How to run Mocha tests written in TypeScript?

Setup: I have a Node project (pure Node, no browser bits) written in TypeScript. I can use the TypeScript compiler (tsc) from the typescript module to compile the code. So far so good.

However, I want to write tests using Mocha, and that's where I'm having trouble. I tried --compilers ts:typescript, but I keep getting errors like:

error TS5023: Unknown compiler option 'compilers'.

It looks like the command line to mocha ends up being passed to tsc, which is obviously not good.

For anybody who has tried and had problems with typescript-require you may want to try ts-node.

$ npm install -g ts-node
$ mocha --require ts-node/register src/**/*.spec.ts

It also appears that there has been some ongoing discussion about deprecating typescript-require in favor of ts-node.

Don't use this answer. typescript-require is unmaintained, and ts-node is its replacement. Leaving this answer here for posterity.

Found it. The typescript module is actually like a "main" function; it runs the compiler as soon as the module is loaded. Not very nice design.

I poked at Mocha's acceptance tests, which show how to use a custom compiler for foo files. They wire it up via the require.extensions mechanism. I was halfway through writing a module that just calls tsc on the command line when I realized that somebody must have done this before. So it's very simple:

$ npm install typescript-require --save-dev
$ mocha --compilers ts:typescript-require

Using the latest version of Mocha and ts-node I was getting an Unexpected token import issue. Using the below settings with ts-mocha worked for me:

tsconfig.json

{
    "files": [
        "src/main.ts"
    ],
    "compilerOptions": {
        "noImplicitAny": true,
        "target": "es2015",
        "types": ["mocha"],
        "module": "commonjs"
    }
}

package.json

"scripts": {
    "mocha": "ts-mocha -p library/tsconfig.json library/test/**/*.ts"
  },

launch.json

{
    "type": "node",
    "request": "launch",
    "name": "Mocha Tests",
    "runtimeArgs": [
        "${workspaceFolder}/node_modules/ts-mocha/bin/ts-mocha",
        "--timeout", "999999",
        "-p",
        "${workspaceFolder}/library/tsconfig.json",
        "${workspaceFolder}/library/test/**/*.ts"
    ],
    "internalConsoleOptions": "openOnSessionStart"
}

and gulp.js just incase you want to use gulp too

const gulp = require('gulp');
const ts = require('gulp-typescript');
const mocha = require('gulp-mocha');

const tsProject = ts.createProject('tsconfig.json');

gulp.task('build', () => tsProject.src()
  .pipe(tsProject())
  .js.pipe(gulp.dest('dist')));

gulp.task('test', () => gulp.src('test/*.spec.ts')
  .pipe(mocha({
    reporter: 'nyan',
    require: ['ts-node/register'],
  })));
/* single command to hook into VS Code */
gulp.task('default', gulp.series('build', 'test'));