TypeScript errorTypeScript: “Object is possibly 'null'” / “'x' is possibly 'undefined'”

Where the null comes from, and the fix

1. 01

   DOM lookups and React refs

   “When strictNullChecks is true, null and undefined have their own distinct types and you’ll get a type error if you try to use them where a concrete value is expected.” TypeScript, TSConfig: strictNullChecks

   document.getElementById and querySelector return null when nothing matches, and useRef<HTMLInputElement>(null) starts as null. Check before use.

   const form = document.getElementById('signup');
   if (!form) return;
   form.addEventListener('submit', onSubmit);
   
   const inputRef = useRef<HTMLInputElement>(null);
   inputRef.current?.focus();

2. 02

   Array.find, Map.get and optional properties

   “TypeScript follows possible paths of execution that our programs can take to analyze the most specific possible type of a value at a given position.” TypeScript, Narrowing

   These return undefined when nothing is found. Decide what should happen then: throw, skip or default.

   const user = users.find((u) => u.id === id);
   if (!user) throw new Error(`No user ${id}`);
   sendEmail(user.email);
   
   const name = profile.nickname ?? profile.fullName;

3. 03

   The check is in another function

   TypeScript narrows within one function. A check done in a helper does not carry over unless the helper is a type guard. Return the checked value, or write the helper as a type predicate.

   function isDefined<T>(value: T | null | undefined): value is T {
     return value != null;
   }
   const ids = maybeIds.filter(isDefined); // string[]

4. 04

   The non-null assertion (!)

   el!.focus() tells TypeScript the value is there and removes the check. It changes nothing at runtime, so if you are wrong the app still crashes. Use it only for values that are guaranteed, such as an element your own markup always renders.

Checked against the vendor’s docs on November 8, 2026: TSConfig: strictNullChecks; TypeScript Handbook: Narrowing.

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Related: “Cannot read properties of undefined” · TypeScript “Argument of type is not assignable to parameter” · TypeScript “Property does not exist on type”