Question

I am trying to read a file type that has a mixture of integers, floats and strings using Javascript.

The file is drag-dropped then, with the File API, read as array buffer and wrapped with a DataView. That takes care of the number types but I had to make my own method for getting text.

DataView.prototype.getAscii = function(byteOffset, byteLength)
{
    var bytes = new Array(byteLength);
    for (var i = 0; i < byteLength; i++) {
        bytes[i] = this.getUint8(byteOffset + i);
    }
    return String.fromCharCode.apply(null, bytes);
}

It works well enough but I worry about the speed of reading individual bytes for large files. Typed arrays can supposedly be used interchangeably with normal arrays so I tried this:

DataView.prototype.getAscii = function(byteOffset, byteLength)
{
    var bytes = new Uint8Array(this.buffer, byteOffset, byteLength);
    return String.fromCharCode.apply(null, bytes);
}

I get a “TypeError: Function.prototype.apply: Arguments list has wrong type” message so it doesn’t like my Uint8Array as a parameter.

Is there a better way of reading many characters at once? FileReader#readAsText() reads the whole file but doesn’t give access to any of the binary methods.

Answer 1

You could slice your file with .webkitSlice/.mozSlice and then use readAsText and readAsArrayBuffer for different ranges.