Make sure to have a file input on your page:

<input type="file" id="upload">

Then in JavaScript:

document.getElementById('upload').addEventListener('change', readFileAsString)
function readFileAsString() {
    var files = this.files;
    if (files.length === 0) {
        console.log('No file is selected');
        return;
    }

    var reader = new FileReader();
    reader.onload = function(event) {
        console.log('File content:', event.target.result);
    };
    reader.readAsText(files[0]);
}

Reading the contents of a file within a web application can be accomplished by utilizing the HTML5 File API. First, add an input with type="file" in your HTML:

<input type="file" id="upload">

Next, we're going to add a change listener on the file-input. This examples defines the listener via JavaScript, but it could also be added as attribute on the input element. This listener gets triggered every time a new file has been selected. Within this callback, we can read the file that was selected and perform further actions (like creating an image with the contents of the selected file):

document.getElementById('upload').addEventListener('change', showImage);

function showImage(evt) {
    var files = evt.target.files;

    if (files.length === 0) {
        console.log('No files selected');
        return;
    }

    var reader = new FileReader();
    reader.onload = function(event) {
        var img = new Image();
        img.onload = function() {
            document.body.appendChild(img);
        };
        img.src = event.target.result;
    };
    reader.readAsDataURL(files[0]);
}

The blob.slice() method is used to create a new Blob object containing the data in the specified range of bytes of the source Blob. This method is usable with File instances too, since File extends Blob.

Here we slice a file in a specific amount of blobs. This is useful especially in cases where you need to process files that are too large to read in memory all in once. We can then read the chunks one by one using FileReader.

/**
* @param {File|Blob} - file to slice
* @param {Number} - chunksAmount
* @return {Array} - an array of Blobs
**/
function sliceFile(file, chunksAmount) {
  var byteIndex = 0;
  var chunks = [];
    
  for (var i = 0; i < chunksAmount; i += 1) {
    var byteEnd = Math.ceil((file.size / chunksAmount) * (i + 1));
    chunks.push(file.slice(byteIndex, byteEnd));
    byteIndex += (byteEnd - byteIndex);
  }

  return chunks;
}

function downloadCsv() {
  var blob = new Blob([csvString]);
  if (window.navigator.msSaveOrOpenBlob){
    window.navigator.msSaveBlob(blob, "filename.csv");
  }
  else {
    var a = window.document.createElement("a");

    a.href = window.URL.createObjectURL(blob, {
      type: "text/plain"
    });
    a.download = "filename.csv";
    document.body.appendChild(a);
    a.click();
    document.body.removeChild(a);
  }
}
var string = "a1,a2,a3";
downloadCSV(string);

Source reference ; https://github.com/mholt/PapaParse/issues/175

The HTML5 file API allows you to restrict which kind of files are accepted by simply setting the accept attribute on a file input, e.g.:

<input type="file" accept="image/jpeg">

Specifying multiple MIME types separated by a comma (e.g. image/jpeg,image/png) or using wildcards (e.g. image/* for allowing all types of images) give you a quick and powerful way to restrict the type of files you want to select. Here's an example for allowing any image or video:

<input type="file" accept="image/*,video*">

---

By default, the file input lets the user select a single file. If you want to enable multiple file selection, simply add the multiple attribute:

<input type="file" multiple>

You can then read all the selected files via the file input's files array. See read file as dataUrl

If you want to get the properties of the file (like the name or the size) you can do it before using the File Reader. If we have the following html piece of code:

<input type="file" id="newFile">

You can access the properties directly like this:

document.getElementById('newFile').addEventListener('change', getFile);

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

    console.log('Name of the file', file.name);
    console.log('Size of the file', file.size);
}

You can also get easily the following attributes: lastModified (Timestamp), lastModifiedDate (Date), and type (File Type)