How to capture no file for fs readFileSync

Question

Within node js readFile   shows how to capture an error  however there is no comment for the readFileSync   function regarding error handling  As such  if I try to use readFileSync   when there is no file  I get the error Error  ENOENT  no such file or directory   How do I capture the exception being thrown  The doco doesn t state what exceptions are thrown  so I don t know what exceptions I need to catch  I should note that I don t like generic  catch every single possible exception  style of try catch statements  In this case I wish to catch the specific exception that occurs when the file doesn t exist and I attempt to perform the readFileSync   Please note that I m performing sync functions only on start up before serving connection attempts  so comments that I shouldn t be using sync functions are not required  -

User · Answer

The JavaScript try   catch mechanism cannot be used to intercept errors generated by asynchronous APIs  A common mistake for beginners is to try to use throw inside an error-first callback     THIS WILL NOT WORK  const fs   require  fs     try     fs readFile   some file that does-not-exist    err  data    gt           Mistaken assumption  throwing here        if  err          throw err                catch  err         This will not catch the throw    console error err      This will not work because the callback function passed to fs readFile   is called asynchronously  By the time the callback has been called  the surrounding code  including the try   catch block  will have already exited  Throwing an error inside the callback can crash the Node js process in most cases  If domains are enabled  or a handler has been registered with process on  uncaughtException    such errors can be intercepted  reference  https   nodejs org api errors html

User · Answer

You have to catch the error and then check what type of error it is   try     var data   fs readFileSync        catch  err         If the type is not what you want  then just throw the error again    if  err code      ENOENT   throw err        Handle a file-not-found error

User · Answer

I prefer this way of handling this  You can check if the file exists synchronously   var file    info json   var content           Check that the file exists locally if  fs existsSync file       console log  File not found          The file  does  exist else        Read the file and do anything you want   content   fs readFileSync file   utf-8           Note  if your program also deletes files  this has a race condition as noted in the comments  If however you only write or overwrite files  without deleting them  then this is totally fine

User · Answer

Basically  fs readFileSync throws an error when a file is not found  This error is from the Error prototype and thrown using throw  hence the only way to catch is with a try   catch block   var fileContents  try     fileContents   fs readFileSync  foo bar      catch  err         Here you get the error when the file was not found       but you also get any other error     Unfortunately you can not detect which error has been thrown just by looking at its prototype chain   if  err instanceof Error    is the best you can do  and this will be true for most  if not all  errors  Hence I d suggest you go with the code property and check its value   if  err code      ENOENT       console log  File not found       else     throw err      This way  you deal only with this specific error and re-throw all other errors   Alternatively  you can also access the error s message property to verify the detailed error message  which in this case is   ENOENT  no such file or directory  foo bar    Hope this helps

User · Answer

Try using Async instead to avoid blocking the only thread you have with NodeJS  Check this example   const util   require  util    const fs   require  fs    const path   require  path    const readFileAsync   util promisify fs readFile    const readContentFile   async  filePath    gt         Eureka  you are using good code practices here    const content   await readFileAsync path join   dirname  filePath         encoding   utf8         return content      Later can use this async function with try catch from any other function   const anyOtherFun   async      gt      try       const fileContent   await readContentFile  my-file txt        catch  err           Here you get the error when the file was not found         but you also get any other error         Happy Coding

User · Answer

I use an immediately invoked lambda for these scenarios    const config         gt      try       return JSON parse fs readFileSync  config json         catch  error        return                 async version    const config   await  async      gt      try       return JSON parse await fs readFileAsync  config json         catch  error        return

[node.js] How to capture no file for fs.readFileSync()?

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