Shadow - Fabric.js

HTML

Click on the canvas, then select the rectangle
<canvas id="c" width="400" height="300"></canvas>

JavaScript

var canvas = new fabric.Canvas('c');
canvas.on('mouse:down', function(options) {
    
    var rect = new fabric.Rect({
        left: options.e.clientX,
        top: options.e.clientY,
        width: 60,
        height: 60,
        fill: 'rgba(255,0,0,0.5)',
    });
    canvas.add(rect);
});