mocha-traceur

2.1.0 verified Fri May 01 auth: no javascript maintenance

A Mocha plugin that compiles JavaScript files through the Traceur transpiler, enabling ES6 (ES2015) features for test files. Version 2.1.0 is the current stable release, depending on 'traceur-runner' for stack traces and optional dependency compilation. It operates via Mocha's --compilers flag. Key differentiator vs alternatives like babel-register: focused solely on Traceur, with no dependency compilation by default, making it lightweight but restrictive. The package is maintained infrequently; latest release 2.1.0 is several years old. Does not bundle Traceur itself, requiring a separate install.

Common errors

error Error: Cannot find module 'traceur' ↓

cause The package does not bundle Traceur; it must be installed separately.

fix

Run: npm install --save-dev traceur

error Error: Unexpected token import ↓

cause Mocha does not natively support ES6 modules. The --compilers flag may not be applied correctly, or the plugin isn't loading.

fix

Use command: mocha --compilers js:mocha-traceur test/*.js (ensure the flag uses the correct path to mocha-traceur).

error SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode ↓

cause Traceur may not be transpiling because the file is not passed through the plugin (e.g., due to incorrect file extension or missing --compilers flag).

fix

Ensure --compilers is set correctly and that your test files match the pattern (e.g., .js). Add 'use strict' if needed.

Warnings

breaking Dependencies under node_modules are NOT compiled with Traceur. If your code imports from an ES6-only package not transpiled, it will fail at runtime. ↓

fix Use only transpiled dependencies, or switch to babel-register which can transpile node_modules via ignore patterns.

deprecated Traceur Compiler is no longer actively maintained. Prefer Babel or TypeScript for ES6+ transpilation. ↓

fix Migrate to a modern transpiler like Babel with @babel/register.

gotcha The package does not include Traceur itself. You must install traceur separately, or use traceur-runner (optional) to add stack trace support. ↓

fix Run: npm install --save-dev traceur or traceur-runner

breaking In v2, dependencies are no longer compiled. If you relied on dependency transpilation in v1, tests may break after upgrading. ↓

fix Ensure all dependencies are pre-transpiled, or pin to v1.x if dependency transpilation is required.

Install

npm install mocha-traceur

yarn add mocha-traceur

pnpm add mocha-traceur

Imports

mocha-traceur (plugin)
wrong
```
require('mocha-traceur')
```
correct
```
mocha --compilers js:mocha-traceur my_test_dir/*.js
```
Not a module import; used solely via Mocha's CLI --compilers flag. No programmatic usage.

Quickstart

Setup and run Mocha tests with Traceur transpilation via --compilers flag, demonstrating ES6 arrow functions and template literals.

// Install dependencies
npm install --save-dev mocha mocha-traceur traceur

// Create a test file 'test/test.js' with ES6 features
import { strictEqual } from 'assert';

const greet = (name) => `Hello, ${name}!`;
describe('greet', () => {
  it('should greet a person', () => {
    strictEqual(greet('World'), 'Hello, World!');
  });
});

// Run tests from command line
mocha --compilers js:mocha-traceur test/*.js