How to run Mocha tests written in TypeScript?

Question

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.

Answer 1

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.

Answer 2

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