Applying polyfill before loading mocha

I am trying to run emulated IE9- tests for my error manager lib using the folowing custom launcher:

            IE9: {
                base: 'IE',
                'x-ua-compatible': 'IE=EmulateIE9',
                flags: ['-extoff']
            },

I got the following error message:

IE 11.0.0 (Windows 7.0.0) ERROR: 'This browser lacks typed array (Uint8Array) support which is required by `buffer` v5.x. Use `buffer` v4.x if you require old browser support.'

IE 11.0.0 (Windows 7.0.0) ERROR
  {
    "message": "'Uint8Array' nincs definiálva\nat node_modules/mocha/mocha.js:6676",
    "str": "'Uint8Array' nincs definiálva\nat node_modules/mocha/mocha.js:6676"
  }

So mocha requires Uint8Array to run and I need a polyfill to fix this. I have found a typedarray polyfill, which might work on IE9- browsers too: https://github.com/inexorabletash/polyfill/blob/master/typedarray.js

I just don’t know how I can apply it before running mocha. Adding it to the files array does not work, since it is loaded after mocha. Any ideas?

[dwightjack]

@maksimr @inf3rno

Thanks! Was able to fix the problem with:

const fixMocha = function(files) {
  files.unshift({
    pattern: path.resolve('./node_modules/core-js/client/core.js'),
    included: true,
    served: true,
    watched: false
  });
};

fixMocha.$inject = ['config.files'];

module.exports = (config) => {
  config.set({
     ...
     frameworks: ['mocha', 'inline-mocha-fix'],

     plugins: [
        'karma-*',
        {
            'framework:inline-mocha-fix': ['factory', fixMocha]
        }
     ],
     ...
  });
};

I’m using core-js but I think it should work with other libraries as well

[jslegers]

Anyway, do I understand correctly that the described approach polyfills everything that is loaded instead of just mocha?

If that is indeed the expected behavior, I don’t think this is a suitable approach for a test runner.

The whole point of testing in IE9 & IE10 is usually to detect whether those browsers don’t break because you rely on features not present in such old browsers. Adding a polyfill for ES6 features in your test environment kinda prevents you from detecting this problem.

I’d rather just use Mocha v4 instead.