JSFiddle - React, Tailwind, and code Playground

HTML

<img...

JavaScript

function grayscale(src){
        var canvas = document.createElement('canvas');
        var ctx = canvas.getContext('2d');
        var imgObj = new Image();
        imgObj.src = src;
        canvas.width = imgObj.width;
        canvas.height = imgObj.height; 
        ctx.drawImage(imgObj, 0, 0); 
        var imgPixels = ctx.getImageData(0, 0, canvas.width, canvas.height);
        for(var y = 0; y < imgPixels.height; y++){
            for(var x = 0; x < imgPixels.width; x++){
                var i = (y * 4) * imgPixels.width + x * 4;
                var avg = (imgPixels.data[i] + imgPixels.data[i + 1] + imgPixels.data[i + 2]) / 3;
                imgPixels.data[i] = avg; 
                imgPixels.data[i + 1] = avg; 
                imgPixels.data[i + 2] = avg;
            }
        }
        ctx.putImageData(imgPixels, 0, 0, 0, 0, imgPixels.width, imgPixels.height);
        return canvas.toDataURL();
    }
function grayscale2(src){
        var canvas = document.createElement('canvas');
        var ctx = canvas.getContext('2d');
        var imgObj = new Image();
        imgObj.src = src;
        canvas.width = imgObj.width;
        canvas.height = imgObj.height; 
        ctx.drawImage(imgObj, 0, 0); 
        var imgPixels = ctx.getImageData(0, 0, canvas.width, canvas.height);
        for(var y = 0; y < imgPixels.height; y++){
            for(var x = 0; x < imgPixels.width; x++){
                var i = (y * 4) * imgPixels.width + x * 4;
                var data = imgPixels.data;
                var avg = ~~(data[i] * 0.3 + data[i + 1] * 0.59 + data[i + 2]* 0.11);
                imgPixels.data[i] = avg; 
                imgPixels.data[i + 1] = avg; 
                imgPixels.data[i + 2] = avg;
            }
        }
        ctx.putImageData(imgPixels, 0, 0, 0, 0, imgPixels.width, imgPixels.height);
        return canvas.toDataURL();
    }

$('test').set('src',grayscale($('test').get('src')));
$('test2').set('src',grayscale2($('test2').get('src')));