what is wrong with my code?

Question

Bellow is my code:

var ctx = canvas.getContext("2d");
var ballRadius = 10;
var x = canvas.width/2;
var y = canvas.height-30;
var dx = 2;

JS says cant read getContext is there something wrong?

Answer 1

That happens most likely because the DOM has not fully loaded when the script is executed, so the canvas variable is still undefined.
Try wrapping the whole initialization process in
window.onload = function() {
...
}