Function overloads in Typescript - Mini Project: Build & Apply

Function overloads

📖 Scenario: You are building a simple calculator that can add numbers or concatenate strings. Depending on the input types, the calculator should behave differently.

🎯 Goal: Create a TypeScript function called combine that works with both numbers and strings using function overloads. It should add numbers or join strings.

📋 What You'll Learn

Create two function overload signatures for combine: one for two numbers, one for two strings

Implement the combine function to handle both cases

Test the function by calling combine with numbers and strings

Print the results of the function calls

💡 Why This Matters

🌍 Real World

Function overloads help create flexible functions that work with different types of inputs, like calculators or formatters.

💼 Career

Many programming jobs require writing clean, reusable code that handles multiple data types safely, especially in TypeScript.

1

Create function overload signatures

Write two function overload signatures for a function called combine. The first signature takes two number parameters and returns a number. The second signature takes two string parameters and returns a string.

Typescript

// Write two function overload signatures for combine
// Your code here

Hint

Function overloads are multiple function declarations with the same name but different parameter types.

2

Implement the combine function

Implement the combine function with parameters a and b of type any. Inside the function, check if both a and b are numbers and return their sum. Otherwise, return their concatenation as strings.

Typescript

function combine(a: number, b: number): number;
function combine(a: string, b: string): string;

// Implement the combine function below
// Your code here

Hint

Use typeof to check the types inside the function and return accordingly.

3

Call the combine function with numbers and strings

Call the combine function twice: once with numbers 5 and 10, and once with strings 'Hello' and 'World'. Store the results in variables called resultNumbers and resultStrings respectively.

Typescript

function combine(a: number, b: number): number;
function combine(a: string, b: string): string;
function combine(a: any, b: any): any {
    if (typeof a === 'number' && typeof b === 'number') {
        return a + b;
    } else {
        return a.toString() + b.toString();
    }
}

// Call combine with numbers and strings and store results
// Your code here

Hint

Call the function with exact values and store the results in variables.

4

Print the results

Print the values of resultNumbers and resultStrings using two separate console.log statements.

Typescript

function combine(a: number, b: number): number;
function combine(a: string, b: string): string;
function combine(a: any, b: any): any {
    if (typeof a === 'number' && typeof b === 'number') {
        return a + b;
    } else {
        return a.toString() + b.toString();
    }
}

const resultNumbers = combine(5, 10);
const resultStrings = combine('Hello', 'World');

// Print the results below
// Your code here

Hint

Use console.log to show the results on the screen.

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