Set p5 canvas to maximum width of parent div

How can I get the width of my parent div?

I need the canvas to bethe maximum width it can be. But the div should still be inside the boundaries set by my CSS-file.

In other words: How do i get the maximum width possible whilst still being inside the divs boundaries?

So:

function setup() {
    //somehow get the width of the div and set the canvas to that
    const myCanvas = createCanvas(400, 400);
    myCanvas.parent('canvasDiv');
}

function draw() {
    background(0);
}

HTML:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Default Page Title</title>
    <link rel="stylesheet" href="blogPost.css">
    <script type="text/javascript" src="../p5.js"></script>
    <script type="text/javascript" src="sketch.js"></script>
</head>
<body>


<div id="containerHeader">
    <h1>Default Page header</h1>
</div>
<div id="canvasDiv"></div>
<div id="container">
    <p>default description</p>
</div>
</body>
</html>

And lastly my CSS:

#canvasDiv {
    margin: 0;
    padding: 0;
}