nodeJs callbacks simple example

Question

can any one give me a a simple example of nodeJs callbacks  I have already searched for the same on many websites but not able to understand it properly  Please give me a simple example   getDbFiles store  function files       getCdnFiles store  function files               I want to do something like that

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A callback is a function passed as an parameter to a Higher Order Function  wikipedia   A simple implementation of a callback would be   const func   callback   gt  callback  Hello World       To call the function  simple pass another function as argument to the function defined   func string   gt  console log string

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const fs   require  fs     fs stat  input txt   function  err  stats        if err           console log err         else           console log stats           console log  Completed Reading File                 fs  is a node module which helps you to read file  Callback function will make sure that your file named  input txt  is completely read before it gets executed  fs stat   function is to get file information like file size  date created and date modified

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Try this example as simple as you can read  just copy save newfile js do node newfile to run the application   function myNew next       console log  Im the one who initates callback        next  nope    success        myNew function err  res       console log  I got back from callback  err  res

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This blog-post has a good write-up   https   codeburst io javascript-what-the-heck-is-a-callback-aba4da2deced   x000D   x000D  function doHomework subject  callback    x000D    alert  Starting my   subject  homework     x000D    callback    x000D    x000D   x000D  function alertFinished    x000D    alert  Finished my homework    x000D    x000D   x000D  doHomework  math   alertFinished   x000D   x000D   x000D

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A callback function is simply a function you pass into another function so that function can call it at a later time  This is commonly seen in asynchronous APIs  the API call returns immediately because it is asynchronous  so you pass a function into it that the API can call when it s done performing its asynchronous task   The simplest example I can think of in JavaScript is the setTimeout   function  It s a global function that accepts two arguments  The first argument is the callback function and the second argument is a delay in milliseconds  The function is designed to wait the appropriate amount of time  then invoke your callback function   setTimeout function        console log  10 seconds later          10000     You may have seen the above code before but just didn t realize the function you were passing in was called a callback function  We could rewrite the code above to make it more obvious   var callback   function        console log  10 seconds later          setTimeout callback  10000     Callbacks are used all over the place in Node because Node is built from the ground up to be asynchronous in everything that it does  Even when talking to the file system  That s why a ton of the internal Node APIs accept callback functions as arguments rather than returning data you can assign to a variable  Instead it will invoke your callback function  passing the data you wanted as an argument  For example  you could use Node s fs library to read a file  The fs module exposes two unique API functions  readFile and readFileSync   The readFile function is asynchronous while readFileSync is obviously not  You can see that they intend you to use the async calls whenever possible since they called them readFile and readFileSync instead of readFile and readFileAsync  Here is an example of using both functions   Synchronous   var data   fs readFileSync  test txt    console log data     The code above blocks thread execution until all the contents of test txt are read into memory and stored in the variable data  In node this is typically considered bad practice  There are times though when it s useful  such as when writing a quick little script to do something simple but tedious and you don t care much about saving every nanosecond of time that you can   Asynchronous  with callback    var callback   function  err  data      if  err  return console error err     console log data      fs readFile  test txt   callback     First we create a callback function that accepts two arguments err and data  One problem with asynchronous functions is that it becomes more difficult to trap errors so a lot of callback-style APIs pass errors as the first argument to the callback function  It is best practice to check if err has a value before you do anything else  If so  stop execution of the callback and log the error   Synchronous calls have an advantage when there are thrown exceptions because you can simply catch them with a try catch block   try     var data   fs readFileSync  test txt      console log data     catch  err      console error err       In asynchronous functions it doesn t work that way  The API call returns immediately so there is nothing to catch with the try catch  Proper asynchronous APIs that use callbacks will always catch their own errors and then pass those errors into the callback where you can handle it as you see fit   In addition to callbacks though  there is another popular style of API that is commonly used called the promise  If you d like to read about them then you can read the entire blog post I wrote based on this answer here

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Here is an example of copying text file with fs readFile and fs writeFile   var fs   require  fs     var copyFile   function source  destination  next         we should read source file first   fs readFile source  function err  data        if  err  return next err      error occurred        now we can write data to destination file     fs writeFile destination  data  next              And that s an example of using copyFile function   copyFile  foo txt    bar txt   function err      if  err           either fs readFile or fs writeFile returned an error     console log err stack    err       else       console log  Success               Common node js pattern suggests that the first argument of the callback function is an error  You should use this pattern because all control flow modules rely on it   next new Error  I cannot do it         error  next null  results      no error occurred  return result

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we are creating a simple function as  callBackFunction  data  function   err  response         console log response          callbackfunction  function callBackFuntion  data  callback         write your logic and return your result as callback    result    if not error callback error        if error

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var myCallback   function data      console log  got data    data       var usingItNow   function callback      callback  get it          Now open node or browser console and paste the above definitions   Finally use it with this next line   usingItNow myCallback     With Respect to the Node-Style Error Conventions  Costa asked what this would look like if we were to honor the node error callback conventions   In this convention  the callback should expect to receive at least one argument  the first argument  as an error  Optionally we will have one or more additional arguments  depending on the context  In this case  the context is our above example   Here I rewrite our example in this convention   var myCallback   function err  data      if  err  throw err     Check for the error and throw if it exists    console log  got data    data      Otherwise proceed as usual      var usingItNow   function callback      callback null   get it        I dont want to throw an error  so I pass null for the error argument      If we want to simulate an error case  we can define usingItNow like this  var usingItNow   function callback      var myError   new Error  My custom error       callback myError   get it        I send my error as the first argument       The final usage is exactly the same as in above    usingItNow myCallback     The only difference in behavior would be contingent on which version of usingItNow you ve defined  the one that feeds a  truthy value   an Error object  to the callback for the first argument  or the one that feeds it null for the error argument

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x000D   x000D    delay callback function x000D  function delay  seconds  callback   x000D      setTimeout      gt   x000D        console log  The long delay ended    x000D        callback  Task Complete    x000D         seconds 1000   x000D    x000D    Execute delay function x000D  delay 1  res   gt      x000D      console log res     x000D     x000D   x000D   x000D

[javascript] nodeJs callbacks simple example

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