Readonly Tuples

A readonly tuple disallows assigning to its positions through that typed reference while preserving the known position types.

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

Readonly tuples remain arrays at runtime. The as const assertion frequently infers readonly tuple types from literal expressions and preserves literal element types, but it is not a deep runtime freeze.

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

const dimensions = [1920, 1080] as const;
const [width, height] = dimensions;
console.log(width * height);
console.log(dimensions.length);

Verified output

2073600
2

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.

as const gives dimensions readonly positions with literal types 1920 and 1080. Multiplying gives 2,073,600; length is 2.

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.

dimensions[0] = 1280 is a type error. Do not claim as const makes a runtime immutable array; use runtime controls when needed.

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 readonly RGB tuple with three numbers. Try assigning to an index, then create a mutable copy with spread.

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.

Readonly Tuples: A readonly tuple disallows assigning to its positions through that typed reference while preserving the known position types.

as const gives dimensions readonly positions with literal types 1920 and 1080. Multiplying gives 2,073,600; length is 2.

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