Answer 1

I think the error is because you are using width as function name. Try using something else like width1 or width2. It's working perfectly.

Answer 2

The problem is:
element.width.value = value;

Instead you need:
element.style.width = value;

function widthChange()
{
var widthValue = slide.value;
slideValue.innerHTML = widthValue;

document.getElementById("rectShape").style.width = slide.value;

}

onload = ()=> {
widthChange();
}