HTML5 Canvas: How to border a fillRect?

I tried on my own and it looks like this:

var x = 100;
var y = 100;
var width = 50;
var height = 50;

var borderWidth = 5;   
var offset = borderWidth * 2;

c.beginPath();
c.fillStyle = 'black';
c.fillRect( x - borderWidth, y -borderWidth, width + offset, height + offset);
c.fillStyle = 'green';
c.fillRect( x, y, width, height);

you can not fill it later without a library. If you want to change something simply redraw. You can use something like that:

ctx.fillStyle = 'blue';
ctx.strokeStyle = 'red';
var fillRect = false;
ctx.rect(20, 20, 150, 100);
if (fillRect) {
  ctx.fill();
}
ctx.stroke();

it will draw only the border, if you change fillRect to true it will be filled. You can update your canvas on every requestAnimationFrame.

But maybe you want to use a library like paper.js. It makes things like clicking on objects much easier and it abstracts draws on canvas to objects you create once and update later, like what you asked for.

Work out the position you want to draw the square with the width and height. Once you have done that simply draw a bigger square first which has wider by 2 and higher by 2 but with the same center point. So you draw a square which is bigger and then you draw the normal square on top, this then gives you the illusion of the square has a border

HTML

<canvas id="canvas1" width="400" height="300" style="border:1px solid #000000;"></canvas>

CSS

#canvas1{
  border: solid 1px black;
}

Javascript

var c=document.getElementById("canvas1");
var ctx=c.getContext("2d");

var rectXPos = 50;
var rectYPos = 50;
var rectWidth = 100;
var rectHeight = 100;

drawBorder(rectXPos, rectYPos, rectWidth, rectHeight)

ctx.fillStyle='#FFF';
ctx.fillRect(rectXPos, rectYPos, rectWidth, rectHeight);

function drawBorder(xPos, yPos, width, height, thickness = 1)
{
  ctx.fillStyle='#000';
  ctx.fillRect(xPos - (thickness), yPos - (thickness), width + (thickness * 2), height + (thickness * 2));
}

jsfiddle link : https://jsfiddle.net/jxgw19sh/2/

-- Update --

Add an extra parameter to drawBorder called thickness the default value is 1 but you can provide any other number for thickness into the function and it will use value instead of 1.