How do I mock a Date() in testcafe?

Question

My test includes a step where the date is set based on the current date (using dayjs()). I need to always get the same, pre-defined date.

dayjs() calls new Date(), so my approach was to mock the global Date() constructor. I've tried it like this:

await t.eval( () => {
  const fixedDate = new Date(2010, 0, 1);
  Date = class extends Date {
    constructor() {
      super();
      return fixedDate;
    }
  };
});

Like this, testcafe can't finish to eval (works in my Chrome though). So far, I only managed to overwrite Date.now() directly, but not the constructor.

I wonder if the approach to modifying Date with eval is the right approach or if there's any better solution how to fixate the current Date.

Answer 1

One solution is to use the mockdate package:

1°) npm install --save mockdate

2°) set up your test like this;

import { ClientFunction } from 'testcafe';
import { readFileSync } from 'fs';
import { join } from 'path';

test('Test', async t => {
  const mockdateJS = readFileSync(join(process.cwd(), 'node_modules','mockdate','src','mockdate.js')).toString();
  const loadJsLib = ClientFunction((js) => {
        window.MockDate = new Function(js);
        window.MockDate();
  });
  const setDate = ClientFunction((date) => window.MockDate.set(date));
    await loadJsLib(mockdateJS); // dynamically load the mockdate lib in browser
    await setDate('2000-11-22'); // mock date in browser
    // now any code in the browser that does new Date() will get '2000-11-22' 

});