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.