Reading client side text file using Javascript

Question

I want to read a file (on the client side) and get the content in an array. It will be just one file. I have the following and it doesn't work. 'query_list' is a textarea where I want to display the content of the file.

<input type="file" id="file" name="file" enctype="multipart/form-data"/>
        
<script>
    document.getElementById('file').addEventListener('change', readFile, false);
    
    function readFile (evt) {
        var files = evt.target.files;
        var file = files[0];
                                                                                                           
        var fh = fopen(file, 0);
        var str = "";
        document.getElementById('query_list').textContent = str;
        if(fh!=-1) {
            length = flength(fh);        
            str = fread(fh, length);     
            fclose(fh);                   
        } 
        document.getElementById('query_list').textContent = str;
    }
</script>

How should I go about it? Eventually I want to loop over the array and run some SQL queries.

Answer 1

You can read files on the client using HTML5's FileReader. The following example reads a text file on the client.

Your example attempts to use fopen which is not available in Javascript.

http://jsfiddle.net/k3j48zmt/

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

function readFile(evt) {
  var files = evt.target.files;
  var file = files[0];
  var reader = new FileReader();
  reader.onload = function(event) {
    console.log(event.target.result);
  }
  reader.readAsText(file)
}

<input id=file type=file />

Answer 2

There is such thing as HTML5 FileReader API to access local files picked by user, without uploading them anywhere. It is quite new feature, but supported by most of modern browsers. I strongly recommend to check out this great article to see, how you can use it.

There is one problem with this, you can't read big files (~400 MB and larger) because straight forward FileAPI functions attempting to load entire file into memory.

If you need to read big files, or search something there, or navigate by line index check my LineNavigator, which allows you to read, navigate and search in files of any size. Try it in jsFiddle! It is super easy to use:

var navigator = new FileNavigator(file);    

navigator.readSomeLines(0, function linesReadHandler(err, index, lines, eof, progress) {    
    // Some error
    if (err) return;
    
    // Process this line bucket
    for (var i = 0; i < lines.length; i++) {
        var line = lines[i]; 
        // Do something with it
    }
    
    // End of file
    if (eof) return;
    
    // Continue reading
    navigator.readSomeLines(index + lines.length, linesReadHandler);
});

Answer 3

I work there, but still wanted to contribute because it works well: You can use the filepicker.io read api to do exactly this. You can pass in an dom element and get the contents back, for text or binary data, even in IE8+

Answer 4

Well I got beat to the answer but its different:

<input type="file" id="fi" />
<button onclick="handleFile(); return false;">click</button>

function handleFile() {
    var preview = document.getElementById('prv')
    var file = document.getElementById('fi').files[0];
    var div = document.body.appendChild(document.createElement("div"));
    div.innerHTML = file.getAsText("utf-8");
}

This will work in FF 3.5 - 3.6, and that's it. FF 4 and WebKit you need to use the FileReader as mentioned by Juan Mendes.

For IE you may find a Flash solution.