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unit-testingjestjspuppeteerweb-component

How to test custom web component with jest?

I would like to test some custom web components and use jest.js as test runner (due to its support for ES6).

Chromium supports commands like

window.customElements.define('my-custom-element', MyCustomElementClass);

to register a custom web component.

However, window.customElements does not seem to be known in the context of jest tests.

As a work around I tried to use jest in combination with puppeteer and express to run the customElements part in Chromium.

However, I have difficulties to inject the custom element class TreezElement in the evaluated code:

treezElement.js:

class TreezElement extends HTMLElement {
    connectedCallback () {
      this.innerHTML = 'Hello, World!';
    }
}

treezElement.test.js:

import TreezElement from '../../src/components/treezElement.js';
import puppeteer from 'puppeteer';
import express from 'express';

describe('Construction', ()=>{

    let port = 3000;

    let browser;
    let page;
    let element;
    const width = 800;
    const height = 800;

    beforeAll(async () => {

        const app = await express()                   
                    .use((req, res) => {                       
                        res.send(
                        `<!DOCTYPE html>
                        <html>            
                            <body>
                            <div id="root"></div>                           
                            </body>
                        </html>`
                        )
                    })
                    .listen(port);

        browser = await puppeteer.launch({
          headless: false,
          slowMo: 80,
          args: [`--window-size=${width},${height}`]
        });

        var pages = await browser.pages();
        page = pages[0]; 

        await page.setViewport({ width, height });        

        await page.goto('http://localhost:3000'); 

        element = await page.evaluate(({TreezElement}) => {
            console.log('TreezElement:')
            console.log(TreezElement);
            window.customElements.define('treez-element', TreezElement);
            var element = document.create('treez-element');
            document.body.appendChild(element);
            return element;           
         }, {TreezElement}); 

    });


    it('TreezElement', ()=>{   

    });    

    afterAll(() => {
        browser.close();
    });        

});

Maybe TreezElement is not serializable and therefore undefined is passed to the function.

If I try to import the custom element class TreezElement directly from within the evaluated code ...

element = await page.evaluate(() => {
            import TreezElement from '../../src/components/treezElement.js';
            console.log('TreezElement:')
            console.log(TreezElement);
            window.customElements.define('treez-element', TreezElement);
            var element = document.create('treez-element');
            document.body.appendChild(element);
            return element;           
         });

... I get the error

'import' and 'export' may only appear at the top level

=> What is the recommended way to test custom web components with jest?

Some related stuff:

Solution

  • JSDOM 16.2 includes basic support for custom elements and is available in Jest 26.5 and above. Here's a simple Jest test that shows it working:

    customElements.define('test-component', class extends HTMLElement {
    constructor() {
        super();
        const p = document.createElement('p')
        p.textContent = 'It works!'
        this.appendChild(p)
    }
})

test('custom elements in JSDOM', () => {
    document.body.innerHTML = `<h1>Custom element test</h1> <test-component></test-component>`
    expect(document.body.innerHTML).toContain('It works!')
})

    Output:

    $ jest

PASS  ./test.js
✓ custom elements in JSDOM (11 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        1.409 s
Ran all test suites.

    Note not all features are supported yet, notably shadow DOM does not work.