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I’m pretty sure when you’ve already come across this exception: `[object ErrorEvent] thrown` when you execute your Karma tests in an Angular project. Not very helpful, right? Let’s take a look how to get more a more useful info and finally resolve it.

Contents are based on Angular version >= 6.0.0

Table of contents

So I was executing my Angular unit tests and all of a sudden the following error message popped up. Here’s a screenshot of it:

I’m executing my tests using Headless Chrome, the same I’m using also on my CI server. Here’s the configuration of my karma.conf.js:

// Karma configuration file, see link for more information
// https://karma-runner.github.io/1.0/config/configuration-file.html
module.exports = function(config) {
    basePath: '',
    frameworks: ['jasmine', '@angular-devkit/build-angular'],
    plugins: [
    client: {
      clearContext: false // leave Jasmine Spec Runner output visible in browser
    coverageIstanbulReporter: {
      dir: require('path').join(__dirname, '../../coverage'),
      reports: ['html', 'lcovonly'],
      fixWebpackSourcePaths: true
    reporters: ['mocha', 'kjhtml'],
    port: 9876,
    colors: true,
    logLevel: config.LOG_INFO,
    autoWatch: true,
    browsers: ['ChromeHeadless'],
    customLaunchers: {
      ChromeHeadless: {
        base: 'Chrome',
        flags: ['--headless', '--disable-gpu', '--no-sandbox', '--disable-extensions', '--remote-debugging-port=9222']
    singleRun: false

So in order to get a better error message there are different possibilities.

Option 1: Debug using the Chrome Devtools

Many times, debugging your test in the Chrome Devtools already helps you out. First of all, if you’re running on Headless Chrome, first switch to a real browser. You can use the --browsers CLI flag

$ ng test --browsers=Chrome

As a result, you should see Chrome starting and opening the Karma running in a webpage.

Click the “Debug” button and then in that new tab window, open the Chrome Devtools (or the corresponding one of your browser) just as you usually do. In the Devtools console you should now see the real error:

That error message is a lot more clear. In fact, within an RxJS Observable pipe function I had a null exception, which caused the [object ErrorEvent] thrown exception to pop up.

Option 2: Disable source maps

If debugging doesn’t help, you might try this approach. There has been (and there’s still a regression) in the Angular CLI which in certain situations gives you a cryptic error message when the source maps are enabled (which is the default). So it might be worth disabling them to see whether the error message improves. We can do that by using one of the many Angular CLI flags.

$ ng test --source-map=false


Hope that helps you out of some tricky debugging session. Have another suggestion? Let me know and just drop me a line on Twitter or on my AMA.

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