Angular tests failing with Failed to execute 'send' on 'XMLHttpRequest'

This is a problem of the new Angular Cli. Run your test with --sourcemaps=false and you will get the right error messages.

See details here: https://github.com/angular/angular-cli/issues/7296

EDIT:

Shorthand for this is:

ng test -sm=false

As of angular 6 the command is:

ng test --source-map=false

I had the same issue using angualar cli 6, I have used this tag to get the right error message :

ng test --source-map=false

Maybe it will help someone :) .

For my case there was a mock data problem and in case of Array, I was returning string from the mock.

someApi = fixture.debugElement.injector.get(SomeApi);
spyOn(someApi, 'someMethod')
  .and.returnValue(Observable.of('this is not a string but array'));

The error message is really distracting and was not telling the actual error. Running ng test --source=false pointed the correct error and line, and helped me to fix it quickly.

Many time it happens when you mock data is incomplete or incorrect.