Array type annotation syntax in Typescript

Introduction

We use array type annotations to tell TypeScript what kind of items an array holds. This helps catch mistakes early and makes code easier to understand.

When you want to store a list of numbers and ensure only numbers go in the array.

When you have a list of strings like names or colors and want to prevent other types from being added.

When you want to declare an empty array but specify what type of items it will hold later.

When you want to create a function that accepts an array of a specific type as input.

When you want to return an array of a certain type from a function.

Syntax

Typescript

let arrayName: type[];

// or

let arrayName: Array<type>;

You can use either the square bracket syntax (type[]) or the generic syntax (Array). Both mean the same.

Replace 'type' with the type of items the array will hold, like number, string, or a custom type.

Examples

An array holding numbers only.

Typescript

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

An array holding strings using the generic syntax.

Typescript

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

An empty array that will hold booleans later.

Typescript

let emptyArray: boolean[] = [];

An array that can hold numbers or strings.

Typescript

let mixed: (number | string)[] = [1, 'two', 3];

Sample Program

This program defines a function that accepts an array of strings and prints each color. It shows how to declare an array with type annotation and how TypeScript ensures only strings are added.

Typescript

function printColors(colors: string[]): void {
  for (const color of colors) {
    console.log(color);
  }
}

const favoriteColors: string[] = ['red', 'green', 'blue'];
console.log('Colors before adding:');
printColors(favoriteColors);

favoriteColors.push('yellow');
console.log('Colors after adding:');
printColors(favoriteColors);

Important Notes

Time complexity for accessing array elements is O(1), which means very fast.

Space complexity depends on the number of items stored in the array.

A common mistake is forgetting to annotate the array type, which can lead to unexpected types being added.

Use array type annotations to catch errors early and make your code easier to read and maintain.

Summary

Array type annotations tell TypeScript what kind of items an array holds.

You can use 'type[]' or 'Array<type>' syntax to declare array types.

This helps prevent mistakes and makes your code clearer.

Practice

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

easy

A. let nums: Array;

B. let nums: number;

C. let nums: [number];

D. let nums: number[];

Solution

1. Step 1: Understand array type syntax

   In TypeScript, to declare an array of a specific type, you can use type[] or Array<type>.

2. Step 2: Check each option

   let nums: number[]; uses number[], which is correct. let nums: number; declares a single number, not an array. let nums: [number]; declares a tuple with one number, not a general array. let nums: Array; is missing the type inside Array<>.

3. Final Answer:

   let nums: number[]; -> Option D

4. Quick Check:

   Array of numbers = number[] [OK]

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

Common Mistakes:

• Forgetting the brackets [] for arrays
• Using just the type without array syntax
• Confusing tuples with arrays

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

easy

A. let names: string[];

B. let names: Array[string];

C. let names: Array<string>;

D. let names: Array();

Solution

1. Step 1: Recall generic array syntax

   The generic syntax for arrays in TypeScript is Array<type>, with angle brackets <>.

2. Step 2: Evaluate options

   let names: Array<string>; correctly uses Array<string>. let names: Array[string]; uses square brackets instead of angle brackets, which is invalid. let names: string[]; is valid syntax but uses the shorthand, not generic syntax. let names: Array(); uses parentheses, which is incorrect.

3. Final Answer:

   let names: Array<string>; -> Option C

4. Quick Check:

   Generic array syntax = Array<type> [OK]

Hint: Generic arrays use angle brackets, not square brackets [OK]

Common Mistakes:

• Using square brackets [] instead of angle brackets <>
• Confusing shorthand and generic syntax
• Using parentheses instead of brackets

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

let fruits: string[] = ['apple', 'banana', 'cherry'];
console.log(fruits[1]);

medium

A. apple

B. banana

C. cherry

D. undefined

Solution

1. Step 1: Understand array indexing

   Arrays in TypeScript (and JavaScript) start at index 0. So, fruits[0] is 'apple', fruits[1] is 'banana'.

2. Step 2: Identify the logged value

   The code logs fruits[1], which is 'banana'.

3. Final Answer:

   banana -> Option B

4. Quick Check:

   Index 1 in array = banana [OK]

Hint: Remember arrays start at index 0 [OK]

Common Mistakes:

• Thinking array indexes start at 1
• Confusing array elements with their indexes
• Expecting the whole array instead of one element

4. Identify the error in this TypeScript array declaration and fix it:

let scores: Array<number> = [10, 20, '30'];

medium

A. Remove quotes from '30' to make it a number

B. Change 'Array<number>' to 'Array<string>'

C. No error, code is correct

D. Change array to 'number[]' syntax

Solution

1. Step 1: Check array type and elements

   The array is declared as Array<number>, so all elements must be numbers.

2. Step 2: Identify the wrong element

   The element '30' is a string because of quotes, which conflicts with the number type.

3. Step 3: Fix the error

   Remove quotes from '30' to make it a number: 30.

4. Final Answer:

   Remove quotes from '30' to make it a number -> Option A

5. Quick Check:

   Array type must match element types [OK]

Hint: Array type enforces element types strictly [OK]

Common Mistakes:

• Mixing string and number types in typed arrays
• Ignoring type errors in array elements
• Changing array syntax instead of fixing element types

5. You want to declare a variable that holds an array of arrays of booleans in TypeScript using the generic Array type. Which is the correct type annotation?

hard

A. let matrix: Array;

B. let matrix: Array<boolean>;

C. let matrix: boolean[][];

D. let matrix: boolean[];

Solution

1. Step 1: Understand nested arrays

   An array of arrays means each element is itself an array. So the type is an array of arrays of booleans.

2. Step 2: Check each option

   let matrix: boolean[][]; uses shorthand nested arrays boolean[][], which is correct but not the generic syntax. let matrix: Array<boolean>; is a simple array of booleans, not nested. let matrix: Array; uses generic syntax for nested arrays: Array<Array<boolean>>, which is correct. let matrix: boolean[]; is a simple array of booleans.

3. Step 3: Choose the generic nested array syntax

   Since the question asks for the correct type annotation (generic style), let matrix: Array; is the best fit.

4. Final Answer:

   let matrix: Array; -> Option A

5. Quick Check:

   Nested arrays = Array<Array<type>> [OK]

Hint: Nested arrays use Array inside Array or double brackets [OK]

Common Mistakes:

• Using single array type for nested arrays
• Confusing shorthand and generic syntax
• Missing one level of array nesting