and not a real browser, there is definitely some things it won't be able to test properly, I suppose WebRTC is one of them.

I have seen some issues in the jest github repo recently about jest slowing down in recent versions.. perhaps its related?

If you are having issues with speed you could try running sequentially: jestjs.io/docs/en/troubleshooting#...