Read-only arrays in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a read-only array in TypeScript?

A read-only array is an array that cannot be changed after creation. You cannot add, remove, or modify its elements.

beginner

How do you declare a read-only array in TypeScript?

Use the readonly keyword before the array type, like readonly number[] or use ReadonlyArray<number>.

beginner

Can you push or pop elements from a read-only array?

No, methods that change the array like push or pop are not allowed on read-only arrays.

beginner

Why use read-only arrays?

They help prevent accidental changes to data, making your code safer and easier to understand.

intermediate

How do you convert a normal array to a read-only array?

You can assign a normal array to a variable typed as a readonly array, or use as const to make it read-only.

Which keyword is used to declare a read-only array in TypeScript?

A immutable

B const

C static

D readonly

The readonly keyword marks an array as read-only.

What happens if you try to use push on a read-only array?

A It throws a compile-time error

B It adds the element successfully

C It removes the last element

D It converts the array to a normal array

Read-only arrays do not allow modification methods like push, causing a compile-time error.

Which of these types is equivalent to readonly number[]?

A Array<number>

B ReadonlyArray<number>

C number[]

D const number[]

ReadonlyArray<number> is the generic form of a read-only array.

Can you reassign an element in a read-only array?

A Yes, anytime

B Only if the array is declared with <code>const</code>

C No, elements cannot be changed

D Only if you use <code>as any</code>

Read-only arrays prevent changing elements after creation.

What does as const do when used on an array?

A Makes the array read-only and its elements literal types

B Makes the array mutable

C Deletes the array

D Converts the array to a string

as const makes the array and its elements read-only with literal types.

Explain what a read-only array is and why it is useful in TypeScript.

Think about how you keep some lists fixed so no one changes them by mistake.

Describe how to declare and use a read-only array in TypeScript with an example.

Show how to write the code and what happens if you try to change it.