Default generic types in Typescript - Time & Space Complexity

Time Complexity: Default generic types

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run code with default generic types changes as the input grows.

How does using default types affect the work done when the code runs?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function wrapInArray<T = number>(item: T): T[] {
  return [item];
}

const result1 = wrapInArray(5);       // Uses default type number
const result2 = wrapInArray('hello'); // Uses string type explicitly

This code defines a function with a default generic type and calls it with different inputs.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Creating a new array with one item.
• How many times: Once per function call.

How Execution Grows With Input

Each call creates a single array with one element, so the work grows directly with how many times the function is called.

Input Size (n)	Approx. Operations
10	10 array creations
100	100 array creations
1000	1000 array creations

Pattern observation: The work grows in a straight line with the number of calls.

Final Time Complexity

Time Complexity: O(n)

This means the time grows directly in proportion to how many times the function is called.

Common Mistake

[X] Wrong: "Using default generic types makes the function slower or more complex."

[OK] Correct: The default generic type only sets a type when none is given; it does not add extra work when the function runs.

Interview Connect

Understanding how default generic types affect performance helps you explain your code choices clearly and shows you know how types relate to runtime work.

Self-Check

"What if the function created an array of size n instead of just one item? How would the time complexity change?"

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