Getting Proper Stack Traces with Mocha and TypeScript

Posted on Thu 20 August 2015

Mocha and TypeScript are awesome when everything's working, but when you have a failing test something's not quite right:

$npm test

  Calculator
    #add
      1) should work


  0 passing (21ms)
  1 failing

  1) Calculator #add should work:

      AssertionError: expected 6 to equal 7
      + expected - actual

      -6
      +7

      at Context.<anonymous> (test/Calculator.spec.js:12:38)

The stacktrace at the end points us to the failing assertion; however the stacktrace is pointing Calculator.spec.js - not our TypeScript test-case.

The TypeScript compiler (tsc) will generate source maps to enable debugging the TypeScript source rather than the transpiled ES5 javascript which gets written out by the compiler. We can enable this with the --sourcemap flag.

Problem is that V8 (the engine that powers Node) doesn't use these sourcemaps when generating stacktraces; source-map-support to the rescue! We can enable it by passing --require source-map-support/register to mocha:

{
    "scripts": {
      "test": "mocha --require source-map-support/register test/**/*.spec.js"
    }
}

Now we get an accurate stacktrace on failure :)

  1) Calculator #add should work:

      AssertionError: expected 6 to equal 7
      + expected - actual

      -6
      +7

      at Context.<anonymous> (test/Calculator.spec.ts:15:22)

Thanks to redditor pieeta who tipped me off :) I've posted the sample project up on Github.