node package manager



Preprocessor to compile ES6 JavaScript on the fly using traceur-compiler.


Install karma-traceur-preprocessor as a devDependency for your project:

npm install karma --save-dev
npm install karma-traceur-preprocessor --save-dev


The following example configuration shows some of the required settings for enabling Traceur support. Traceur compiles source files into either AMD or Node.js modules. Since Karma is testing in a browser you need to configure RequireJS.

// karma.conf.js 
module.exports = function(config) {
    // at a minimum need requirejs + traceur 
    frameworks: ['jasmine', 'requirejs', 'traceur'],
    preprocessors: {
      'src/**/*.es6': ['traceur']
    files: [
      {pattern: 'src/**/*.es6', included: false},
      {pattern: 'test/**/*Spec.es6', included: false},
    // default configuration, not required 
    traceurPreprocessor: {
      // options passed to the traceur-compiler 
      // see traceur --longhelp for list of options 
      options: {
        sourceMaps: false,
        modules: 'amd'
      // custom filename transformation function 
      transformPath: function(path) {
        return path.replace(/\.es6$/, '.js');

Source Maps

If you set the sourceMaps preprocessor option to true then the generated source map will be inlined as a data-uri.

Source maps allow the browser to map the generated JavaScript back to the original ES6 code. You can then set breakpoints in the source ES6 code instead of the generated code. In the browser you should see two files for each source file: .es6 and .js. The .js is the compiled output from Traceur and .es6 is the original source file.

Supported Karma Launchers

Traceur is an ES6 to ES5 transpiler, and as such, does not support older browsers without ES5 support like IE8 or PhantomJS. (This is the case for any Traceur-compiled code, not just that produced by karma-traceur-preprocessor.)

For more information on Karma see the homepage.

For an example of a project configured to test ES6 code check out karma-traceur-test.