Extract type in Typescript - Time & Space Complexity

Time Complexity: Extract type

O(n)

Understanding Time Complexity

Let's see how the time it takes to run code using the Extract type changes as the input grows.

We want to know how the work done scales when extracting types from bigger sets.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

type Extract<T, U> = T extends U ? T : never;

// Example usage:
type Result = Extract<'a' | 'b' | 'c', 'a' | 'c'>;
// Result is 'a' | 'c'
    

This code picks types from T that are assignable to U, filtering the union types.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Checking each member of the union type T against U.
• How many times: Once for each member in the union T.

How Execution Grows With Input

Each additional type in T means one more check against U.

Input Size (n)	Approx. Operations
10	10 checks
100	100 checks
1000	1000 checks

Pattern observation: The work grows directly with the number of types in T.

Final Time Complexity

Time Complexity: O(n)

This means the time to extract types grows linearly with the number of types in the input.

Common Mistake

[X] Wrong: "Extract type runs instantly no matter how many types are in the union."

[OK] Correct: Each type in the union must be checked, so more types mean more work.

Interview Connect

Understanding how type operations scale helps you reason about code performance and complexity in real projects.

Self-Check

"What if we changed the Extract type to check against multiple types in U? How would the time complexity change?"

Practice

1. What does the TypeScript Extract<T, U> utility type do?

easy

A. It combines types T and U into a union type.

B. It removes types from T that are assignable to U.

C. It creates a new type by picking types from T that are assignable to U.

D. It converts a type T into a string literal type.

Solution

1. Step 1: Understand the purpose of Extract

   Extract<T, U> picks from T only those types that can be assigned to U.

2. Step 2: Compare with other utility types

   Unlike Exclude<T, U> which removes types, Extract keeps matching types.

3. Final Answer:

   It creates a new type by picking types from T that are assignable to U. -> Option C

4. Quick Check:

   Extract = picks matching types [OK]

Hint: Extract keeps only types from T assignable to U [OK]

Common Mistakes:

• Confusing Extract with Exclude
• Thinking Extract combines types
• Assuming Extract converts types to strings

2. Which of the following is the correct syntax to extract only string types from a union type U = string | number | boolean?

easy

A. type OnlyString = Extract;

B. type OnlyString = Extract;

C. type OnlyString = Extract;

D. type OnlyString = Extract;

Solution

1. Step 1: Identify the correct order of parameters

   Extract<T, U> takes the big union T first, then the type to pick U.

2. Step 2: Match the desired extracted type

   We want to pick string from U = string | number | boolean, so Extract<U, string> is correct.

3. Final Answer:

   type OnlyString = Extract<U, string>; -> Option A

4. Quick Check:

   Extract<Union, PickType> syntax [OK]

Hint: First param is full union, second is type to pick [OK]

Common Mistakes:

• Swapping the order of parameters
• Picking wrong type in second parameter
• Using Extract with unrelated types

3. Given the code:

type Mixed = string | number | boolean;
type Result = Extract;

What is the type of Result?

medium

A. string

B. never

C. string | number | boolean

D. number | boolean

Solution

1. Step 1: Understand the union types involved

   Mixed is string | number | boolean. We want to extract types assignable to number | boolean.

2. Step 2: Extract matching types

   From Mixed, only number and boolean match number | boolean. string is excluded.

3. Final Answer:

   number | boolean -> Option D

4. Quick Check:

   Extract picks matching types [OK]

Hint: Extract keeps only types assignable to second param [OK]

Common Mistakes:

• Including types not in second parameter
• Confusing Extract with union
• Thinking Extract returns never if partial match

4. Identify the error in this code snippet:

type A = string | number;
type B = Extract;

What will be the type of B?

medium

A. boolean

B. never

C. string | number

D. Error: incompatible types

Solution

1. Step 1: Analyze the Extract parameters

   A is string | number, and we try to extract boolean types.

2. Step 2: Determine matching types

   Since boolean is not assignable to string or number, no types match, so the result is never.

3. Final Answer:

   never -> Option B

4. Quick Check:

   No matching types means never [OK]

Hint: No matches in Extract means type is never [OK]

Common Mistakes:

• Expecting an error instead of never
• Assuming Extract returns original union
• Confusing Extract with Exclude

5. You have a union type type Data = string | number | null | undefined. You want to create a type that excludes null and undefined but only keeps string and number. Which Extract usage achieves this?

hard

A. type CleanData = Extract;

B. type CleanData = Extract;

C. type CleanData = Extract;

D. type CleanData = Extract;

Solution

1. Step 1: Identify the types to keep

   We want to keep only string and number, excluding null and undefined.

2. Step 2: Use Extract with correct second parameter

   Using Extract<Data, string | number> picks only string and number from Data.

3. Final Answer:

   type CleanData = Extract<Data, string | number>; -> Option A

4. Quick Check:

   Extract filters union by matching types [OK]

Hint: Extract with desired types filters out unwanted ones [OK]

Common Mistakes:

• Including null or undefined in second param
• Using Exclude instead of Extract
• Confusing Extract with union