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.

Practice

1. What does a readonly array in TypeScript do?

easy

A. Only prevents changing the array length

B. Allows adding new elements but not removing existing ones

C. Makes the array elements optional

D. Prevents any changes to the array elements after creation

Solution

1. Step 1: Understand the readonly keyword

   The readonly keyword before an array type means the array cannot be changed after it is created.

2. Step 2: Effects on the array

   This means you cannot add, remove, or modify elements in the array once it is set.

3. Final Answer:

   Prevents any changes to the array elements after creation -> Option D

4. Quick Check:

   readonly arrays are immutable [OK]

Hint: Readonly arrays block all changes to elements [OK]

Common Mistakes:

• Thinking readonly only blocks length changes
• Assuming elements can be modified individually
• Confusing readonly with optional elements

2. Which of the following is the correct syntax to declare a read-only array of numbers in TypeScript?

easy

A. let nums: readonly number[] = [1, 2, 3];

B. let nums: number[] readonly = [1, 2, 3];

C. let nums: number readonly[] = [1, 2, 3];

D. let nums: number[] = readonly [1, 2, 3];

Solution

1. Step 1: Recall the correct readonly syntax

   In TypeScript, readonly comes before the array type, like readonly number[].

2. Step 2: Check each option

   let nums: readonly number[] = [1, 2, 3]; uses readonly number[], which is correct syntax. Others place readonly incorrectly.

3. Final Answer:

   let nums: readonly number[] = [1, 2, 3]; -> Option A

4. Quick Check:

   readonly before type [OK]

Hint: Put readonly before the array type, not after [OK]

Common Mistakes:

• Placing readonly after the array brackets
• Using readonly as a variable modifier
• Trying to put readonly inside the array literal

3. What will be the output of this TypeScript code?

const arr: readonly string[] = ['a', 'b', 'c'];
// arr.push('d');
console.log(arr.length);

medium

A. 4

B. undefined

C. 3

D. Error: Property 'push' does not exist on type 'readonly string[]'

Solution

1. Step 1: Understand the array declaration

   The array arr is declared as readonly string[] with 3 elements: 'a', 'b', 'c'.

2. Step 2: Check the commented out push and output

   The push line is commented out, so it does not run. The length remains 3.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   Readonly array length unchanged = 3 [OK]

Hint: Commented push means length stays same [OK]

Common Mistakes:

• Assuming push works on readonly arrays
• Expecting length to increase despite push commented
• Confusing runtime errors with compile-time errors

4. Identify the error in this TypeScript code using a read-only array:

const nums: readonly number[] = [1, 2, 3];
nums[0] = 10;

medium

A. Type 'number' is not assignable to type 'readonly number[]'

B. Cannot assign to '0' because it is a read-only property

C. Syntax error: missing semicolon

D. No error, code runs fine

Solution

1. Step 1: Understand the readonly array behavior

   The array nums is readonly, so its elements cannot be changed after creation.

2. Step 2: Analyze the assignment to an element

   Trying to assign nums[0] = 10 causes an error because elements are read-only.

3. Final Answer:

   Cannot assign to '0' because it is a read-only property -> Option B

4. Quick Check:

   Readonly elements cannot be changed [OK]

Hint: Readonly arrays block element assignment [OK]

Common Mistakes:

• Thinking only array structure is readonly, not elements
• Ignoring TypeScript error messages
• Assuming semicolon missing causes error

5. Given a function that accepts a read-only array, which of these operations inside the function is allowed?

function process(items: readonly number[]) {
  // Which line is valid?
  // A) items.push(5);
  // B) items[0] = 10;
  // C) const first = items[0];
  // D) items.length = 0;
}

hard

A. const first = items[0];

B. items.push(5);

C. items[0] = 10;

D. items.length = 0;

Solution

1. Step 1: Understand read-only array restrictions

   Read-only arrays do not allow modifying methods like push, element assignment, or length changes.

2. Step 2: Check each operation

   Only reading elements, like const first = items[0];, is allowed. Others cause errors.

3. Final Answer:

   const first = items[0]; -> Option A

4. Quick Check:

   Reading elements allowed, modifying disallowed [OK]

Hint: Readonly arrays allow reading, not modifying [OK]

Common Mistakes:

• Trying to push or assign elements in readonly arrays
• Attempting to change array length
• Confusing read-only with mutable arrays