How to write asynchronous functions for Node js

Question

I ve tried to research on how exactly asynchronous functions should be written  After a lot of plowing through a lot of documentation  it s still unclear to me   How do I write asynchronous functions for Node  How should I implement error event handling correctly   Another way to ask my question would be this  How should I interpret the following function   var async function   function val  callback       process nextTick function            callback val                Also  I found this question on SO   How do I create a non-blocking asynchronous function in node js    interesting  I don t feel like it has been answered yet

User · Answer

I ve dealing too many hours for such task in for node js  I m mainly front-end guy   I find this quite important  because all node methods asyncronous deal with callback  and transform it into Promise is better to handle it   I Just want to show a possible outcome  more lean and readable  Using ECMA-6 with async you can write it like this    async function getNameFiles  dirname      return new Promise  resolve  reject    gt        fs readdir dirname   err  filenames    gt          err      undefined    null    reject err    resolve filenames                  the  undefined    null  is for repl  read event print loop  scenarios  using undefined also work

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If you KNOW that a function returns a promise  i suggest using the new async await features in JavaScript  It makes the syntax look synchronous but work asynchronously  When you add the async keyword to a function  it allows you to await promises in that scope   async function ace       var r   await new Promise  resolve  reject    gt        resolve true           console log r      true     if a function does not return a promise  i recommend wrapping it in a new promise that you define  then resolve the data that you want   function ajax call url  method      return new Promise  resolve  reject    gt        fetch url    method         then resp   gt  resp json         then json   gt    resolve json               async function your function       var json   await ajax call  www api-example com some data    GET      console log json        status  200  data            Bottom line  leverage the power of Promises

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Try this  it works for both node and the browser   isNode    typeof exports      undefined    amp  amp   typeof module      undefined    amp  amp   typeof module exports      undefined    amp  amp   typeof navigator      undefined     typeof navigator appName      undefined     true   false  asyncIt    isNode   function  func      process nextTick function          func              function  func      setTimeout func  5

User · Answer

You should watch this  Node Tuts episode 19 - Asynchronous Iteration Patterns  It should answers your questions

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Just passing by callbacks is not enough  You have to use settimer for example  to make function async   Examples  Not async functions   function a       var a   0        for i 0  i lt 10000000  i          a           b        function b       var a   0        for i 0  i lt 10000000  i          a               c        function c       for i 0  i lt 10000000  i             console log  async finished         a    console log  This should be good      If you will run above example  This should be good  will have to wait untill those functions will finish to work   Pseudo multithread  async  functions   function a       setTimeout   function         var a   0        for i 0  i lt 10000000  i            a               b         0       function b       setTimeout   function         var a   0        for i 0  i lt 10000000  i            a                 c         0       function c       setTimeout   function         for i 0  i lt 10000000  i                 console log  async finished          0       a    console log  This should be good      This one will be trully async  This should be good will be writen before async finished

User · Answer

You seem to be confusing asynchronous IO with asynchronous functions  node js uses asynchronous non-blocking IO because non blocking IO is better  The best way to understand it is to go watch some videos by ryan dahl      How do I write asynchronous functions for Node    Just write normal functions  the only difference is that they are not executed immediately but passed around as callbacks      How should I implement error event handling correctly   Generally API s give you a callback with an err as the first argument  For example  database query  something   function err  result      if  err  handle err     doSomething result         Is a common pattern   Another common pattern is on  error    For example  process on  uncaughtException   function  err      console log  Caught exception      err         Edit   var async function   function val  callback       process nextTick function            callback val                The above function when called as  async function 42  function val      console log val      console log 43     Will print 42 to the console asynchronously  In particular process nextTick fires after the current eventloop callstack is empty  That call stack is empty after async function and console log 43  have run  So we print 43 followed by 42   You should probably do some reading on the event loop

[javascript] How to write asynchronous functions for Node.js

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