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Writing mocha tests with style - OOP style:

import { suite, test, slow, timeout } from "mocha-typescript";
@suite class Hello {
    @test world() {
        assert.equal(1, 2, "Expected one to equal two.");

The test UI will register a suite with tests for the @suite and @test decorators. When the tests run, the class will be instantiated once for each @test method and the method will be called.

Build Status


Test UI

The test interface provides support for mocha's built-in tdd, bdd: describe/suite, it/test, timeout, slow, it.only and it.skip; as well as TypeScript decorators based test UI for classes. You can mix and match:

import { suite, test, slow, timeout } from "mocha-typescript";
suite("one", () => {
    test("test", () => {});
@suite class Two {
    @test method() {}

Similarly you can use describe/it:

import { suite, test, slow, timeout } from "mocha-typescript";
describe("one", () => {
    it("test", () => {});
@suite class Two {
    @test method() {}

Or even mix the two approaches to get nested suites:

import { suite, test, slow, timeout } from "mocha-typescript";
describe("suite one", () => {
    it("test one", () => {});
    @suite class TestTwo {
        @test method() {}


The mocha-typescript comes with a watcher script that runs the TypeScript compiler in watch mode, and upon successful compilations runs the mocha tests, concatenating the output of both. This in combination with the support for "only":

@suite class One {
    @test.only method1() {}
    @test mothod2() {}

Allows for rapid development of both new functionality and unit tests.

Please note, the built in mocha watcher should work with mocha-typescript UI and the awesome-typescript-loader.

Thanks to

  • Haringat for the async support in before and after methods.
  • godart for taking the extra step to support non-default test file paths.

Setting Up

Adding Mocha-TypeScript to Existing Project

If you already have an npm package with mocha testing integrated just install mocha-typescript:

npm i mocha-typescript --save-dev

Then require the mocha-typescript in your test files and you will be good to go:

import { suite, test, slow, timeout } from "mocha-typescript";
@suite class Two {
    @test method() {}

Setting up New Project With Custom UI


Fork the mocha-typescript-seed repo, or clone it:

git clone

Don't forget to edit the package.json, and check the license.

From that point on, you could:

npm i
npm test
npm run watch

Manual Steps

Create a folder, cd in the folder, npm init, npm install:

npm init
npm install mocha typescript mocha-typescript @types/mocha chai @types/chai source-map-support nyc --save-dev

Edit the package.json and set the scripts section to:

  "scripts": {
    "pretest": "tsc",
    "test": "nyc mocha",
    "watch": "mocha-typescript-watch",
    "prepare": "tsc"

You may omit the nyc tool and have "test": "mocha" instead, nyc is the instanbul code coverage reporting tool.

Add a tsconfig.json file with settings similar to:

    "compilerOptions": {
        "target": "es6",
        "module": "commonjs",
        "sourceMap": true,
        "experimentalDecorators": true,
        "lib": [ "es6" ]

Create test folder and add test/mocha.opts file.

--ui mocha-typescript
--require source-map-support/register
  • Sets the mocha-typescript as custom ui
  • Optionally require the source-map-support/register to have typescript stack traces for Errors
  • Optionally provide test files list, point to specific dist fodler, or skip this to use mocha's defaults Add your first test file test/test.ts:
// Reference mocha-typescript's global definitions:
/// <reference path="../node_modules/mocha-typescript/globals.d.ts" />

@suite(timeout(3000), slow(1000))
class Hello {
    @test world() {

From that point on, you could either:

npm test
npm run watch

To run the tests once manually or run all tests. Keep in mind you can use add .only to run a single test.

Setting Up Dev Test Watcher

There is a watcher script in the package, that runs tsc -w process and watches its output for successful compilation, upon compilation runs a mocha process.

You will need a tsconfig.json, and at least test.ts mocha entrypoint.

Install mocha, typescript and mocha-typescript as dev dependencies (required):

npm install mocha typescript mocha-typescript --save-dev

Add the following npm script to package.json:

    "dev-test-watch": "mocha-typescript-watch"

And run the typescript mocha watcher from the terminal using npm run dev-test-watch.

You can use the watcher with plain describe, it functions. The decorator based interface is not required for use with the watcher.

