Make image Black and White Javascript

I am making an app which will convert users uploaded image to black and white and then return a base64 string of image

Like:

<input type="file" onchange="handleFiles(this.file)" />

function handleFiles(f) { var o=[];

for(var i =0;i<f.length;i++)
{
    var reader = new FileReader();
    reader.onload = (function(theFile) {
    return function(e) {
        gCtx.clearRect(0, 0, gCanvas.width, gCanvas.height);
        //i want e.target.result to have base64 of black and white image not colorful image.
        //My processing with e.target.result image
    };
    })(f[i]);
    reader.readAsDataURL(f[i]); 
}

}

e.target.result contains colordful image base 64 string i want e.target.result to have black and white base 64 string