Handling Nullable Values

Nullable unions represent values that may be absent. Under strictNullChecks, string | null must be checked before string-only operations.

What you will learn: the purpose of handling nullable values, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

Use explicit null checks, optional chaining, and nullish coalescing according to the intended behavior. The ?? operator applies a fallback only to null or undefined, preserving valid values such as 0, false, and the empty string.

Key distinction: TypeScript checks the declared types during development; emitted JavaScript carries out the value-level operation at runtime. A type declaration alone never validates untrusted runtime input.

Read this illustration from the first labeled part to the last. Compare it with the example and identify which parts are type checks and which parts run in JavaScript.

This self-contained example uses fixed values, so it runs without reading input or depending on an external service. Paste it into the TypeScript Playground or save it as main.ts.

TypeScript program

type Profile = { displayName?: string | null };
function nameOf(profile: Profile): string {
  return profile.displayName?.trim() ?? "Anonymous";
}
console.log(nameOf({ displayName: " Mira " }));
console.log(nameOf({ displayName: "" }));
console.log(nameOf({}));

Verified output

Mira

Anonymous

Run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. For projects with a tsconfig.json, build using that configuration instead.

Mira is trimmed to Mira. For an empty string, optional chaining still calls trim and returns an empty string, which ?? preserves. Missing displayName is undefined, so ?? returns Anonymous.

Check each step: identify the input values, follow the JavaScript operation in execution order, and match the final expression with the exact output shown. An annotation can reject invalid source before execution, but does not change the value printed.

Do not replace ?? with || unless empty strings and zero should trigger the fallback too.

How to debug: reproduce the issue in a minimal example. For compiler errors, inspect the inferred and declared types; for a runtime surprise, log the actual values and follow their references or branches.

Make a score formatter that uses 0 as a valid score but prints N/A when the score is nullish.

Practice routine: predict the result on paper, edit one part of the code, compile under --strict, run the JavaScript, and explain whether TypeScript caught an error or JavaScript simply behaved differently.

Handling Nullable Values: Nullable unions represent values that may be absent.

Mira is trimmed to Mira. For an empty string, optional chaining still calls trim and returns an empty string, which ?? preserves. Missing displayName is undefined, so ?? returns Anonymous.

Before continuing, explain one valid use case, one common pitfall, and the separation between compile-time checking and runtime behavior for this feature.