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function fakeAjax(url, cb) {
	var fake_responses = {
		file1: 'The first text',
		file2: 'The middle text',
		file3: 'The last text'
	};
	var randomDelay = (Math.round(Math.random() * 1e4) % 8000) + 1000;

	console.log('Requesting: ' + url);

	setTimeout(function() {
		cb(fake_responses[url]);
	}, randomDelay);
}

function output(text) {
	console.log(text);
}

function getFile(file) {
	return new Promise(function(resolve) {
		fakeAjax(file, resolve);
	});
}

['file1', 'file2', 'file3']
	.map(getFile)
	.reduce((chain, pr) => chain.then(() => pr).then(output), Promise.resolve())
	.then(() => output('Complete'));