Generic array syntax in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an array of numbers using generic syntax.

Typescript

let numbers: Array<[1]> = [1, 2, 3];

Drag options to blanks, or click blank then click option'

Astring

Bboolean

Cany

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' or 'boolean' instead of 'number' for the array type.

The generic array syntax uses Array<Type>. Here, number specifies the array holds numbers.

2fill in blank

medium

Complete the code to declare an array of strings using generic syntax.

Typescript

const fruits: Array<[1]> = ['apple', 'banana', 'cherry'];

Drag options to blanks, or click blank then click option'

Astring

Bobject

Cboolean

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'number' or 'object' instead of 'string' for the array type.

The array holds strings, so the generic type should be string.

3fill in blank

hard

Fix the error in the generic array declaration.

Typescript

let flags: Array[1] = [true, false, true];

Drag options to blanks, or click blank then click option'

A[boolean]

B<boolean>

C(boolean)

D{boolean}

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets or parentheses instead of angle brackets.

The correct generic syntax uses angle brackets: Array<boolean>.

4fill in blank

hard

Fill both blanks to declare an array of objects with a generic type.

Typescript

const users: Array<[1]> = [[2]];

Drag options to blanks, or click blank then click option'

A{ name: string; age: number }

B{ name: 'Alice', age: 30 }

C{ name: 'Bob', age: 25 }

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string type instead of an object type for the generic.

Putting the object type inside the array instead of an object instance.

The generic type defines the shape of objects in the array. The array contains objects matching that shape.

5fill in blank

hard

Fill all three blanks to declare and initialize a generic array of booleans.

Typescript

let flags: Array<[1]> = [[2], [3]];

Drag options to blanks, or click blank then click option'

Aboolean

Btrue

Cfalse

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' as the generic type.

Using non-boolean values inside the array.

The generic type is boolean. The array contains boolean values true and false.

Practice

1. Which of the following is a correct way to declare an array of numbers using generic array syntax in TypeScript?

easy

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

B. let nums: Array<number> = [1, 2, 3];

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

D. let nums: number[] = 123;

Solution

1. Step 1: Understand generic array syntax

   In TypeScript, Array<Type> declares an array holding elements of Type.

2. Step 2: Check each option for correct syntax and type

   let nums: Array<number> = [1, 2, 3]; uses Array<number> with a number array, which is correct. Others have wrong types or assignments.

3. Final Answer:

   let nums: Array<number> = [1, 2, 3]; -> Option B

4. Quick Check:

   Generic array syntax = Array<Type> [OK]

Hint: Use Array<Type> or Type[] for arrays [OK]

Common Mistakes:

• Forgetting to specify the type inside Array<>
• Assigning a non-array value to an array type
• Using Array without type parameter

2. Which of the following is the correct syntax to declare an array of strings using the shorthand syntax in TypeScript?

easy

A. let names: string[] = ['Alice', 'Bob'];

B. let names: Array = ['Alice', 'Bob'];

C. let names: string = ['Alice', 'Bob'];

D. let names: [string] = ['Alice', 'Bob'];

Solution

1. Step 1: Recall shorthand array syntax

   In TypeScript, Type[] declares an array of Type.

2. Step 2: Validate each option

   let names: string[] = ['Alice', 'Bob']; correctly uses string[] for an array of strings. Others are incorrect or incomplete.

3. Final Answer:

   let names: string[] = ['Alice', 'Bob']; -> Option A

4. Quick Check:

   Shorthand array syntax = Type[] [OK]

Hint: Use Type[] for quick array declarations [OK]

Common Mistakes:

• Omitting the type inside Array<>
• Using tuple syntax instead of array
• Assigning array to a non-array type

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

function firstElement<T>(arr: Array<T>): T | undefined {
  return arr[0];
}

const nums: number[] = [10, 20, 30];
console.log(firstElement(nums));

medium

A. Error

B. undefined

C. 10

D. 30

Solution

1. Step 1: Understand the generic function behavior

   The function firstElement returns the first element of the array or undefined if empty.

2. Step 2: Apply function to given array

   The array nums has elements [10, 20, 30], so arr[0] is 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   firstElement([10,20,30]) = 10 [OK]

Hint: Generic function returns first array item or undefined [OK]

Common Mistakes:

• Assuming function returns last element
• Confusing undefined with zero
• Thinking generic causes error

4. Identify the error in the following TypeScript code snippet:

let fruits: Array<string> = ['apple', 'banana', 5];

medium

A. The array contains a number instead of a string

B. Array<string> syntax is invalid

C. Missing semicolon at the end

D. Array should be declared as string[]

Solution

1. Step 1: Check array element types

   The array is declared to hold strings only.

2. Step 2: Identify type mismatch

   The element 5 is a number, which violates the string type constraint.

3. Final Answer:

   The array contains a number instead of a string -> Option A

4. Quick Check:

   Array<string> must hold only strings [OK]

Hint: Array type must match all element types [OK]

Common Mistakes:

• Mixing types inside a typed array
• Ignoring type errors in arrays
• Confusing syntax errors with type errors

5. You want to create a function in TypeScript that accepts an array of any type and returns a new array with the same elements repeated twice. Which is the correct generic array syntax for the function parameter and return type?

hard

A. function repeatArray<T>(arr: Array): Array<T>

B. function repeatArray(arr: T[]): T[]

C. function repeatArray<T>(arr: T): T[]

D. function repeatArray<T>(arr: Array<T>): Array<T>

Solution

1. Step 1: Define generic parameter and array type

   The function uses a generic type T and takes an array of T elements.

2. Step 2: Ensure return type matches input array type

   The function returns an array of T, so Array<T> is correct for both parameter and return type.

3. Final Answer:

   function repeatArray<T>(arr: Array<T>): Array<T> -> Option D

4. Quick Check:

   Generic function array syntax = Array<T> [OK]

Hint: Use <T> and Array<T> for generic array functions [OK]

Common Mistakes:

• Omitting generic type parameter <T>
• Using non-generic Array without type
• Incorrect parameter or return types