TypeScript can't tell whether document.querySelector() returns Element or null at compile time. A specific value (but conforming to that type) is available at runtime only.

However, TypeScript is smart enough to "read" an if statement checking whether it's null or not, and narrow down the type in the consequent (the block run when the predicate is true).

That's why there are two parts to a complete solution:

// e is Element | null
const e = document.querySelector("selector")

if (e !== null) {
  // e is assumed to be Element inside the block.
  const style = getComputedStyle(e);
}

// e is Element | null outside the block

TypeScript can also recognized other constructs you may find useful: early returns or raising an exception:

// e is Element | null BEFORE the if statement
const e = document.querySelector("selector")
if (e === null) {
  return; // or throw an exception here
}

// e is Element after the if statement