Tổng hợp Top 10+ callback javascript là gì hàng đầu

Hình như ngược ngược rồi anh ơi, em mới newbie nodejs thôi mà em đọc cuốn “node for beginner” thì dùng callback để biến đồng bộ thành bất đồng bộ. Trích trong trang 16 17 của cuốn này( anh lên gg search node for begginer download để down lậu cuốn này chứ ko hiểu sao copy bị lỗi)

Event-drivenasynchronouscallbacks To understand why Node.js applications have to be written this way, we need to understand how Node.js executes our code. Node’s approach isn’t unique, but the underlying execution modelisdifferentfromruntimeenvironmentslikePython,Ruby, PHP or Java. Let’s take a very simple piece of code like this:

var result = database.query(“SELECT * FROM hugetable”); console.log(“Hello World”);

Please ignore for now that we haven’t actually talked about connecting to databases before – it’s just an example. The first line queries a database for lots of rows, the second line puts ”Hello World” to the console. Let’sassumethat the databasequeryisreallyslow, thatit hasto read an awful lot of rows, which takes several seconds. Building the application stack 16 The way we have written this code, the JavaScript interpreter of Node.jsfirsthastoreadthecompleteresultsetfromthedatabase, and then it can execute the console.log() function. If this piece of code actually was, say, PHP, it would work the sameway: readalltheresultsatonce,thenexecutethenextline of code. If this code would be part of a web page script, the user would have to wait several seconds for the page to load. However,intheexecutionmodelofPHP,thiswouldnotbecome a”global”problem: thewebserverstartsitsownPHPprocessfor everyHTTPrequestitreceives. Ifoneoftheserequestsresultsin theexecutionofaslowpieceofcode,itresultsinaslowpageload for this particular user, but other users requesting other pages would not be affected. The execution model of Node.js is different – there is only one single process. If there is a slow database query somewhere in thisprocess,thisaffectsthewholeprocess-everythingcomesto a halt until the slow query has finished. To avoid this, JavaScript, and therefore Node.js, introduces the conceptofevent-driven,asynchronouscallbacks, byutilizingan event loop. Wecanunderstandthisconceptbyanalyzingarewrittenversion of our problematic code:

database.query(“SELECT * FROM hugetable”, function(rows) { var result = rows; }); console.log(“Hello World”);

Here, instead of expecting database.query() to directly return Building the application stack 17 a result to us, we pass it a second parameter, an anonymous function. In its previous form, our code was synchronous: first do the database query, and only when this is done, then write to the console. Now, Node.js can handle the database request asynchronously. Provided that database.query() is part of an asynchronous library,thisiswhatNode.jsdoes: justasbefore,ittakesthequery and sends it to the database. But instead of waiting for it to be finished, it makes a mental note that says ”When at some point in the future the database server is done and sends the result of the query, then I have to execute the anonymous function that was passed to database.query().” Then, it immediately executes console.log(), and afterwards, it enters the event loop. Node.js continuously cycles through this loop again and again whenever there is nothing else to do, waitingforevents. Eventslike,e.g.,aslowdatabasequeryfinally delivering its results.

Top 15 callback javascript là gì biên soạn bởi Nhà Xinh