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

// Copyright Joyent, Inc. and other Node contributors.
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'use strict';
const common = require('../common');
const assert = require('assert');
const EventEmitter = require('events');

const e = new EventEmitter();

e.once('hello', common.mustCall());

e.emit('hello', 'a', 'b');
e.emit('hello', 'a', 'b');
e.emit('hello', 'a', 'b');
e.emit('hello', 'a', 'b');

function remove() {
  assert.fail('once->foo should not be emitted');
}

e.once('foo', remove);
e.removeListener('foo', remove);
e.emit('foo');

e.once('e', common.mustCall(function() {
  e.emit('e');
}));

e.once('e', common.mustCall());

e.emit('e');

// Verify that the listener must be a function
assert.throws(() => {
  const ee = new EventEmitter();

  ee.once('foo', null);
}, /^TypeError: "listener" argument must be a function$/);