Default parameters with types in Typescript - Mini Project: Build & Apply

Default parameters with types

📖 Scenario: You are creating a simple greeting function for a website. Sometimes users provide their name, and sometimes they don't. You want to make sure the function works well in both cases.

🎯 Goal: Build a TypeScript function called greet that takes a name parameter with a default value and a type. The function should return a greeting message.

📋 What You'll Learn

Create a function named greet with a parameter name of type string

Set the default value of name to "Guest"

Return a greeting string using the name parameter

Call the function twice: once without arguments and once with the argument "Alice"

Print the results of both calls

💡 Why This Matters

🌍 Real World

Default parameters help make functions flexible and easier to use when some information is optional.

💼 Career

Understanding default parameters with types is important for writing clean, safe, and maintainable TypeScript code in real projects.

1

Create the greet function with a typed name parameter

Write a function called greet that takes one parameter named name of type string. Do not add a default value yet.

Typescript

function greet(name: string) {
  // Your code here
}

Hint

Use function greet(name: string) to define the function and return a greeting string using backticks.

2

Add a default value "Guest" to the name parameter

Modify the greet function so that the name parameter has a default value of "Guest".

Typescript

function greet(name: string = "Guest") {
  return `Hello, ${name}!`;
  // Your code here
}

Hint

Add = "Guest" after the parameter name: string to set the default.

3

Call greet twice: once without arguments and once with "Alice"

Write two lines: call greet() without arguments and store the result in greeting1, then call greet("Alice") and store the result in greeting2.

Typescript

function greet(name: string = "Guest") {
  return `Hello, ${name}!`;
}

const greeting1 = greet();
const greeting2 = greet("Alice");
// Your code here

Hint

Use const greeting1 = greet() and const greeting2 = greet("Alice") to call the function.

4

Print the greetings stored in greeting1 and greeting2

Write two console.log statements to print greeting1 and greeting2.

Typescript

function greet(name: string = "Guest") {
  return `Hello, ${name}!`;
}

const greeting1 = greet();
const greeting2 = greet("Alice");

console.log(greeting1);
console.log(greeting2);
// Your code here

Hint

Use console.log(greeting1) and console.log(greeting2) to show the greetings.

Practice

1.

What is the main purpose of default parameters in TypeScript functions?

easy

A. To provide fallback values when arguments are missing

B. To enforce strict typing on function return values

C. To declare variables inside functions

D. To create anonymous functions

Solution

1. Step 1: Understand default parameters

   Default parameters give a value to a function argument if no value is passed.

2. Step 2: Identify the purpose

   This helps avoid errors when arguments are missing by providing fallback values.

3. Final Answer:

   To provide fallback values when arguments are missing -> Option A

4. Quick Check:

   Default parameters = fallback values [OK]

Hint: Default parameters fill missing arguments automatically [OK]

Common Mistakes:

• Confusing default parameters with return type enforcement
• Thinking default parameters declare variables
• Mixing default parameters with anonymous functions

2.

Which of the following is the correct syntax for a TypeScript function with a default parameter of type number?

function multiply(x: number, y: number = 2) { return x * y; }

easy

A. function multiply(x: number, y = 2: number) { return x * y; }

B. function multiply(x = 2: number, y: number) { return x * y; }

C. function multiply(x: number, y: number = 2) { return x * y; }

D. function multiply(x:, y: number = 2) { return x * y; }

Solution

1. Step 1: Check parameter type and default value syntax

   In TypeScript, the type annotation comes before the default value: param: type = default.

2. Step 2: Verify each option

   function multiply(x: number, y: number = 2) { return x * y; } correctly uses y: number = 2. Others have syntax errors or wrong order.

3. Final Answer:

   function multiply(x: number, y: number = 2) { return x * y; } -> Option C

4. Quick Check:

   Type before default value = correct syntax [OK]

Hint: Type annotation before default value: param: type = value [OK]

Common Mistakes:

• Placing default value before type annotation
• Omitting type annotation on default parameter
• Assigning default to wrong parameter

3.

What is the output of the following TypeScript code?

function greet(name: string = "Guest") {
  return `Hello, ${name}!`;
}

console.log(greet());
console.log(greet("Alice"));

medium

A. Hello, undefined!\nHello, Alice!

B. Hello, Guest!\nHello, Alice!

C. Hello, !\nHello, Alice!

D. Error: Missing argument for name

Solution

1. Step 1: Analyze default parameter usage

   Calling greet() without argument uses default "Guest".

2. Step 2: Check outputs for both calls

   First call prints "Hello, Guest!"; second call prints "Hello, Alice!".

3. Final Answer:

   Hello, Guest!\nHello, Alice! -> Option B

4. Quick Check:

   Default parameter used when argument missing [OK]

Hint: Default parameter fills missing argument in function call [OK]

Common Mistakes:

• Expecting undefined instead of default value
• Thinking missing argument causes error
• Ignoring template literal output format

4.

Identify the error in this TypeScript function and choose the correct fix:

function calculateArea(width: number = 5, height: number) {
  return width * height;
}

medium

A. Move the parameter with default value to the end: function calculateArea(height: number, width: number = 5) {...}

B. Remove the default value from width: function calculateArea(width: number, height: number) {...}

C. Add a default value to height: function calculateArea(width: number = 5, height: number = 10) {...}

D. No error; function is correct as is

Solution

1. Step 1: Understand parameter order rules

   In TypeScript, parameters with default values must come after parameters without defaults.

2. Step 2: Fix parameter order

   Moving width with default to the end fixes the error.

3. Final Answer:

   Move the parameter with default value to the end: function calculateArea(height: number, width: number = 5) {...} -> Option A

4. Quick Check:

   Default parameters must be last [OK]

Hint: Put default parameters after required ones [OK]

Common Mistakes:

• Placing default parameters before required ones
• Ignoring TypeScript parameter order rules
• Assuming no error when order is wrong

5.

Consider this function that merges two objects with default parameters:

function mergeOptions(options: { verbose?: boolean } = { verbose: false }, defaults: { verbose: boolean } = { verbose: true }) {
  return { ...defaults, ...options };
}

console.log(mergeOptions());
console.log(mergeOptions({ verbose: true }));

What is the output of the two console.log calls?

hard

A. Error due to conflicting default parameters

B. { verbose: true }\n{ verbose: true }

C. { verbose: true }\n{ verbose: false }

D. { verbose: false }\n{ verbose: true }

Solution

1. Step 1: Understand default parameter values

   Both options and defaults have default objects with verbose flags.

2. Step 2: Analyze object merging

   Using spread, { ...defaults, ...options } merges with options overriding defaults.

3. Step 3: Evaluate calls

   First call uses defaults: options = { verbose: false }, defaults = { verbose: true }, merged result is { verbose: false } because options overwrites defaults.

4. Step 4: Check second call

   Second call passes { verbose: true } as options, so merged result is { verbose: true }.

5. Final Answer:

   { verbose: false }\n{ verbose: true } -> Option D

6. Quick Check:

   Options override defaults in merge [OK]

Hint: Spread merges with later objects overriding earlier ones [OK]

Common Mistakes:

• Confusing which object overrides which in spread
• Assuming default parameters merge automatically
• Ignoring optional property in type