String and Numeric Literal Types

String and numeric literal types constrain a value to a small set of exact strings or numbers.

What you will learn: the purpose of string and numeric literal types, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

Use literals for explicit modes, ratings, or protocol codes. A union such as 1 | 2 | 3 accepts exactly those numeric values, not an arbitrary number. Narrow literal types make incorrect call sites visible before execution.

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 Priority = 1 | 2 | 3;
type Channel = "email" | "sms";
function format(priority: Priority, channel: Channel): string {
  return `P${priority} via ${channel}`;
}
console.log(format(1, "email"));
console.log(format(3, "sms"));

Verified output

P1 via email
P3 via sms

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.

The first call combines numeric literal 1 with email. The second combines 3 with sms. Other numbers or strings fail a type check, but the generated code still handles plain JS values.

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 confuse numeric literals like 1 | 2 with a numeric range: the type does not automatically include numbers between arbitrary endpoints.

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.

Create a Speed type of 30 | 60 | 90 and print a label for each speed.

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.

String and Numeric Literal Types: String and numeric literal types constrain a value to a small set of exact strings or numbers.

The first call combines numeric literal 1 with email. The second combines 3 with sms. Other numbers or strings fail a type check, but the generated code still handles plain JS values.

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