js.ts

// default tagName is 'div'

// The problem

function createElement(tagName: string): Element {
  // very broken
  return document.createElement(tagName || 'div');
}

// you might want to use defaults

function createElement(tagName: string = "div"): Element {
  return document.createElement(tagName);
}

createElement(); // <div>
createElement(null); // <null>
  
// but maybe you are just going to keep doing it inline
  
// let's look at the semantics of ?? where it only coalesces undefined
  
function createElement(tagName: string): Element {
  return document.createElement(tagName ?? "div");
}
  
// in this case, it has the same behavior as the previous example
  
// however, maybe you are TRYING to coalesce away nulls

function createElement(tagName: string): Element {
  return document.createElement(tagName == undefined ? 'div' : tagName);
}
  
// what happens if we make ?? coalesce null AND undefined
  
function createElement(tagName: string): Element {
  return document.createElement(tagName ?? "div");
}
  
// however, what if you are trying to match the behavior of the built-in default syntax
  
function createElement(tagName: string): Element {
  return document.createElement(tagName === undefined ? 'div' : tagName);
}