Difference between readFile and readFileSync

Question

The following code outputs the content of the index html  it just contains the text hello world  to the browser  However  when I replace readFile   with readFileSync    the request times out  What am I missing  Is a different kind of buffer required  I am using node 0 61 and express 2 4  var express   require  express    var fs   require  fs     var app   express createServer express logger      app get      function request  response        fs readFile  index html   function err  data           response send data toString                  var port   process env PORT    5000  app listen port  function       console log  quot Listening on  quot    port

User · Answer

use strict  var fs   require  fs             implementation of readFileSync     var data   fs readFileSync  input txt    console log data toString     console log  Program Ended             implementation of readFile      fs readFile  input txt   function  err  data        if  err  return console error err      console log data toString          console log  Program Ended      For better understanding run the above code and compare the results

User · Answer

readFileSync   is synchronous and blocks execution until finished  These return their results as return values  readFile   are asynchronous and return immediately while they function in the background  You pass a callback function which gets called when they finish  let s take an example for non-blocking       following method read a file as a non-blocking way   var fs   require  fs    fs readFile filename   utf8   function err  data            if  err  throw err          console log data            following is read a file as blocking or synchronous way    var data   fs readFileSync filename        LOL   If you don t want readFileSync   as blocking way then take   reference from the following code   Native    var fs   require  fs    function readFileAsSync        new Promise  resolve  reject   gt           fs readFile filename   utf8   function err  data                    if  err  throw err                  resolve data                          async function callRead        let data   await readFileAsSync        console log data      callRead         it s mean behind scenes readFileSync   work same as above promise  base

User · Answer

fs readFile takes a call back which calls response send as you have shown - good   If you simply replace that with fs readFileSync  you need to be aware it does not take a callback so your callback which calls response send will never get called and therefore the response will never end and it will timeout   You need to show your readFileSync code if you re not simply replacing readFile with readFileSync   Also  just so you re aware  you should never call readFileSync in a node express webserver since it will tie up the single thread loop while I O is performed   You want the node loop to process other requests until the I O completes and your callback handling code can run

[javascript] Difference between readFile() and readFileSync()

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