Why is that my JavaScript code doesnot run as expected?

Question

This happens especially when I try manipulating html elements from JavaScript.
Below is some example of code like that. Please help me find the problem.

https://code.sololearn.com/WcpzYPFy62qs/?ref=app

Answer 1

It's because you declared the variable for the canvas before the page loaded, so it became undefined.
You should do it like this:

window.onload = () => {
//your code here
}

Answer 2

Thank you Airree.
It worked like a charm