Generic array syntax in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the generic array syntax in TypeScript?

It is a way to define arrays with a specific type using angle brackets, like Array<Type>. For example, Array<number> means an array of numbers.

beginner

How do you declare an array of strings using generic array syntax?

You write <code>let names: Array&lt;string&gt; = ['Alice', 'Bob'];</code> which means <strong>names</strong> is an array that only holds strings.

beginner

What is the difference between string[] and Array<string> in TypeScript?

Both mean the same: an array of strings. string[] is shorthand, and Array<string> is the generic syntax. You can use either based on preference.

intermediate

Can you use generic array syntax with custom types?

Yes! For example, if you have a type Person, you can write Array<Person> to make an array of Person objects.

intermediate

Why use generic array syntax instead of the shorthand?

Generic syntax is clearer when working with complex types or nested generics. It helps readability and consistency in some cases.

Which of the following declares an array of numbers using generic array syntax?

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

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

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

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

Option D correctly uses Array<number> to declare an array of numbers.

What does Array<string> mean in TypeScript?

A An array of strings

B A function returning a string

C A string variable

D An array that can hold any type

Array<string> means an array that holds only strings.

Which is the shorthand for Array<boolean>?

A boolean[]

B bool[]

C Array<boolean[]>

D boolean<>

boolean[] is the shorthand for Array<boolean>.

Can you use generic array syntax with a custom type Car?

A No, only built-in types work

B Only if Car is a string

C Yes, like <code>Array&lt;Car&gt;</code>

D Only with interfaces, not types

Generic array syntax works with any type, including custom types like Car.

Why might you choose Array<Type> over Type[]?

A It is required by TypeScript

B It is clearer with complex or nested types

C It is shorter to write

D It runs faster

Generic syntax can be clearer and easier to read with complex or nested types.

Explain how to declare an array of numbers using generic array syntax in TypeScript.

Think about how you tell TypeScript what type the array holds using < and >.

Describe the difference and similarity between string[] and Array<string>.

Both are ways to say 'array of strings' but look different.

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