Parameters type in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'Parameters type' mean in TypeScript?

It means specifying the type of values that a function expects as inputs, so the function knows what kind of data it will receive.

beginner

How do you write a function in TypeScript that takes a number as a parameter?

You write the parameter name followed by a colon and the type. Example: <pre>function double(x: number) { return x * 2; }</pre>

beginner

What happens if you pass a wrong type to a function with typed parameters in TypeScript?

TypeScript will show an error before running the code, helping you fix the mistake early.

intermediate

Can function parameters in TypeScript have optional types?

Yes, you can mark parameters as optional with a question mark. Example: <pre>function greet(name?: string) { ... }</pre>

beginner

What is the benefit of using parameter types in functions?

It helps catch mistakes early, makes code easier to understand, and improves code quality by ensuring correct data is used.

How do you specify a string parameter named 'text' in a TypeScript function?

A function example(text) {}

B function example(string text) {}

C function example(text = string) {}

D function example(text: string) {}

In TypeScript, you write the parameter name followed by a colon and the type.

What does the question mark (?) mean in a parameter like 'name?: string'?

A The parameter is optional

B The parameter is required

C The parameter is a number

D The parameter is a default value

The question mark marks the parameter as optional, so it can be omitted.

What will TypeScript do if you pass a number to a function expecting a string parameter?

A Crash the program at runtime

B Convert the number to string automatically

C Show a type error before running

D Ignore the type and run anyway

TypeScript checks types before running and shows errors if types don't match.

Which of these is a correct way to define a function with two parameters: a number and a boolean?

A function check(num, flag) {}

B function check(num: number, flag: boolean) {}

C function check(number num, boolean flag) {}

D function check(num: boolean, flag: number) {}

You specify each parameter name followed by its type separated by commas.

Why is it helpful to use parameter types in functions?

A To catch errors early and understand code better

B To make code run faster

C To allow any type of input without errors

D To make functions shorter

Parameter types help catch mistakes early and improve code clarity.

Explain how to define a function with typed parameters in TypeScript and why it is useful.

Think about how you tell the function what kind of data it should expect.

Describe what happens if you pass the wrong type of argument to a function with typed parameters in TypeScript.

Consider how TypeScript helps you avoid bugs.

Practice

1. What does specifying a parameter type in a TypeScript function do?
function greet(name: string) {}

easy

A. It tells the function what type of value to expect for that parameter.

B. It changes the return type of the function automatically.

C. It makes the parameter optional.

D. It converts the parameter to a string.

Solution

1. Step 1: Understand parameter type declaration

   In TypeScript, writing name: string means the function expects a string value for name.

2. Step 2: Effect of parameter type

   This helps catch errors if a different type is passed and makes the code clearer.

3. Final Answer:

   It tells the function what type of value to expect for that parameter. -> Option A

4. Quick Check:

   Parameter type = expected value type [OK]

Hint: Parameter types define expected input data [OK]

Common Mistakes:

• Thinking parameter type changes return type
• Assuming parameter becomes optional automatically
• Believing parameter type converts data

2. Which of the following is the correct way to declare a function parameter of type number in TypeScript?

easy

A. function add(x number) {}

B. function add(x: number) {}

C. function add(number x) {}

D. function add(x = number) {}

Solution

1. Step 1: Check TypeScript parameter syntax

   The correct syntax uses a colon between the parameter name and its type: x: number.

2. Step 2: Identify correct option

   function add(x: number) {} matches this syntax exactly, others have syntax errors.

3. Final Answer:

   function add(x: number) {} -> Option B

4. Quick Check:

   Parameter syntax = name: type [OK]

Hint: Use colon between parameter and type [OK]

Common Mistakes:

• Omitting colon between name and type
• Placing type before parameter name
• Using equals sign instead of colon

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

function multiply(a: number, b: number) {
  return a * b;
}
console.log(multiply(3, 4));

medium

A. 12

B. 34

C. undefined

D. Error: Type mismatch

Solution

1. Step 1: Understand function parameters and operation

   The function takes two numbers and returns their product.

2. Step 2: Calculate the output

   Calling multiply(3, 4) returns 3 * 4 = 12.

3. Final Answer:

   12 -> Option A

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Multiply parameters as numbers to get output [OK]

Common Mistakes:

• Concatenating numbers as strings
• Expecting error due to types
• Confusing return value with parameter list

4. Identify the error in this TypeScript function declaration:

function greet(name string) {
  console.log('Hello, ' + name);
}

medium

A. Parameter name cannot be 'name'.

B. Parameter type should be after the function name.

C. Function must return a value.

D. Missing colon between parameter name and type.

Solution

1. Step 1: Check parameter syntax

   The parameter declaration name string is missing a colon between name and string.

2. Step 2: Correct syntax

   It should be name: string to be valid TypeScript.

3. Final Answer:

   Missing colon between parameter name and type. -> Option D

4. Quick Check:

   Parameter syntax requires colon [OK]

Hint: Always put colon between parameter and type [OK]

Common Mistakes:

• Omitting colon in parameter type
• Thinking parameter type goes after function name
• Assuming function must return value

5. Given this function, which call will cause a TypeScript error due to parameter types?

function setFlag(flag: boolean) {
  return flag ? 'ON' : 'OFF';
}

hard

A. setFlag(true)

B. setFlag(false)

C. setFlag('true')

D. setFlag(Boolean(0))

Solution

1. Step 1: Check parameter type expected

   The function expects a boolean parameter.

2. Step 2: Analyze each call

   Calls with true, false, and Boolean(0) are booleans. Passing 'true' (a string) causes a type error.

3. Final Answer:

   setFlag('true') -> Option C

4. Quick Check:

   String passed instead of boolean causes error [OK]

Hint: Pass correct type, not string for boolean parameters [OK]

Common Mistakes:

• Passing string instead of boolean
• Confusing Boolean() constructor with string
• Assuming any truthy value is allowed