Default parameters with types in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set a default value for the parameter name.

Typescript

function greet(name: string = [1]) {
  return `Hello, ${name}!`;
}

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

Anull

B"Guest"

Cundefined

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using null or undefined as default for a string parameter.

Not using quotes around the default string value.

The default value for name is set to the string "Guest". This means if no argument is passed, name will be "Guest".

2fill in blank

medium

Complete the function to set a default number parameter with type.

Typescript

function multiply(x: number, y: number = [1]): number {
  return x * y;
}

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

Anull

B"2"

C2

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string "2" instead of number 2.

Using null or undefined as default for a number parameter.

The default value for y is 2, a number matching the parameter type number.

3fill in blank

hard

Fix the error by completing the default parameter with the correct type.

Typescript

function setFlag(flag: boolean = [1]): string {
  return flag ? "On" : "Off";
}

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

Atrue

B"true"

C1

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using string "true" instead of boolean true.

Using number 1 or null as default for boolean.

The default value for flag must be a boolean true, not a string or number.

4fill in blank

hard

Fill both blanks to create a function with a default string parameter and return type.

Typescript

function welcome(message: [1] = [2]): string {
  return message;
}

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

Astring

B"Hello"

Cnumber

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using number or boolean as type for a string parameter.

Not quoting the default string value.

The parameter message is typed as string with a default value "Hello". The function returns a string.

5fill in blank

hard

Fill all three blanks to define a function with a default boolean parameter and a typed return.

Typescript

function isActive(status: [1] = [2]): [3] {
  return status;
}

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

Astring

Btrue

Cboolean

D"active"

Attempts:

3 left

💡 Hint

Common Mistakes

Using string type or string default for boolean parameter.

Mismatching return type with parameter type.

The parameter status is typed as boolean with default true. The function returns a boolean.

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