DOM Scripting By Example,The result should be in pixels (for example, the function could return: "50 px").

Assuming you need help figuring out how to return the width of element

First we need to get the element that is using the specified id, we can do this via the document.getElementById function. This returns an element with the associated id (the first one it comes across because their should only be a unique id per document)

function getWidth(id) {
  let element = document.getElementById(id);
}

Next we need to get the width of that element, there are multiple ways to do this. We can get the offsetWidth if we want to return the actual calculated width of the element as a number. Or we can return the actual width entered in a style in the element's css by targeting it's style attribute, which will be returned as a string with the value and measurement.

In this case I think they want us to return the style as a value, so let's do that by targeting the style attribute in the element.

function getWidth(id) {
  let element = document.getElementById(id);
  //We can return any css attribute here but it has to be camel case. For example 
  //background-color would be returned as style.backgroundColor
  return element.style.width; 
}

Now we can optimize the function to have a single return statement instead of saving a space in memory for that element variable. This isn't necessary but it's good programming practices to get into, it'll work with the above code as well.

function getWidth(id) {
  return document.getElementById(id).style.width;
}