whydoyouwork.js

function doReplaceASync() {
	const input = 'haha';
	const result = input.replace(/a/g, function() {
		let result;
		myAsyncFunc(function(err, data) {
			result = data;
		});
		return result;
	});
	
	console.log(result);
};

function myAsyncFunc(cb) {
	let total = 5;
	for(let i = 0; i < 1000000; i++){
		total++;
	}
	cb(null, 'e');
}

doReplaceASync();