{"id":66,"date":"2019-09-21T21:40:45","date_gmt":"2019-09-21T21:40:45","guid":{"rendered":"https:\/\/www.kindsonthegenius.com\/nodejs\/?p=66"},"modified":"2020-08-06T09:55:47","modified_gmt":"2020-08-06T09:55:47","slug":"07-node-js-event-loops","status":"publish","type":"post","link":"https:\/\/www.kindsonthegenius.com\/nodejs\/07-node-js-event-loops\/","title":{"rendered":"Node.js – Event Loops"},"content":{"rendered":"

In the previous lesson, we talked about Callbacks<\/a>. Now, we would learn of something a bit similar. That is Event Loops.<\/p>\n

Event loops and callbacks are features used by Node.js to support concurrency. This is because Node.js is single-threaded application.<\/p>\n

 <\/p>\n

Events in Node.js<\/strong><\/p>\n

Node.js is an event-driven program. This means that the main program loops listens for events to occurs. Then when an event occurs, a callback is triggered. Already, you know that callbacks are functions that are called when an event is detects.<\/p>\n

This is what is also called the observer pattern.<\/strong><\/p>\n

This means that a loop (event loop) is maintained which keeps listening(observing) for events. This is exactly what happens in event-driven<\/strong> programs. This make sense too because, throughout the life of a program, events keep occurring. Hence the name event loop. This is illustrated in the figure 1 below:<\/p>\n

 <\/p>\n

\"Event<\/a>
Figure 1: Event Loop in Node.js<\/figcaption><\/figure>\n

 <\/p>\n

Events vs Callbacks<\/strong><\/p>\n

As you know, a callback fires when an asynchronous function returns data.For instance, the readFile() function is an async functions that returns the content of a file. In this case it is callback.The code is given below
\n<\/p>\n

var<\/span> fs =<\/span> require(\"fs\"<\/span>)\r\nvar<\/span> path =<\/span> \"D:\/nodefiles\/testinput.txt\"<\/span>;\r\nfs.readFile(path, function<\/span>(err, data){ \r\n   \/\/perform some action<\/span>\r\n});\r\n<\/pre>\n
 <\/p>\n
However, in the case of event, the function (observer) listens for events to occur. Then when an even occurs, then the event listener executes. In Node.js, there are many in-built events. You can find them in the events module and EventEmitter class. Both are used to bind events to event-listeners.<\/p>\n
An example of event is file open. So we can have a listener that listens to file-open event. And when it detects it, it fires an event listener.<\/p>\n
var<\/span> fs =<\/span> require('fs'<\/span>);\r\nvar<\/span> path =<\/span> 'D:\/nodefiles\/testinput.txt'<\/span>;\r\nvar<\/span> rs =<\/span> fs.createReadStream(path);\r\nrs.on('open'<\/span>, function<\/span> () {\r\n  console.log('A file have been opened'<\/span>);\r\n});\r\n<\/pre>\n
 <\/p>\n
Let’s now write another program that uses a complete event loop. The program is as given below;<\/p>\n
 <\/p>\n\n\n\n
\n
 1\r\n 2\r\n 3\r\n 4\r\n 5\r\n 6\r\n 7\r\n 8\r\n 9\r\n10\r\n11\r\n12\r\n13\r\n14\r\n15\r\n16\r\n17\r\n18\r\n19\r\n20\r\n21\r\n22\r\n23\r\n24\r\n25\r\n26<\/pre>\n<\/td>\n
\n
\/\/ Import the events module<\/span>\r\nvar<\/span> events =<\/span> require(\"events\"<\/span>);\r\n\r\n\/\/Create an eventEmitter variable<\/span>\r\nvar<\/span> eventEmitter =<\/span> new<\/span> events.EventEmitter();\r\n\r\n\/\/ Create and eventHandler function<\/span>\r\nvar<\/span> connectHandler =<\/span> function<\/span> connected() {\r\n    console.log(\"connection succeeded\"<\/span>);\r\n\r\n    \/\/Then fire the data_recieved event as follows<\/span>\r\n    eventEmitter.emit(\"data_received\"<\/span>);\r\n}\r\n\r\n\/\/ Bind the connection even with the connectHandler<\/span>\r\neventEmitter.on(\"connection\"<\/span>, connectHandler);\r\n\r\n\/\/ Bind the data_recieved event with a function<\/span>\r\neventEmitter.on(\"data_received\"<\/span>, function<\/span>(){\r\n    console.log(\"Data was received!\"<\/span>);\r\n});\r\n\r\n\/\/ Now let's fire the connection event<\/span>\r\neventEmitter.emit(\"connection\"<\/span>);\r\n\r\nconsole.log(\"End of Program\"<\/span>);\r\n<\/pre>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Listing 1 – Event Loop<\/p>\n
 <\/p>\n
Explaining Listing 1<\/strong><\/p>\n
Let’s try to understand what is happening in Listing 1. I have added line numbers to simplify the explanation.<\/p>\n
In line 24, we fired the connection event. This calls the connectHandler in lines 7 to 13.<\/p>\n
The connectHandler fires data_recieved event. This calls the data_recieved handler function in line 19. This is an anonymous function.<\/p>\n
Also note that in lines 16 and 19, we how how the events are bound to the corresponding eventListeners (handlers).<\/p>\n
 <\/p>\n
I save this file as events2.js. The output is shown below:<\/p>\n
 <\/p>\n
\"Event<\/a><\/p>\n
 <\/p>\n
I would recommend you watch the video to get a clearer understanding.<\/p>\n
 <\/p>\n","protected":false},"excerpt":{"rendered":"
In the previous lesson, we talked about Callbacks. Now, we would learn of something a bit similar. That is Event Loops. 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