Check for file format using magic numbers in file

fileformatcheck.js

var blob = $(this).get(0).files[0]; 
    var fileReader = new FileReader();
    fileReader.readAsArrayBuffer(blob);
    fileReader.onloadend = function(e) {
      var arr = (new Uint8Array(e.target.result)).subarray(0, 4);
      var header = "";
      for(var i = 0; i < arr.length; i++) {
         header += arr[i].toString(16);
      }
      console.log(header);

      // Check the file signature against known types
      //https://en.wikipedia.org/wiki/List_of_file_signatures
    };

can we do the same thing without the onloadend event.

Thanks a lot for this.