Why typed functions matter in Typescript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a function that adds two numbers and returns a number.

Typescript

function add(a: number, b: number): [1] {
  return a + b;
}

Drag options to blanks, or click blank then click option'

Aboolean

Bstring

Cnumber

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' or 'void' as return type causes errors.

The function returns a number because it adds two numbers.

2fill in blank

medium

Complete the code to declare a function that takes a string and returns its length as a number.

Typescript

function getLength(text: string): [1] {
  return text.length;
}

Drag options to blanks, or click blank then click option'

Astring

Bvoid

Cboolean

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 'string' or 'void' causes type errors.

The length property of a string is a number.

3fill in blank

hard

Fix the error in the function by specifying the correct return type.

Typescript

function isEven(num: number): [1] {
  return num % 2 === 0;
}

Drag options to blanks, or click blank then click option'

Aboolean

Bstring

Cnumber

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'number' or 'string' as return type causes errors.

The function returns true or false, so the return type is boolean.

4fill in blank

hard

Fill both blanks to declare a function that takes an array of numbers and returns the sum as a number.

Typescript

function sumArray(arr: [1]): [2] {
  return arr.reduce((acc, val) => acc + val, 0);
}

Drag options to blanks, or click blank then click option'

Anumber[]

Bstring[]

Cnumber

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string[]' or 'boolean' for the array or return type causes errors.

The function takes an array of numbers (number[]) and returns a number which is the sum.

5fill in blank

hard

Fill all three blanks to declare a function that takes a string and a number, and returns a string repeated that many times.

Typescript

function repeatText(text: [1], times: [2]): [3] {
  return text.repeat(times);
}

Drag options to blanks, or click blank then click option'

Astring

Bnumber

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up parameter types or return type causes errors.

The function takes a string and a number, and returns a string repeated multiple times.

Practice

1. Why is it important to specify types for function parameters and return values in TypeScript?

easy

A. It helps catch errors early by ensuring correct input and output types.

B. It makes the code run faster at runtime.

C. It allows functions to run without any input.

D. It automatically fixes bugs in the code.

Solution

1. Step 1: Understand the role of typed functions

   Typed functions define what types of inputs they accept and what type they return, helping the developer know how to use them correctly.

2. Step 2: Recognize the benefit of early error detection

   By specifying types, TypeScript can catch mistakes before running the code, preventing bugs caused by wrong data types.

3. Final Answer:

   It helps catch errors early by ensuring correct input and output types. -> Option A

4. Quick Check:

   Typed functions = early error detection [OK]

Hint: Typed functions catch type mistakes before running code [OK]

Common Mistakes:

• Thinking types speed up runtime
• Believing typed functions fix bugs automatically
• Assuming functions don't need inputs

2. Which of the following is the correct way to type a function in TypeScript that takes a number and returns a string?

easy

A. function convert(num: number) { return num.toString(); }

B. function convert(num: number): string { return num.toString(); }

C. function convert(num: string): number { return num.length; }

D. function convert(num): string { return num.toString(); }

Solution

1. Step 1: Check parameter and return types

   The function should accept a number parameter and return a string, so the parameter must be typed as number and the return type as string.

2. Step 2: Identify the correct syntax

   function convert(num: number): string { return num.toString(); } correctly types the parameter as number and the return type as string. function convert(num): string { return num.toString(); } misses parameter type, C swaps types, and A misses return type.

3. Final Answer:

   function convert(num: number): string { return num.toString(); } -> Option B

4. Quick Check:

   Parameter: number, Return: string = function convert(num: number): string { return num.toString(); } [OK]

Hint: Parameter and return types must match function purpose [OK]

Common Mistakes:

• Omitting parameter types
• Swapping parameter and return types
• Not specifying return type

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

function add(a: number, b: number): number {
  return a + b;
}

const result = add(5, 10);
console.log(result);

medium

A. 15

B. 510

C. undefined

D. Error: Type mismatch

Solution

1. Step 1: Analyze function parameters and return

   The function add takes two numbers and returns their sum as a number.

2. Step 2: Calculate the result of add(5, 10)

   Adding 5 and 10 gives 15, so result is 15 and console.log prints 15.

3. Final Answer:

   15 -> Option A

4. Quick Check:

   5 + 10 = 15 [OK]

Hint: Add numbers to get sum, not string concat [OK]

Common Mistakes:

• Thinking numbers concatenate as strings
• Expecting undefined return
• Assuming type error without mismatch

4. Identify the error in this TypeScript function and choose the fix:

function greet(name: string): string {
  return 'Hello, ' + name;
}

greet(42);

medium

A. Remove the return type to fix the error.

B. Change function parameter to number: greet(name: number).

C. No error, code runs fine.

D. Change greet(42) to greet('42') to match string parameter.

Solution

1. Step 1: Check function parameter type

   The function expects a string parameter named name.

2. Step 2: Identify the call with wrong argument type

   The call greet(42) passes a number, which conflicts with the string type.

3. Step 3: Fix the call to match expected type

   Changing greet(42) to greet('42') passes a string, fixing the type mismatch.

4. Final Answer:

   Change greet(42) to greet('42') to match string parameter. -> Option D

5. Quick Check:

   Parameter type mismatch fixed by passing string [OK]

Hint: Match argument types to parameter types exactly [OK]

Common Mistakes:

• Ignoring type mismatch errors
• Removing return type instead of fixing input
• Changing parameter type without reason

5. You want a function that accepts either a string or number and returns a string describing the input type. Which typed function is correct?

hard

A. function describe(input: any): string {
return `Input: ${input}`;
}

B. function describe(input: string): string {
return `String: ${input}`;
}

C. function describe(input: string | number): string {
if (typeof input === 'string') return `String: ${input}`;
else return `Number: ${input}`;
}

D. function describe(input: number): string {
return `Number: ${input}`;
}

Solution

1. Step 1: Understand the input types needed

   The function must accept either string or number, so union type string | number is required.

2. Step 2: Check function logic and typing

   function describe(input: string | number): string { if (typeof input === 'string') return `String: ${input}`; else return `Number: ${input}`; } uses union type and checks input type to return correct description. Options C and D accept only one type, A uses any which loses type safety.

3. Final Answer:

   function describe(input: string | number): string { if (typeof input === 'string') return `String: ${input}`; else return `Number: ${input}`; } -> Option C

4. Quick Check:

   Union type with type check = function describe(input: string | number): string { if (typeof input === 'string') return `String: ${input}`; else return `Number: ${input}`; } [OK]

Hint: Use union types and type checks for multiple input types [OK]

Common Mistakes:

• Using any type losing safety
• Accepting only one type instead of both
• Not checking input type before returning