HTML5 + JAVASCRIPT asynchronous form validation reading the first four bytes of the file


I'm new to working with asynchronous tasks and I'm freaking out. When I finish what I have in hand I will be able to study more about this, but now I am short of time.

My question is the following: I need to validate a form from the client (I will also do it from the server, but I want double security). What I want to validate is when uploading a file, verify that this is an image . There are many methods out there that parse the extension or return the mime / type but it is wrong, since you can try it by changing the extension to an mp3 and jpg and it will tell you that the MIME is jpg.

I started by seeing an example that read 4 bits of the file and then depending on those 4 bytes you can know what type of file it is . What happens that the code that I started to do not know how to implement it.

What I want it to do is the following: from the form you select an image, and when you select one, it reads its initial 4 bytes, compares it and if it really is among those admitted, then a preview is shown … if not, then an error message is displayed saying invalid file.

What I need is that the checkFileType function returns a value to $('input[type=file]#imagencabecera').change(function() and so after validation if it is correct to show the preview on the screen.

This is the missing code to tweak:

    var file = (this.files[0].name).toString();
    var type = (this.files[0].type).toString();
    var reader = new FileReader();


    reader.onload = function (e){
        $('#filepreview img').attr('src',;


function checkFileType(file){
    if (window.FileReader && window.Blob)
    // All the File APIs are supported. Si soporta HTML5 FileReader y Blob
        var slice = file.slice(0,4);      // Get the first 4 bytes of a file
        var reader = new FileReader();    // Create instance of file reader. It is asynchronous!
        reader.readAsArrayBuffer(slice);  // Read the chunk file and return to blob

        reader.onload = function(e) {
            var buffer = reader.result;          // The result ArrayBuffer
            var view = new DataView(buffer);     // Get access to the result bytes
            var signature = view.getUint32(0, false).toString(16);  // Read 4 bytes, big-endian,return hex string

            switch(signature) // Every file has a unique signature, we can collect them and create a data lib
                case "89504e47": file.verified_type = "image/png"; break;
                case "47494638": file.verified_type = "image/gif"; break;
                case "FFd8FFe0": file.verified_type = "image/jpeg"; break;
                case "FFd8FFe1": file.verified_type = "image/jpeg"; break;
                case "FFd8FFe2": file.verified_type = "image/jpeg"; break;                  
                case "FFd8FFe3": file.verified_type = "image/jpeg"; break;
                case "FFd8FFe8": file.verified_type = "image/jpeg"; break;
                default: file.verified_type = 0;

    // File and Blob are not supported


Thank you so much for everything.


The simplest is to use promises . To do this, you must first define a promise which will execute the checkFileType function. Then, consume the promise so that when the execution of checkFileType , whatever you want is executed. This is how the JS asynchrony is solved.

var checkFileTypePromise = new Promise(function(resolve, reject) {
  // Ejecutamos la función y obtenemos el 'verified_type'
  verified_type = checkFileType(file);

  // Si queremos resolver (finalizar correctamente) la promesa,
  // utilizamos la función 'resolve(resultado)'.
  // Si queremos lanzar algún error, utilizamos 'reject(razón)'

  .then(function(result) {
    // La promesa ha finalizado correctamente.
    // En 'result' tenemos el valor que nos devuelve la función
  .catch(function(reason) {
    // La promesa ha finalizado con algún error.

This code can be inserted in $('input[type=file]#imagencabecera').change() .

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