Multiple generic parameters in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are multiple generic parameters in TypeScript?

They are placeholders for types that allow a function, class, or interface to work with more than one type, making the code flexible and reusable.

beginner

How do you declare a function with two generic parameters in TypeScript?

Use angle brackets with two type variables separated by a comma, like , before the function parameters.

beginner

Example: What does this function do? function pair(first: T, second: U): [T, U] { return [first, second]; }

It takes two values of any types T and U, and returns a tuple (pair) containing both values in order.

intermediate

Why use multiple generic parameters instead of just one?

Because you might want to work with two or more different types at once, keeping their types separate and flexible.

intermediate

Can generic parameters have constraints when using multiple generics?

Yes, each generic parameter can have its own constraint to limit what types it can accept, improving type safety.

How do you declare a function with two generic types T and U in TypeScript?

A function example<T & U>() {}

B function example<T U>() {}

C function example<T, U>() {}

D function example<T; U>() {}

The correct syntax uses a comma between generic parameters inside angle brackets: .

What does this function return? function combine(a: T, b: U): [T, U] { return [a, b]; }

A An array with elements of types T and U

B A single value of type T

C A single value of type U

D A string combining a and b

It returns a tuple (array) containing the two values with their respective types.

Can each generic parameter have its own constraint in multiple generics?

A Yes, each can have different constraints

B No, constraints apply to all generics together

C Only one generic can have a constraint

D Constraints are not allowed with multiple generics

Each generic parameter can have its own constraint to restrict its type.

Why use multiple generic parameters instead of one?

A To make code slower

B To force all types to be the same

C To avoid using types

D To handle multiple different types separately

Multiple generics allow working with different types independently.

Which of these is a valid generic function with two parameters?

A function f<T; U>(x: T, y: U) {}

B function f<T, U>(x: T, y: U) {}

C function f<T U>(x: T, y: U) {}

D function f<T & U>(x: T, y: U) {}

Only option C uses the correct syntax with a comma separating generic parameters.

Explain how to declare and use a function with multiple generic parameters in TypeScript.

Think about how you can pass two different types and keep them separate.

Describe why multiple generic parameters are useful and how constraints can be applied to them.

Consider real-life situations where you want to keep two things different but related.

Practice

1.

What does using multiple generic parameters in TypeScript allow you to do?

Example: function example<T, U>(arg1: T, arg2: U): void {}

easy

A. Only use one type at a time

B. Work with different types together in a single function or class

C. Automatically convert types to strings

D. Disable type checking for function arguments

Solution

1. Step 1: Understand generic parameters

   Generic parameters like T and U let you define placeholders for types.

2. Step 2: Analyze multiple generics usage

   Using multiple generics allows a function or class to accept different types for different parameters, making it flexible and type-safe.

3. Final Answer:

   Work with different types together in a single function or class -> Option B

4. Quick Check:

   Multiple generics = different types together [OK]

Hint: Multiple generics mean multiple type placeholders [OK]

Common Mistakes:

• Thinking generics convert types automatically
• Assuming only one generic can be used
• Believing generics disable type checking

2.

Which of the following is the correct syntax to declare a function with two generic parameters T and U in TypeScript?

easy

A. function example<T & U>(arg1: T, arg2: U): void {}

B. function example<T U>(arg1: T, arg2: U): void {}

C. function example<T; U>(arg1: T, arg2: U): void {}

D. function example<T, U>(arg1: T, arg2: U): void {}

Solution

1. Step 1: Recall generic syntax

   Generic parameters are listed inside angle brackets separated by commas, like <T, U>.

2. Step 2: Check each option

   The correct syntax uses function example<T, U>(arg1: T, arg2: U): void {} with generic parameters separated by commas inside angle brackets. The distractors use invalid delimiters: <T & U>, <T U> (no comma), and <T; U> (semicolon).

3. Final Answer:

   function example<T, U>(arg1: T, arg2: U): void {} -> Option D

4. Quick Check:

   Generics separated by commas inside <> [OK]

Hint: Separate generics with commas inside angle brackets [OK]

Common Mistakes:

• Omitting commas between generic parameters
• Using semicolons instead of commas
• Using & instead of commas

3.

What is the output of the following TypeScript code?

function pairToString<T, U>(pair: [T, U]): string {
  return `${pair[0]} and ${pair[1]}`;
}

const result = pairToString<number, string>([5, 'apples']);
console.log(result);

medium

A. "[5, apples]"

B. Type error because types differ

C. "5 and apples"

D. "number and string"

Solution

1. Step 1: Understand the function behavior

   The function takes a tuple with two generic types and returns a string combining both elements.

2. Step 2: Analyze the call with types and values

   Calling with <number, string> and tuple [5, 'apples'] returns the string "5 and apples".

3. Final Answer:

   "5 and apples" -> Option C

4. Quick Check:

   Tuple values combined as string = "5 and apples" [OK]

Hint: Generics keep types but output string combines values [OK]

Common Mistakes:

• Expecting a type error due to different types
• Thinking output is the tuple stringified
• Confusing generic types with runtime values

4.

Identify the error in this TypeScript function using multiple generic parameters:

function combine<T, U>(a: T, b: U): T {
  return b;
}

medium

A. Return type is T but function returns b of type U

B. Missing comma between generic parameters

C. Function should not use generics

D. Parameters a and b must be the same type

Solution

1. Step 1: Check function signature

   The function declares it returns type T but returns b which is type U.

2. Step 2: Identify type mismatch

   Returning b violates the declared return type T, causing a type error.

3. Final Answer:

   Return type is T but function returns b of type U -> Option A

4. Quick Check:

   Return type mismatch = error [OK]

Hint: Return type must match returned value type [OK]

Common Mistakes:

• Ignoring return type mismatch
• Thinking generics auto-convert types
• Assuming parameters must share the same type

5.

You want to create a generic function mergeObjects that takes two objects of different types and returns a new object combining both. Which is the correct way to declare this function with multiple generic parameters?

hard

A. function mergeObjects<T, U>(obj1: T, obj2: U): T & U { return {...obj1, ...obj2}; }

B. function mergeObjects<T>(obj1: T, obj2: T): T { return {...obj1, ...obj2}; }

C. function mergeObjects(obj1: object, obj2: object): object { return {...obj1, ...obj2}; }

D. function mergeObjects<T, U>(obj1: T, obj2: U): object { return {...obj1, ...obj2}; }

Solution

1. Step 1: Understand the goal

   The function should accept two objects of different types and return their combined type.

2. Step 2: Analyze return type

   Using T & U as return type means the result has properties from both types, which is correct.

3. Step 3: Check options

   function mergeObjects<T, U>(obj1: T, obj2: U): T & U { return {...obj1, ...obj2}; } correctly uses two generics and returns T & U. function mergeObjects<T>(obj1: T, obj2: T): T { return {...obj1, ...obj2}; } uses one generic and same type for both, which is less flexible. function mergeObjects(obj1: object, obj2: object): object { return {...obj1, ...obj2}; } uses plain objects without generics, losing type safety. function mergeObjects<T, U>(obj1: T, obj2: U): object { return {...obj1, ...obj2}; } returns just object, losing specific type info.

4. Final Answer:

   function mergeObjects<T, U>(obj1: T, obj2: U): T & U { return {...obj1, ...obj2}; } -> Option A

5. Quick Check:

   Multiple generics + intersection return type = correct [OK]

Hint: Use intersection type T & U for combined object return [OK]

Common Mistakes:

• Using one generic for both objects
• Returning just object losing type info
• Not using intersection type for return