Javascript read external text file, pass text file data as parameter

Please check this fiddle: https://jsfiddle.net/dp0y4hrw/16/

This is JS to find longest compound word in an array of strings.

Instead of an array, I want this program to read a local txt file, consisting of over 100,000 lines of text. Then find longest compound words. Each line has one word.

I've tried using FileReader to get the data. I was able to correctly pass the data, but gave me trouble with some shared variables between 'addPrefix' and 'findPrefixes'.

I also tried using a promise, to account for the asynchronous behavior:

function readFile(event) {  
  var file = event.target.files[0]; 

  if (file) {   
    new Promise(function(resolve, reject) {      
      var reader = new FileReader();      
      reader.onload = function (evt) {      
      resolve(evt.target.result);      
      };     

      reader.readAsText(file);     
      reader.onerror = reject;   
     })    

     .then(findLongestWord)   
     .catch(function(err) {     
       console.log(err)
     });  
   }
 }

document.getElementById('file').addEventListener('change', readFile, false);

function findLongestWord(data) {
...

This still gives me an issue. What would be the best way to read the file so I can process the contents correctly in this situation?

EDIT:

// adds word as a prefix
var addPrefix = function (word) {
  var i  = 0;
  var current = prefixes;
  var char;

  while (char = word[i++]) {
    if (!current[char]) {
      current[char] = {};
    }
      current = current[char];      
  }
  current.word = true;
  return current.word; //RETURNING CURRENT WORD HERE
};

// Finds the longest prefix we can make using the word.
var findPrefixes = function (word) {
  var prefix = '';
  var current = prefixes;
  var found  = [];
  var i  = 0;
  var char;

  while (char = word[i++]) {
    if (!current[char]) { 
      break; 
    }
    // Move to the next character and add to the prefix.
    current = current[char];
    prefix += char;

    if(current.word)
    {
      found.push(prefix);
    }
  }
  return found;
};

//for each word in list, add to prefix
list.forEach(function (word) {
  var prefix;

// If we can find a closest possible word, it may be possible to create a
// compound word - but we won't be able to check until we reach the end.
if ((prefix = findPrefixes(addPrefix())) && prefix.length) { //FINDPREFIXES USING ADDPREFIX HERE
  prefixMatch.push([ word, prefix ]);

}

// Insert the word into the prefix tree.
addPrefix(word);
});

EDIT 2: This is example of input text file:

cat
cats
catsdogcats
dog
dogcatsdog
hippopotamuses
rat
ratcatdogcat
catratdograt
dogcatscats

Expected result is: longest: ratcatdogcat,catratdograt...2nd longest: catsdogcats,dogcatscats...number of compound words: 5