Cypress hangs in loop when running custom Chai assertion - chai

Question

I have been trying to create my own custom chai assertion (based on the Cypress recipe template: https://github.com/cypress-io/cypress-example-recipes/blob/master/examples/extending-cypress__chai-assertions/cypress/support/index.js).
What I have found with the code below is that when it is run I end up with a constant loop of WRAP, if I swap this.obj with element it then results in a constant stream of GET. I do not seem to ever progress further than getRect(first).then((actual)
If anyone could help me out I'd be very grateful.
cypress/integration/test.js
describe('testing custom chai', () => {
it('uses a custom chai helper', () => {
cy.visit('https://www.bbc.co.uk/news');
cy.get('#orb-modules > header').should('be.leftAligned', '#orb-header');
});
});
cypress/support/index.js
function getRect(selector) {
if (selector === '&document') {
return cy.document().then(doc => doc.documentElement.getBoundingClientRect());
} if (typeof selector === 'string') {
return cy.get(selector).then($elem => $elem[0].getBoundingClientRect());
}
return cy.wrap(selector).then(elem => Cypress.$(elem)[0].getBoundingClientRect());
}
function getRects(first, second) {
return getRect(first).then((actual) => {
getRect(second).then(expected => [actual, expected]);
});
}
const aligned = (_chai, utils) => {
function leftAligned(element) {
getRects(element,this.obj).then((rects) => {
this.assert(
rects[0].left === rects[1].left,
'expected #{this} to be equal',
'expected #{this} to not be equal',
this._obj,
);
});
}
_chai.Assertion.addMethod('leftAligned', leftAligned);
};
chai.use(aligned);

Answer 1

The basic problem is that the async commands cy.get(), cy.wrap(), cy.document() can't be used in the custom assertion. My best guess is that the auto-retry mechanism is going bananas and giving you the constant loop.
Instead, you can use Cypress.$() which is the synchronous version (essentially jquery exposed on the Cypress object).
The following seems to work ok. (I renamed getRects() param to subject, as sometimes it's a selector and sometimes it's the object passed in to .should()).
Note also this._obj instead of this.obj.
function getRect(subject) {
if (subject === '&document') {
return Cypress.$(document).context.documentElement.getBoundingClientRect();
}
if (typeof subject === 'string') { // the selector passed in to assertion
return Cypress.$(subject)[0].getBoundingClientRect();
}
if (typeof subject === 'object') { // the element from cy.get() i.e this._obj
return subject[0].getBoundingClientRect();
}
return null; // something unkown
}
function getRects(first, second) {
const actual = getRect(first)
const expected = getRect(second)
return [actual, expected];
}
const aligned = (_chai, utils) => {
function leftAligned(element) {
const rects = getRects(element, this._obj)
this.assert(
rects[0].left === rects[1].left,
'expected #{this} to be equal',
'expected #{this} to not be equal',
this._obj,
);
}
_chai.Assertion.addMethod('leftAligned', leftAligned);
};
chai.use(aligned);
I was unable to test your BBC page directly, as there's a cross-origin problem occurring
Refused to display 'https://www.bbc.com/news' in a frame because it set 'X-Frame-Options' to 'sameorigin'
but it does work with a mockup page
cypress/app/bbc-sim.html
<div id="orb-modules">
<header>
<h1>Brexit: Boris Johnson's second attempt to trigger election fails</h1>
</header>
</div>
and testing like so
it('uses a custom chai helper', () => {
cy.visit('app/bbc-sim.html')
cy.get('#orb-modules > header').should('be.leftAligned', '#orb-modules');
});