NewPromiseCapability.js

'use strict';

var $TypeError = require('es-errors/type');

var IsCallable = require('./IsCallable');
var IsConstructor = require('./IsConstructor');

// https://262.ecma-international.org/6.0/#sec-newpromisecapability

module.exports = function NewPromiseCapability(C) {
	if (!IsConstructor(C)) {
		throw new $TypeError('C must be a constructor'); // step 1
	}

	var resolvingFunctions = { '[[Resolve]]': void undefined, '[[Reject]]': void undefined }; // step 3

	var promise = new C(function (resolve, reject) { // steps 4-5
		if (typeof resolvingFunctions['[[Resolve]]'] !== 'undefined' || typeof resolvingFunctions['[[Reject]]'] !== 'undefined') {
			throw new $TypeError('executor has already been called'); // step 4.a, 4.b
		}
		resolvingFunctions['[[Resolve]]'] = resolve; // step 4.c
		resolvingFunctions['[[Reject]]'] = reject; // step 4.d
	}); // step 4-6

	if (!IsCallable(resolvingFunctions['[[Resolve]]']) || !IsCallable(resolvingFunctions['[[Reject]]'])) {
		throw new $TypeError('executor must provide valid resolve and reject functions'); // steps 7-8
	}

	return {
		'[[Promise]]': promise,
		'[[Resolve]]': resolvingFunctions['[[Resolve]]'],
		'[[Reject]]': resolvingFunctions['[[Reject]]']
	}; // step 9
};