Generic function syntax in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a generic function named identity.

Typescript

function identity[1](arg: T): T {
  return arg;
}

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

A<T>

B(T)

C[T]

D{T}

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses or square brackets instead of angle brackets for generic parameters.

Forgetting to add the generic parameter after the function name.

Generic functions use angle brackets <> to declare type parameters.

2fill in blank

medium

Complete the code to call the generic function identity with a string argument.

Typescript

let output = identity[1]("hello");

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

A[string]

B(string)

C{string}

D<string>

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of angle brackets to specify the type argument.

Omitting the type argument when calling the generic function.

When calling a generic function, specify the type argument inside angle brackets.

3fill in blank

hard

Fix the error in the generic function declaration by completing the missing generic parameter syntax.

Typescript

function getArray[1](items: T[]): T[] {
  return new Array().concat(items);
}

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

A<T>

B(T)

C[T]

D{T}

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses or braces instead of angle brackets for generic parameters.

Forgetting to declare the generic parameter.

Generic parameters must be declared inside angle brackets after the function name.

4fill in blank

hard

Fill both blanks to declare a generic function wrapInArray that takes a value and returns it inside an array.

Typescript

function wrapInArray[1](value: [2]): [2][] {
  return [value];
}

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

A<T>

BT

CU

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using different generic type names for argument and return type.

Not declaring the generic parameter.

Declare the generic parameter <T> and use T as the type for the argument and return array.

5fill in blank

hard

Fill all three blanks to declare a generic function merge that takes two objects and returns their merged result.

Typescript

function merge[1](obj1: [2], obj2: [3]): [2] & [3] {
  return { ...obj1, ...obj2 };
}

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

A<T, U>

BT

CU

Dobject

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same generic type for both arguments.

Not declaring generic parameters properly.

Declare two generic parameters <T, U> and use them as types for the two arguments and the return type as their intersection.

Practice

1. What does the <T> represent in a TypeScript generic function?

easy

A. A placeholder for a type that will be specified later

B. A variable name for a function parameter

C. A special keyword to declare a constant

D. A syntax error in TypeScript

Solution

1. Step 1: Understand generic syntax

   The <T> syntax is used to declare a generic type parameter in TypeScript functions.

2. Step 2: Identify the role of T

   It acts as a placeholder for any type that will be provided when the function is called, making the function reusable for different types.

3. Final Answer:

   A placeholder for a type that will be specified later -> Option A

4. Quick Check:

   Generic <T> = Type placeholder [OK]

Hint: Remember <T> means 'type to be decided' [OK]

Common Mistakes:

• Thinking T is a variable name
• Confusing <T> with a function argument
• Assuming <T> is a TypeScript keyword

2. Which of the following is the correct syntax to declare a generic function named identity that returns the same type it receives?

easy

A. function identity<T>(arg): T { return arg; }

B. function identity(arg: T): T { return arg; }

C. function identity<T>(arg: T): T { return arg; }

D. function identity<T>(arg: T): void { return arg; }

Solution

1. Step 1: Check generic declaration

   The generic type <T> must be declared before the function parameters.

2. Step 2: Verify parameter and return types

   The parameter arg should be of type T, and the function should return the same type T.

3. Final Answer:

   function identity<T>(arg: T): T { return arg; } -> Option C

4. Quick Check:

   Generic function syntax = function identity<T>(arg: T): T { return arg; } [OK]

Hint: Generic type <T> goes before parameters [OK]

Common Mistakes:

• Omitting <T> before parameters
• Not specifying type for arg
• Using void as return type incorrectly

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

function wrapInArray<T>(value: T): T[] { return [value]; }
const result = wrapInArray(5);
console.log(result);

medium

A. TypeError at runtime

B. 5

C. SyntaxError

D. [5]

Solution

1. Step 1: Understand function behavior

   The function wrapInArray takes a value of any type T and returns an array containing that value.

2. Step 2: Analyze the call with number 5

   Calling wrapInArray(5) returns an array with one element: [5].

3. Final Answer:

   [5] -> Option D

4. Quick Check:

   wrapInArray(5) = [5] [OK]

Hint: Generic returns array with input inside brackets [OK]

Common Mistakes:

• Expecting just the value without array
• Confusing output with error
• Misreading return type as void

4. Identify the error in this generic function declaration:

function getFirstElement<T>(arr: T): T {
  return arr[0];
}

medium

A. Parameter arr should be an array type T[], not T

B. Return type should be T[] instead of T

C. Generic type <T> is missing

D. Function should not use generics here

Solution

1. Step 1: Check parameter type

   The parameter arr is declared as type T, but the function tries to access arr[0], which requires arr to be an array.

2. Step 2: Correct parameter type

   To access the first element, arr should be of type T[] (array of T), not just T.

3. Final Answer:

   Parameter arr should be an array type T[], not T -> Option A

4. Quick Check:

   Array parameter needs T[] type [OK]

Hint: Array parameters need T[] type, not just T [OK]

Common Mistakes:

• Using T instead of T[] for arrays
• Returning wrong type
• Forgetting to declare generic <T>

5. You want to write a generic function mergeObjects that takes two objects of possibly different types and returns a new object combining both. Which is the correct syntax to declare this function?

hard

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

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

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

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

Solution

1. Step 1: Understand the requirement

   The function should accept two objects of different types and return a combined object containing properties of both.

2. Step 2: Check generic parameters and return type

   Using two generic types T and U allows different input types. The return type T & U means an object with properties from both types.

3. Step 3: Verify function body

   The spread operator {...obj1, ...obj2} correctly merges the two objects.

4. Final Answer:

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

5. Quick Check:

   Two generics + intersection return = function mergeObjects(obj1: T, obj2: U): T & U { return {...obj1, ...obj2}; } [OK]

Hint: Use two generics and return type intersection T & U [OK]

Common Mistakes:

• Using same generic type for both parameters
• Returning just object loses type info
• Not using intersection type for return