global variable values set within fs.readfile are not reatained outside the function

Question

Array values logged at the end console.log(arr) are empty;

const fs = require("fs");
    const fss = require("fs");
    
      const data = fs.readFileSync("files_list.txt","utf-8");
      const lines = data.split(/\r?\n/);
    let arr = [];
    
    for (var i = 0; i < lines.length; i++) {
      const val = lines[i];
      const startLength = val.indexOf(';')+1;
      const endLength = lines[i].length;
    const hour = val.substring(startLength,endLength);
    const fileName = val.substring(0,endLength-2);
    fs.readFile("available.txt", function (err, dt){
      if (err) throw err;
      if(dt.includes(fileName)){
        var r = {
          filName:fileName,
          batchTiming:hour,
          status:'OK'
        };
        arr.push(r);
    }
    else
    {
        var r = {
        filName:fileName,
        batchTiming:hour,
        status:'OK'
      };
      arr.push(r);
    }
    
    });
    console.log(arr);
    }

jfriend00 · Answer 1 · 2021-03-22T06:51:49.807

You are attempting to log the contents of arr BEFORE anything has been put in it. fs.readFile() is non-blocking and asynchronous so your console.log(arr) runs before fs.readFile() calls its callback.

The ONLY place you can use the results of fs.readFile() is in the callback itself or in a function you call from within that callback and pass the result to that function.

P.S. This code is a little odd because you're reading the same file over and over again inside a for loop. That doesn't really make sense. You can read the content once before your loop and then use the content over and over again inside the for loop.

global variable values set within fs.readfile are not reatained outside the function

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