Default parameters with types in Typescript - Step-by-Step Execution

Concept Flow - Default parameters with types

Function call

↓

Check if argument provided

↓

Use arg

↓

Execute function body

↓

Return result

When a function is called, it checks if an argument is given. If not, it uses the default value with the specified type.

Execution Sample

Typescript

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

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

This function greets a person using a default name if none is provided.

Execution Table

Step	Function Call	Parameter 'name'	Action	Output
1	greet()	undefined	Use default 'Friend'	Hello, Friend!
2	greet("Alice")	"Alice"	Use provided argument	Hello, Alice!
3	End	-	-	-

💡 Function calls complete; default used when argument missing.

Variable Tracker

Variable	Start	After Call 1	After Call 2	Final
name	undefined	"Friend"	"Alice"	-

Key Moments - 2 Insights

Why does the function use "Friend" when no argument is given?

Because the parameter 'name' has a default value "Friend" defined, so when the argument is missing (see execution_table row 1), it uses that default.

What happens if we pass an argument to the function?

The function uses the provided argument instead of the default (see execution_table row 2), so 'name' becomes the passed value.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'name' during the first function call?

A "Alice"

B "Friend"

C undefined

D null

💡 Hint

Check the 'Action' column in execution_table row 1.

At which step does the function use the argument passed by the caller?

A Step 2

B Step 1

C Step 3

D Never

💡 Hint

Look at the 'Action' column in execution_table row 2.

If we remove the default value, what would happen when calling greet() with no arguments?

A It uses undefined as name

B It uses "Friend" anyway

C It throws an error

D It returns an empty string

💡 Hint

Think about TypeScript requiring arguments when no default is set.

Concept Snapshot

function f(param: type = defaultValue) {
  // function body
}

- If argument missing, defaultValue is used.
- Parameter type enforces argument type.
- Helps avoid undefined errors and simplifies calls.

Full Transcript

This visual trace shows how TypeScript functions handle default parameters with types. When the function greet is called without an argument, it uses the default string "Friend" as the parameter 'name'. When called with an argument like "Alice", it uses that value instead. The execution table tracks each call, showing parameter values and outputs. The variable tracker shows how 'name' changes. Key moments clarify why defaults are used and what happens when arguments are passed. The quiz tests understanding of parameter values and behavior when defaults are removed.

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