readonly tuple

Learn immutable tuples with readonly tuple.

readonly tuple

To prevent a tuple's elements from being modified later, you put the readonly keyword in front of the type: readonly [string, number]. You can read elements of such a tuple but you cannot assign to them.

let t: readonly [string, number] = ["a", 1];
console.log(t[0]);   // "a"
t[0] = "b";          // compile error

A readonly tuple is used when data must stay unchanged after creation.

When to use a readonly tuple

A readonly tuple fits data that must not change later, such as configuration values, fixed coordinates or protected return data from a function. Like readonly arrays, accidental modification is blocked at compile time.

let ayar: readonly [string, boolean] = ["debug", true];
console.log(ayar[0]);  // "debug"

A tuple's length and order are already fixed; readonly also locks content changes.

Exercise Configuration with a readonly tuple

Define a readonly tuple named config with type readonly [string, number] and value ["port", 3000]. Print the second element with console.log.

Show example solution

let config: readonly [string, number] = ["port", 3000];
console.log(config[1]);

Test yourself

Which code produces a TypeScript compile error?
- let t: readonly [string] = ["x"]; console.log(t[0]);
- let t: [string, number] = ["a", 1]; console.log(t.length);
- let t: [number, number] = [1, 2]; t[0] = 5;
- let t: readonly [number, number] = [1, 2]; t[0] = 5;
Show answer

let t: readonly [number, number] = [1, 2]; t[0] = 5; — A <code>readonly</code> tuple does not allow element assignment, so <code>t[0] = 5</code> gives a compile error. The other options either lack <code>readonly</code> or only read.

Complete the readonly tuple type

let ayar: readonly [string, ____] = ["debug", true];

Show answer

let ayar: readonly [string, boolean] = ["debug", true];

readonly and mutation

readonly [string, number]

Immutable tuple type
t[0] = "x"

Element assignment (mutation)
readonly tuple

Elements cannot be reassigned later
readonly number[]

Immutable array type

Next lesson: Summary - Tuple

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies