Creating a file only if it doesn t exist in Node js

Question

We have a buffer we d like to write to a file  If the file already exists  we need to increment an index on it  and try again  Is there a way to create a file only if it doesn t exist  or should I just stat files until I get an error to find one that doesn t exist already   For example  I have files a 1 jpg and a 2 jpg  I d like my method to try creating a 1 jpg and a 2 jpg  and fail  and finally successfully create a 3 jpg   The ideal method would look something like this   fs writeFile path  data    overwrite  false    function  err      if  err  throw err    console log  It  s saved           or like this   fs createWriteStream path    overwrite  false       Does anything like this exist in node s fs library   EDIT  My question isn t if there s a separate function that checks for existence  It s this  is there a way to create a file if it doesn t exist  in a single file system call

User · Answer

This method is no longer recommended  fs exists is deprecated  See comments    Here are some options   1  Have 2  fs  calls  The first one is the  fs exists  call  and the second is  fs write   read  etc     checks if the file exists     If it does  it just calls back    If it doesn t  then the file is created  function checkForFile fileName callback        fs exists fileName  function  exists            if exists                        callback             else                       fs writeFile fileName   flag   wx    function  err  data                                  callback                                        function writeToFile         checkForFile  file dat  function                  It is now safe to write read to file dat        fs readFile  file dat   function  err data                        do stuff                        2  Or Create an empty file first   --- Sync     If you want to force the file to be empty then you want to use the  w  flag   var fd   fs openSync filepath   w       That will truncate the file if it exists and create it if it doesn t     Wrap it in an fs closeSync call if you don t need the file descriptor it returns   fs closeSync fs openSync filepath   w       --- ASync   var fs   require  fs    fs open path   wx   function  err  fd           handle error     fs close fd  function  err               handle error               3  Or use  touch   https   github com isaacs node-touch

User · Answer

You can do something like this   function writeFile i       var i   i    0      var fileName    a     i     jpg       fs exists fileName  function  exists            if exists               writeFile   i             else               fs writeFile fileName

User · Answer

Todo this in a single system call you can use the fs-extra npm module  After this the file will have been created as well as the directory it is to be placed in   const fs   require  fs-extra    const file     tmp this path does not exist file txt  fs ensureFile file  err   gt        console log err       gt  null       Another way is to use ensureFileSync which will do the same thing but synchronous   const fs   require  fs-extra    const file     tmp this path does not exist file txt  fs ensureFileSync file

User · Answer

With async   await and Typescript I would do  import   as fs from  fs   async function upsertFile name  string      try          try to read file     await fs promises readFile name      catch  error           create empty file  because it wasn t found     await fs promises writeFile name

User · Answer

As your intuition correctly guessed  the naive solution with a pair of exists   writeFile calls is wrong  Asynchronous code runs in unpredictable ways  And in given case it is   Is there a file a txt      No   File a txt gets created by another program  Write to a txt if it s possible      Okay    But yes  we can do that in a single call  We re working with file system so it s a good idea to read developer manual on fs  And hey  here s an interesting part       w  - Open file for writing  The file is created  if it does not   exist  or truncated  if it exists         wx  - Like  w  but fails if path exists    So all we have to do is just add wx to the fs open call  But hey  we don t like fopen-like IO  Let s read on fs writeFile a bit more      fs readFile filename   options   callback        filename String      options Object           encoding String   Null default   null          flag String default    r          callback Function   That options flag looks promising  So we try  fs writeFile path  data    flag   wx     function  err        if  err  throw err      console log  It s saved           And it works perfectly for a single write  I guess this code will fail in some more bizarre ways yet if you try to solve your task with it  You have an atomary  check for a   jpg existence  and write there if it s empty  operation  but all the other fs state is not locked  and a 1 jpg file may spontaneously disappear while you re already checking a 5 jpg  Most  file systems are no ACID databases  and the fact that you re able to do at least some atomic operations is miraculous  It s very likely that wx code won t work on some platform  So for the sake of your sanity  use database  finally   Some more info for the suffering  Imagine we re writing something like memoize-fs that caches results of function calls to the file system to save us some network cpu time  Could we open the file for reading if it exists  and for writing if it doesn t  all in the single call  Let s take a funny look on those flags  After a while of mental exercises we can see that a  does what we want  if the file doesn t exist  it creates one and opens it both for reading and writing  and if the file exists it does so without clearing the file  as w  would   But now we cannot use it neither in  smth File  nor in create Smth Stream functions  And that seems like a missing feature   So feel free to file it as a feature request  or even a bug  to Node js github  as lack of atomic asynchronous file system API is a drawback of Node  Though don t expect changes any time soon   Edit  I would like to link to articles by Linus and by Dan Luu on why exactly you don t want to do anything smart with your fs calls  because the claim was left mostly not based on anything

User · Answer

What about using the a option   According to the docs       a   - Open file for reading and appending  The file is created if it does not exist    It seems to work perfectly with createWriteStream

[node.js] Creating a file only if it doesn't exist in Node.js

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