Function overloads in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a function overload for a function named combine that takes two numbers and returns a number.

Typescript

function combine(a: number, b: number): [1];

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

Anumber

Bvoid

Cboolean

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using string as return type when inputs are numbers.

Leaving the return type as void.

The function combine takes two numbers and returns a number, so the return type should be number.

2fill in blank

medium

Complete the code to declare a function overload for combine that takes two strings and returns a string.

Typescript

function combine(a: string, b: string): [1];

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

Aboolean

Bnumber

Cstring

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using number as return type for string inputs.

Using any unnecessarily.

The function combine takes two strings and returns a string, so the return type should be string.

3fill in blank

hard

Fix the error in the implementation signature of the overloaded function combine.

Typescript

function combine(a: [1], b: [1]): [1] {
  if (typeof a === 'string' && typeof b === 'string') {
    return a + b;
  }
  return a + b;
}

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

Anumber

Bany

Cstring

Dunknown

Attempts:

3 left

💡 Hint

Common Mistakes

Using string or number only, which causes type errors.

Using unknown without type checks.

The implementation signature must be compatible with all overloads, so using any for parameters and return type allows handling both strings and numbers.

4fill in blank

hard

Fill both blanks to complete the function overloads for combine that accept either two numbers or two strings.

Typescript

function combine(a: [1], b: [1]): [1];
function combine(a: [2], b: [2]): [2];

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

Anumber

Bstring

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing parameter and return types incorrectly.

Using any in overload declarations.

The first overload takes two numbers and returns a number, the second takes two strings and returns a string.

5fill in blank

hard

Fill all three blanks to complete the overloaded function combine with implementation that handles both numbers and strings.

Typescript

function combine(a: [1], b: [1]): [2];
function combine(a: [3], b: [3]): [3];
function combine(a: any, b: any): any {
  if (typeof a === 'string' && typeof b === 'string') {
    return a + b;
  }
  return a + b;
}

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

Anumber

Bstring

Cany

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using any in overload declarations instead of specific types.

Mismatching parameter and return types.

The first overload uses number parameters, the second uses string. Both return the type matching their parameters.

Practice

1. What is the main purpose of function overloads in TypeScript?

easy

A. To allow a single function to handle different types of input with clear signatures

B. To create multiple functions with the same name but different implementations

C. To automatically generate function documentation

D. To enforce that functions only accept one specific type

Solution

1. Step 1: Understand function overloads concept

   Function overloads let one function have multiple type signatures to handle different input types clearly.

2. Step 2: Compare options with concept

   Only To allow a single function to handle different types of input with clear signatures correctly describes this purpose. Other options describe different or incorrect ideas.

3. Final Answer:

   To allow a single function to handle different types of input with clear signatures -> Option A

4. Quick Check:

   Function overloads = multiple input types handled clearly [OK]

Hint: Overloads = multiple input types, one function name [OK]

Common Mistakes:

• Thinking overloads create multiple separate functions
• Believing overloads generate docs automatically
• Assuming overloads restrict to one type only

2. Which of the following is the correct syntax for declaring function overloads in TypeScript?

easy

A. function foo(x: any): any; function foo(x: any) { return x; }

B. function foo(x: number | string): number | string { return x; }

C. function foo(x: number): number { } function foo(x: string): string { }

D. function foo(x: number): number; function foo(x: string): string; function foo(x: any) { return x; }

Solution

1. Step 1: Identify correct overload syntax

   Overloads require multiple declarations without bodies, then one implementation with a body.

2. Step 2: Check each option

   function foo(x: number): number; function foo(x: string): string; function foo(x: any) { return x; } shows multiple signatures without bodies, then one implementation with a body. Others either combine signatures or have bodies in all declarations.

3. Final Answer:

   function foo(x: number): number; function foo(x: string): string; function foo(x: any) { return x; } -> Option D

4. Quick Check:

   Overloads = multiple signatures + one implementation [OK]

Hint: Overloads: signatures no body, one implementation with body [OK]

Common Mistakes:

• Putting function bodies in all overload declarations
• Using union types instead of overloads
• Declaring only one function signature

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

function combine(a: number, b: number): number;
function combine(a: string, b: string): string;
function combine(a: any, b: any) {
  return a + b;
}

console.log(combine(5, 10));
console.log(combine('Hi, ', 'there!'));

medium

A. '510' and 'Hi, there!'

B. 15 and 'Hithere!'

C. 15 and 'Hi, there!'

D. Error: No matching overload

Solution

1. Step 1: Analyze overload signatures and implementation

   Two overloads: one for numbers returning number, one for strings returning string. Implementation uses + operator.

2. Step 2: Evaluate console.log calls

   combine(5, 10) adds numbers: 5 + 10 = 15. combine('Hi, ', 'there!') concatenates strings: 'Hi, ' + 'there!' = 'Hi, there!'.

3. Final Answer:

   15 and 'Hi, there!' -> Option C

4. Quick Check:

   Number add = 15, String concat = 'Hi, there!' [OK]

Hint: Overloads match input types; + adds numbers or joins strings [OK]

Common Mistakes:

• Thinking + always concatenates strings only
• Expecting error due to any type in implementation
• Confusing output as string '510' for numbers

4. Identify the error in the following TypeScript function overload code:

function greet(name: string): string {
  return 'Hello, ' + name;
}
function greet(age: number): string {
  return 'You are ' + age + ' years old';
}

medium

A. Return types must be void for overloads

B. Function overloads must have multiple signatures without bodies, then one implementation

C. Function names must be different for overloads

D. Overloads cannot use string or number types

Solution

1. Step 1: Review overload syntax rules

   Overloads require multiple declarations without bodies, followed by one implementation with a body.

2. Step 2: Check given code

   Both functions have bodies and same name, which is invalid overload syntax.

3. Final Answer:

   Function overloads must have multiple signatures without bodies, then one implementation -> Option B

4. Quick Check:

   Overloads = signatures no body + one implementation [OK]

Hint: Overloads: only one function has body, others are signatures [OK]

Common Mistakes:

• Writing multiple functions with bodies for overloads
• Using different function names for overloads
• Thinking return type must be void

5. Given the following overloaded function signatures, which implementation correctly handles both overloads and returns the expected types?

function parseInput(input: string): number;
function parseInput(input: number): string;
// Implementation here

hard

A. function parseInput(input: any) {
if (typeof input === 'string') return Number(input);
else return input.toString();
}

B. function parseInput(input: string | number) {
return input;
}

C. function parseInput(input: string | number): string | number {
return input + 1;
}

D. function parseInput(input: any) {
return input;
}

Solution

1. Step 1: Understand overload signatures

   Input string returns number; input number returns string.

2. Step 2: Check each implementation

   function parseInput(input: any) { if (typeof input === 'string') return Number(input); else return input.toString(); } checks input type and converts accordingly, matching overloads. Others either return input unchanged or perform invalid operations.

3. Final Answer:

   function parseInput(input: any) { if (typeof input === 'string') return Number(input); else return input.toString(); } -> Option A

4. Quick Check:

   Type check + convert = correct overload implementation [OK]

Hint: Check input type, convert to match overload return type [OK]

Common Mistakes:

• Ignoring input type and returning input directly
• Using union types without type checks
• Returning wrong types for given inputs