readonly arrays

Learn the immutability of readonly arrays.

readonly arrays

The readonly modifier marks an array as immutable. With readonly number[], you cannot add or remove elements via push, pop, etc.; the array is read-only. This prevents accidental data changes.

let sabit: readonly number[] = [1, 2, 3];
// sabit.push(4); // Error
console.log(sabit[0]);  // 1

readonly vs normal array

A normal number[] is mutable; a readonly number[] is not. TypeScript warns even when assigning a readonly array to a normal array, because a mutable copy can break data integrity. Readonly is especially useful in function parameters to protect data.

function goster(d: readonly number[]) {
    // d.push(5); // Error
    return d[0];
}

Exercise Days array

Define a readonly string[] array named gunler with the values "pzt", "sal", "çar". Then print the array with console.log.

Show example solution

let gunler: readonly string[] = ["pzt", "sal", "çar"];
console.log(gunler);

Test yourself

Which operation is valid on an array declared as `readonly string[]`?
- Changing the first element of the array
- Shrinking the array with pop
- Adding an element with push
- Reading the array (e.g. sabit[0])
Show answer

Reading the array (e.g. sabit[0]) — A readonly array can only be read; `push`, `pop` and element assignment give compile errors.

Define a readonly array

let puanlar: ____ = [90, 85, 70];

Show answer

let puanlar: readonly number[] = [90, 85, 70];

readonly behaviors

readonly number[]

Immutable number array
number[]

Mutable number array
sabit.push(4)

Errors on a readonly array
sabit[0]

Valid on a readonly array

Next lesson: Multidimensional arrays

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