playwright/tests/library/events/prepend.spec.ts

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// Modifications copyright (c) by Microsoft Corporation
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import { EventEmitter } from '../../../packages/playwright-core/lib/client/eventEmitter';
import { test, expect } from '@playwright/test';

test('EventEmitter functionality', () => {
  const myEE = new EventEmitter();
  let m = 0;
  // This one comes last.
  myEE.on('foo', () => {
    expect(m).toEqual(2);
  });

  // This one comes second.
  myEE.prependListener('foo', () => {
    expect(m++).toEqual(1);
  });

  // This one comes first.
  myEE.prependOnceListener('foo', () => {
    expect(m++).toEqual(0);
  });

  myEE.emit('foo');
});

test('Verify that the listener must be a function', () => {
  expect(() => {
    const ee = new EventEmitter();
    ee.prependOnceListener('foo', null);
  }).toThrow(TypeError);
});