Array type annotation syntax in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this TypeScript code with array type annotation?

Consider the following TypeScript code snippet. What will be logged to the console?

Typescript

function sumNumbers(numbers: number[]): number {
  return numbers.reduce((acc, val) => acc + val, 0);
}

console.log(sumNumbers([1, 2, 3, 4]));

A 10

B TypeError at runtime

C undefined

D Compilation error due to wrong type annotation

💡 Hint

Look at how the array is typed and how reduce sums the values.

The function sumNumbers takes an array of numbers and returns their sum using reduce. The input array [1, 2, 3, 4] sums to 10, so the output is 10.

❓ Predict Output

intermediate

What error does this TypeScript code raise?

What error will this TypeScript code produce when compiled?

Typescript

let names: string[] = ["Alice", "Bob", 42];

A Runtime error: Cannot mix types in array

B SyntaxError: Unexpected number in array

C No error, compiles fine

D Type 'number' is not assignable to type 'string'.

💡 Hint

Check the array type and the types of elements inside.

The array is declared to hold strings only, but 42 is a number. TypeScript raises a type error during compilation.

🔧 Debug

advanced

Why does this array type annotation cause a compilation error?

Identify the reason for the compilation error in this TypeScript code snippet.

Typescript

const values: Array<number> = [1, 2, "3", 4];

A Array<number> syntax is invalid in TypeScript.

B The string "3" is not assignable to type number in the array.

C Missing semicolon after array declaration causes error.

D The array must be declared with square brackets, not Array<>.

💡 Hint

Check the types of elements inside the array compared to the annotation.

The array is typed as Array, but contains a string "3" which is not allowed. This causes a type error.

📝 Syntax

advanced

Which option correctly annotates an array of booleans in TypeScript?

Choose the correct TypeScript syntax to declare a variable named flags as an array of booleans.

A let flags: boolean[] = [true, false, true];

B let flags: [boolean] = [true, false, true];

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

D let flags: boolean = [true, false, true];

💡 Hint

Remember the two common ways to annotate arrays in TypeScript.

Option A uses the correct syntax boolean[] for an array of booleans. Option A misses the generic type parameter. Option A declares a tuple of one boolean, not an array. Option A assigns an array to a boolean variable.

🚀 Application

expert

How many items are in the resulting array after this TypeScript code runs?

Given the following TypeScript code, how many items does the variable result contain?

Typescript

const mixedArray: (string | number)[] = ["a", 1, "b", 2];
const result: string[] = mixedArray.filter((item): item is string => typeof item === "string");

A 0

B 4

C 2

D Type error at compile time

💡 Hint

Look at the filter condition and the types in the original array.

The filter keeps only strings from mixedArray. The original array has two strings: "a" and "b". So result contains 2 items.

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