原来node6.2的时候，我用async.js控制并发，，完全达到预期效果 现在升级到8.2，直接报错了

async.mapLimit(pages, 10, function(url, callback) {
	fetchUrl(url, callback);
}, function(err, result) {
	console.log('final:');
	console.log(result);
});

var concurrencyCount = 0;
function fetchUrl(url, callback) {
	concurrencyCount++;
	console.log('现在的并发数是', concurrencyCount, '，正在抓取的是', url);

	SpiderPage(url);

	setTimeout(function() {
		concurrencyCount--;
		callback(null, url + ' html content');
	}, 360);
};