Generic array syntax in Typescript

Introduction

Generic array syntax helps you create arrays that can hold any type of data safely. It makes your code flexible and type-safe.

When you want an array that can store numbers, strings, or any type without losing type information.

When writing functions that work with arrays of different types and you want to keep type safety.

When creating reusable components or data structures that handle arrays of various types.

When you want to avoid errors by ensuring all items in an array are of the same type.

Syntax

Typescript

let arrayName: Array<Type> = [value1, value2, ...];

// or using shorthand
let arrayName: Type[] = [value1, value2, ...];

Array<Type> and Type[] mean the same thing in TypeScript.

Use the generic form Array<Type> when you want to be explicit, and Type[] for shorter syntax.

Examples

An array of numbers using generic syntax.

Typescript

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

An array of strings using shorthand syntax.

Typescript

let strings: string[] = ['apple', 'banana', 'cherry'];

An empty array that will hold boolean values later.

Typescript

let emptyArray: Array<boolean> = [];

An array that can hold numbers or strings using a union type.

Typescript

let mixed: Array<number | string> = [1, 'two', 3];

Sample Program

This program defines a generic function that prints all items in an array. It works with arrays of numbers and strings, showing how generic array syntax keeps type safety and flexibility.

Typescript

function printArrayItems<T>(items: Array<T>): void {
  for (const item of items) {
    console.log(item);
  }
}

const numberArray: Array<number> = [10, 20, 30];
const stringArray: string[] = ['hello', 'world'];

console.log('Number array items:');
printArrayItems(numberArray);

console.log('String array items:');
printArrayItems(stringArray);

Important Notes

Using generic arrays keeps your code safe by checking types at compile time.

Time complexity for accessing array items is O(1).

Common mistake: mixing types in arrays without using union types can cause errors.

Use generic arrays when you want reusable code that works with any data type.

Summary

Generic array syntax lets you create arrays that hold any type safely.

You can write Array<Type> or Type[] to declare arrays.

It helps make your code flexible and prevents type errors.

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