node/test/parallel/test-event-emitter-listeners.js

'use strict';

require('../common');
const assert = require('assert');
const events = require('events');

function listener() {}
function listener2() {}

{
  const ee = new events.EventEmitter();
  ee.on('foo', listener);
  const fooListeners = ee.listeners('foo');
  assert.deepStrictEqual(ee.listeners('foo'), [listener]);
  ee.removeAllListeners('foo');
  assert.deepStrictEqual(ee.listeners('foo'), []);
  assert.deepStrictEqual(fooListeners, [listener]);
}

{
  const ee = new events.EventEmitter();
  ee.on('foo', listener);
  const eeListenersCopy = ee.listeners('foo');
  assert.deepStrictEqual(eeListenersCopy, [listener]);
  assert.deepStrictEqual(ee.listeners('foo'), [listener]);
  eeListenersCopy.push(listener2);
  assert.deepStrictEqual(ee.listeners('foo'), [listener]);
  assert.deepStrictEqual(eeListenersCopy, [listener, listener2]);
}

{
  const ee = new events.EventEmitter();
  ee.on('foo', listener);
  const eeListenersCopy = ee.listeners('foo');
  ee.on('foo', listener2);
  assert.deepStrictEqual(ee.listeners('foo'), [listener, listener2]);
  assert.deepStrictEqual(eeListenersCopy, [listener]);
}

{
  const ee = new events.EventEmitter();
  ee.once('foo', listener);
  assert.deepStrictEqual(ee.listeners('foo'), [listener]);
}

{
  const ee = new events.EventEmitter();
  ee.on('foo', listener);
  ee.once('foo', listener2);
  assert.deepStrictEqual(ee.listeners('foo'), [listener, listener2]);
}