The mocha-typescript-watch script is designed as a command line tool. You can provide the arguments in the package.json's script. In case you are not using the default test.js file as entrypoint for mocha, you can list the test suite files as arguments to mocha-typescript-watch and they will be passed to mocha. For example:

    "dev-test-watch": "mocha-typescript-watch -p tsconfig.test.json -o mocha.opts dist/test1.js dist/test2.js"

For complete list with check ./node_modules/.bin/mocha-typescript-watch --help:

  -p, --project  Path to tsconfig file or directory containing tsconfig, passed
                 to `tsc -p <value>`.                    [string] [default: "."]
  -t, --tsc      Path to executable tsc, by default points to typescript
                 installed as dev dependency. Set to 'tsc' for global tsc
                         [string] [default: "./node_modules/typescript/bin/tsc"]
  -o, --opts     Path to mocha.opts file containing additional mocha
                 configuration.          [string] [default: "./test/mocha.opts"]
  -m, --mocha    Path to executable mocha, by default points to mocha installed
                 as dev dependency.
                           [string] [default: "./node_modules/mocha/bin/_mocha"]
  -g, --grep     Passed down to mocha: only run tests matching <pattern>[string]
  -f, --fgrep    Passed down to mocha: only run tests containing <string>
  -h, --help     Show help                                             [boolean]




JetBrain's stellar WebStorm now (since WebStorm 2017.3 EAP) supports the mocha-typescript mocha UI. Featuring:

  • Test Explorer - Detects tests in the TypeScript using static analysis.
  • Test Runner - WebStorm has a Mocha test runner that can be configured to also do a TypeScript compilation before test run.
  • Code Editor Integration - In the TypeScript code editor, tests are prefixed with an icon, that lets you:
    • Run a specific test or suite
    • Debug a specific test or suite

The mocha-typescript-seed has been preconfigured (see the .idea folder in the repo) with UnitTests task that will run all mocha tests with the mocha-typescript UI. The UnitTests is configured to run mocha, with TypeScript compilation before launch, use the mocha-typescript mocha UI, as well as include tests in the test folder recursively.


The WebStorm has its own way to define tasks so the configuration for the project is duplicated at few places. Here are some side-effects it would be good for you to be aware of.

Should running/debugging a single test/unit from the TypeScript code editor fail due missing ts-node, consider installing npm i ts-node --save-dev to your repo. WebStorm is using ts-node to transpile the file you are testing. This may omit proper type checking or using settings in your tsconfg, but that would rarely be an issue.

Should running/debugging a single test/unit run the test twice, that's because WebStorm provides the file you are editing to mocha as .ts file, but mocha also reads the test/mocha.opts where additional files may be specified. You can either:

  • Nevermind running the test twice
  • Edit the automatically generated single test config from the top tasks menu in WebStorm and change the file extension it points to from .ts to .js, this will use the JavaScript files produced by the TypeScript compilation of your project. But you will have to change the extension by hand each time you debug or run a single test.
  • Change the test/mocha.opts file so it won't reference any files (e.g. delete the --recursive test from it). In that case you may need to fix the package.json build scripts.

At few occasions when misxing BDD and the mocha-typescript decorators based UI, trying to run a single BDD test would cause WebStorm to generate a mocha task that would run using BDD ui, instead of mocha-typescript. In these cases the tests may fail as there is no suite or test functions defined in the BDD UI. To fix this you may edit the default Mocha task, and configure it to use mocha-typescript UI explicitly. From that point on, when you try to run a single test, event BDD one, WebStorm will create Mocha tasks that will use the mocha-typescript UI.


Please note that the methods and decorators used below are introduced through importing from the mocha-typescript module:

import { suite, test, slow, timeout } from "mocha-typescript";

Or by installing mocha-typescript as custom mocha test UI.

