Notes on testing Polymer elements with Jasmine

Dealing with dom-if
When selecting by ID an Element that is dynamically added to the DOM with dom-if, use parentElement.querySelector('#id') instead of parentElement.$['id'].
When you have elements only visible through a toggle, and you would like it to be available during a test, don't forget to flush after appending your Component to the document.body:
<template is="dom-if" if="[[showButton]]">
<button id="button"> Button </button>
</template>
// Script.
@property({type: Boolean}) showButton: boolean = false;
// Test.
it('should make use of button correctly', () => {
  const comp = new MyPolymerComponent();
  comp.set('showButton', true);
  document.body.appendChild(comp);
  // A flush is needed to resolve `dom-if`s.
  flush();
// ...
Dealing with async behavior
In each execution, all tests of a suite are performed on a single document. This means that you should clean up the document just before each test finishes, ensuring that you leave the document in the same state it was before conducting this very test. This may imply the following:
If you have appended an Element to the document.body, remember to document.body.removeChild this element.
Be cautious if your business logic code has setTimeout: Tests are (generally) performed really fast, completing and quitting well before a delayed function is called. Of course, you can wait for the delayed function to be called in your test code, but remember that tests are supposed to be run frequently, and the additional delay could accummulate to a considerable waste of time. Therefore, I would generally warn against using setTimeout at all.
You might feel like to test @polymer/app-route, which interacts with the URL. This can be really tricky, as the URL of the testing document is probably some random HTML file. I still have not yet found a nice way to test routing behaviors.
Failed attempts working with Jasmine test
How to test <app-location>?
Jasmine (or it's actually karma_web_test_suite?) will execute tests in some '/context.html' (this will be your window.location.pathname). This will make testing <app-location> and <app-route> painful.
This is NOT working :
// Hijack `window.location.pathname` to always return `/grid`.
Object.defineProperty(window.location, 'pathname', {
  configurable: true,
});
spyOnProperty(window.location, 'pathname').and.returnValue('/grid');
Also NOT working:
function setLocation(url: string) {
  window.history.pushState({}, '', url);
  window.dispatchEvent(new Event('location-changed'));
}
​
setLocation('/grid');
flush();
PaperListboxElement.select is not a function in Jasmine?!
It probably has something to do with our creating our Polymer Component dynamically and appendChild-ing to document.body -- I suppose using fixture will make this problem go away. Methods tried and didn't work for selecting an item in a <paper-listbox> programatically:
item.click(); // doesn't work
// The next tricks may work those who have not overriden `@iron-select`, but not for me.
box.listbox!.select('a');
box.listbox!.selected = 'a';
box.listbox!.selected = 1;
box.listbox!.selectIndex(0);
​
// The next two tricks worked for other teams.
item.dispatchEvent(new Event('iron-select'));
box.listbox!.dispatchEvent(new Event('iron-select'));

Notes on testing Polymer elements with Jasmine

Dealing with `dom-if`

Dealing with `async` behavior

Failed attempts working with Jasmine test

How to test `<app-location>`?

`PaperListboxElement.select` is not a function in Jasmine?!