JSFiddle - React, Tailwind, and code Playground

HTML

<canvas id='c'>

</canvas>

CSS

canvas {
  background-color: white;
  height: 100%;
  width: 100%;
}

JavaScript

var Circle = function() {
  this.start = [];
  this.end = [];
}
Circle.prototype.draw = function(canvas) {
  var me = this;

  if (canvas.getContext) {
    var ctx = canvas.getContext('2d');
    var mid = me.getcenter();
    var rad = me.getradius();
    console.log(mid, rad);
    ctx.beginPath();
    console.log(mid['x'], mid['y']);
    ctx.arc(mid['x'], mid['y'], rad, 0, 360);
    ctx.stroke();
  }

};

Circle.prototype.getcenter = function() {
  var me = this;
  //Check the start and end are set
  var centerX = (me.start['x'] + me.end['x']) / 2;
  var centerY = (me.start['y'] + me.end['y']) / 2;

  return {
    'x': centerX,
    'y': centerY
  };
};

Circle.prototype.getradius = function() {
  var me = this;
  var distX = Math.abs(me.start['x'] - me.end['x']);
  var distY = Math.abs(me.start['y'] - me.end['y']);

  return distX / 2;
};

var circle;
var canvas = $('#c');
canvas.get(0).width = canvas.width();
canvas.get(0).height = canvas.height();

$('#c').mousedown(function(event) {
  var parentOffset = $(this).offset();
  circle = new Circle();
  circle.start['x'] = event.pageX - parentOffset.left;
  circle.start['y'] = event.pageY - parentOffset.top;

});

$('#c').mouseup(function(event) {
  var parentOffset = $(this).offset();
  circle.end['x'] = event.pageX - parentOffset.left;
  circle.end['y'] = event.pageY - parentOffset.top;
  circle.draw(canvas[0]);

});