Declarative Suites and Tests

Declaring suites is done using the @suite decorator and tests within the suite using the @test decorator:

@suite class Suite {
    @test test1() {}

When used without parameters, the names are infered from the class and method name. Complex names can be provided as arguments to the @suite or @test decorator:

@suite("A suite")
class Suite {
    @test("can have tests") {}
    @test "typescript also supports this syntax for method naming"() {}

Generated Suites and Tests

Mocha's simple interface is very flexible when tests have to be dynamically generated. If tests for classes have to be generated dynamically here is an example:

[{ title: 'google', url: '' },
 { title: 'github', url: '' }
].forEach(({title, url}) => {
    @suite(`Http ${title}`) class GeneratedTestClass {
        @test login() {}
        @test logout() {}

Before and After Actions

By default, before and after test actions are implemented with instance and static before and after methods. The static before and after methods are invoked before the suite and after the suite, the instance before and after methods are invoked before and after each test method.

@suite class Suite {
    static before() { /* 1 */ }
    before() { /* 2, 5 */ }
    @test one() { /* 3 */ }
    @test one() { /* 6 */ }
    after() { /* 4, 7 */ }
    static after() { /* 8 */ }

Async Tests, Before and After Actions

The methods that accept a done callback or return a Promise are considered async similar and their execution is similar to the one in mocha.

  • For done, calling it without params marks the test as passed, calling it with arguments fails the test.
  • For returned Promise, the test passes is the promise is resolved, the test fails if the promise is rejected.
@suite class Suite {
    @test async1(done) {
        setTimeout(done, 1000);
    @test async2() {
        return new Promise((resolve, reject) => setTimeout(resolve, 1000));
    @test async async3() {
        // async/await FTW!
        await something();

Skipped and Only Suite and Tests

Marking a test as pending or marking it as the only one to execute declaratively is done using @suite.skip, @suite.only, @test.skip or @test.only similar to the mocha interfaces:

@suite.only class SuiteOne {
    @test thisWillRun() {}
    @test.skip thisWillNotRun() {}
@suite class SuiteTwo {
    @test thisWillNotRun() {}

The signatures for the skip and only are the same as the suite and test counterpart so you can switch between @suite.only(args) and @suite(args) with ease.

If running in watch mode it may be common to focus a particular test file in your favourite IDE (VSCode, vim, whatever), and mark the suite or the tests you are currently developing with only so that the mocha-typescript watcher would trigger just the tests you are focused on. When you are ready, remove the only to have the watcher execute all tests again.

Timing - Timeout, Slow

Controlling the time limits, similar to the it("test", function() { this.slow(ms); /* ... */ }); is done using suite or test traits, these are modifiers passed as arguments to the @suite() and @test() decorators:

@suite(slow(1000), timeout(2000))
class Suite {
    @test first() {}
    @test(slow(2000), timeout(4000)) second() {}

The slow and timeout traits were initially working as decorators (e.g. @suite @timeout(200) class Test {}), but this behavior may be dropped in future major versions as it generates too much decorators that clutter the syntax. They are still useful though for setting timeouts on before and after methods (e.g. @suite class Test { @timeout(100) before() { /* ... */ }}).


I would not recomend retrying failed tests multiple times to ensure green light but I also wouldn't judge, here it goes mocha-typescript retries:

class Suite {
    static tries1 = 0;
    static tries2 = 0;
    @test first() {
        assert.isAbove(Suite.tries1++, 2);
    @test(retries(5)) second() {
        assert.isAbove(Suite.tries1++, 3);

The retries can also be used as a decorator similar to timeout and slow - @test @retries(3) testMethod() {}.

Extending Test Behavior

Accessing the Mocha Context Within Class Methods

There are various aspects of the suites and tests that can be altered via the mocha context. Within the default mocha 'BDD' style this is done through the callback's this object. That object is exposed to the TypeScript decorators based UI through a field decorated with the @context decorator:

@suite class MyClass {
  @context mocha; // Set for instenace methods such as tests and before/after
  static @context mocha; // Set for static methods such as static before/after (mocha bdd beforeEach/afterEach)
  after() {

Skipping Tests In Suite After First Failure - skipOnError

In functional testing it is sometimes fine to skip the rest of the suite when one of the tests fail. Consider the case of a web site tests where the login test fail and subsequent tests that depend on the login will hardly pass. This can be done with the skipOnError suite trait:

class StockSequence {
    @test step1() {}
    @test step2() { throw new Error("Failed"); }
    @test step3() { /* will be skipped */ }
    @test step4() { /* will be skipped */ }