Read-only arrays in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a read-only array of numbers.

Typescript

const numbers: [1] = [1, 2, 3];

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Areadonly number[]

BArray<number>

Cnumber[]

Dconst number[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using just number[] allows modification.

Trying to use const inside the type is incorrect.

Using readonly number[] declares an array that cannot be changed after creation.

2fill in blank

medium

Complete the code to prevent modification of the array elements.

Typescript

function printFirst(numbers: [1]) {
  console.log(numbers[0]);
}

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AArray<number>

Breadonly number[]

Cnumber[]

Dconst number[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using number[] allows modification inside the function.

Using const in the type is invalid.

Using readonly number[] ensures the function cannot modify the array.

3fill in blank

hard

Fix the error by completing the code to assign a read-only array.

Typescript

const arr: readonly string[] = ['a', 'b'];
// Error: Cannot assign to 'arr'
arr = [1];

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Aarr

B['a', 'b']

C['c', 'd']

Dnew Array('c', 'd')

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to assign a new array literal to a read-only variable.

Using new Array() to assign a new array.

You cannot assign a new array to a read-only variable. Assigning arr to itself avoids the error.

4fill in blank

hard

Fill both blanks to create a read-only array and access its first element.

Typescript

const fruits: [1] = ['apple', 'banana'];
const firstFruit: string = fruits[2];

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Areadonly string[]

Bnumber

C[0]

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using number as the type for the array.

Trying to access the first element with .length.

The array is declared as readonly string[] and the first element is accessed with [0].

5fill in blank

hard

Fill all three blanks to declare a read-only array, map its elements, and prevent modification.

Typescript

const letters: [1] = ['x', 'y', 'z'];
const upperLetters = letters.map(letter => letter.[2]());
// Error if uncommented: letters.[3]('a');

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Areadonly string[]

BtoUpperCase

Cpush

Dstring[]

Attempts:

3 left

💡 Hint

Common Mistakes

Declaring the array as mutable string[].

Using a wrong method name instead of toUpperCase.

Trying to use push on a read-only array.

The array is read-only (readonly string[]), mapping uses toUpperCase(), and push is not allowed on read-only arrays.