Default generic types in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are default generic types in TypeScript?

Default generic types are preset types given to generic parameters so that if no type is provided when using the generic, the default is used automatically.

beginner

How do you specify a default generic type in TypeScript?

You specify a default generic type by using an equals sign after the generic parameter name, like <T = string>.

beginner

Consider this code:

function example<T = number>(value: T): T { return value; }

What happens if you call example() without specifying a type?

The function uses the default generic type number, so TypeScript treats value as a number.

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Why are default generic types useful?

They make generics easier to use by providing a fallback type, so users don’t always have to specify the type explicitly.

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Can you override a default generic type when calling a generic function?

Yes, you can provide a specific type when calling the function, which replaces the default generic type.

What does this syntax mean in TypeScript? function foo() {}

A T is a function returning string

B T must always be string

C T is a variable of type string

D T is a generic type defaulting to string if not specified

The syntax means T is a generic type parameter with a default type of string.

If a generic function has a default type, what happens when you call it without specifying a type?

A It uses the default generic type

B It causes a compile error

C It uses any type

D It uses void type

When no type is specified, the default generic type is used automatically.

How do you override a default generic type when calling a generic function?

A By passing the type as a function argument

B By specifying the type in angle brackets after the function name

C By changing the default type in the function definition

D You cannot override the default type

You override the default by specifying the type explicitly, like foo().

Which of these is a benefit of default generic types?

A They reduce the need to specify types every time

B They make code run faster

C They prevent any type errors

D They force all generics to be strings

Default generic types make generics easier to use by providing a fallback type.

What happens if you specify a generic type when a default is already set?

A TypeScript throws an error

B The default type is used anyway

C The specified type replaces the default

D The function ignores the generic type

The explicitly provided type overrides the default generic type.

Explain what default generic types are and why they are useful in TypeScript.

Think about how generics work and what happens if you don't specify a type.

Describe how to declare a default generic type and how to override it when calling a generic function.

Remember the equals sign in the generic parameter and how to call generic functions.

Practice

1.

What is the main purpose of default generic types in TypeScript?

easy

A. To provide a fallback type when no type argument is specified

B. To enforce a specific type always

C. To disable generics in a function

D. To convert types to strings automatically

Solution

1. Step 1: Understand generic types

   Generic types allow functions or classes to work with any type specified by the user.

2. Step 2: Role of default generic types

   Default generic types provide a fallback type if the user does not specify one explicitly.

3. Final Answer:

   To provide a fallback type when no type argument is specified -> Option A

4. Quick Check:

   Default generic types = fallback type [OK]

Hint: Defaults fill in missing generic types automatically [OK]

Common Mistakes:

• Thinking default generics enforce a fixed type
• Confusing default generics with type conversion
• Believing default generics disable generics

2.

Which of the following is the correct syntax to set a default generic type T to string in a function?

function example<T = string>()

easy

A. function example<T extends string>()

B. function example<T: string>()

C. function example<T = string>()

D. function example<T? = string>()

Solution

1. Step 1: Recall TypeScript generic syntax

   Default generic types use the equals sign (=) to assign a fallback type.

2. Step 2: Analyze options

   function example<T = string>() uses <T = string> which is correct syntax for default generic types.

3. Final Answer:

   function example<T = string>() -> Option C

4. Quick Check:

   Default generic syntax uses = [OK]

Hint: Use = to assign default generic types [OK]

Common Mistakes:

• Using colon (:) instead of equals (=)
• Adding question mark (?) incorrectly
• Confusing extends with default assignment

3.

What will be the output of the following TypeScript code?

function wrap<T = number>(value: T): T[] {
  return [value];
}

const result = wrap('hello');
console.log(result);

medium

A. ["number"]

B. ["hello"]

C. ["value"]

D. Type error

Solution

1. Step 1: Understand default generic type usage

   The function wrap has a default generic type T = number, but the caller passes a string 'hello', so T becomes string.

2. Step 2: Analyze function output

   The function returns an array containing the passed value, so it returns ["hello"].

3. Final Answer:

   ["hello"] -> Option B

4. Quick Check:

   Explicit type overrides default generic [OK]

Hint: Passed type overrides default generic type [OK]

Common Mistakes:

• Assuming default generic always applies
• Expecting a type error for different type
• Confusing value with type name

4.

Identify the error in this TypeScript code using default generic types:

function identity<T = number>(arg: T): T {
  return arg.length;
}

medium

A. Property 'length' may not exist on type 'T'

B. No error, code is correct

C. Return type should be number, not T

D. Default generic type cannot be 'number'

Solution

1. Step 1: Analyze the generic type and usage

   The generic type T defaults to number, but number does not have a length property.

2. Step 2: Identify the error

   Accessing arg.length causes a TypeScript error because length may not exist on type T.

3. Final Answer:

   Property 'length' may not exist on type 'T' -> Option A

4. Quick Check:

   Accessing unknown properties on generic types causes errors [OK]

Hint: Check if generic type supports accessed properties [OK]

Common Mistakes:

• Ignoring property existence on generic types
• Thinking default generic restricts properties
• Assuming no error if default is number

5.

Given this generic function with a default type, how can you call it to get an array of booleans?

function createArray<T = string>(length: number, value: T): T[] {
  return Array(length).fill(value);
}

Choose the correct call:

hard

A. createArray<boolean>(true, 3)

B. createArray(3, 'true')

C. createArray<number>(3, true)

D. createArray<boolean>(3, true)

Solution

1. Step 1: Understand default generic and call signature

   The function defaults T to string, but to get an array of booleans, we must specify T = boolean.

2. Step 2: Analyze options

   createArray<boolean>(3, true) explicitly sets T to boolean and passes true as value, which matches the type.

3. Final Answer:

   createArray<boolean>(3, true) -> Option D

4. Quick Check:

   Specify generic type to override default [OK]

Hint: Specify generic type to override default generic [OK]

Common Mistakes:

• Relying on default generic without specifying type
• Passing arguments in wrong order
• Mismatching generic type and value type