Extract type in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

It creates a new type by selecting from type T only those types that are assignable to type U. Think of it as filtering T to keep only the parts that fit U.

beginner

How would you use Extract<string | number | boolean, number | boolean>?

This extracts the types from string | number | boolean that are assignable to number | boolean. The result is number | boolean.

intermediate

Why is Extract<T, U> useful in real code?

It helps you narrow down types when you have a union and want to keep only certain parts. For example, when filtering event types or extracting specific keys from a union of string literals.

intermediate

What happens if no types in T are assignable to U in Extract<T, U>?

The result is never, meaning no types match the condition.

beginner

Show a simple example of Extract<T, U> with string literal types.

Example: <br>type T = 'a' | 'b' | 'c';<br>type U = 'a' | 'c' | 'd';<br>type Result = Extract<T, U>; // 'a' | 'c'<br>This keeps only the common string literals between T and U.

What is the result of Extract<string | number | boolean, number>?

A number

B string | number | boolean

C string

D boolean

Extract keeps only types from the first union assignable to the second. Only number fits.

If T has no types assignable to U, what does Extract<T, U> return?

A never

B unknown

C any

D void

When no types match, Extract returns never, meaning no possible value.

Which utility type is the opposite of Extract<T, U>?

A Omit&lt;T, U&gt;

B Pick&lt;T, U&gt;

C Exclude&lt;T, U&gt;

D Partial&lt;T&gt;

Exclude<T, U> removes types assignable to U from T, opposite of Extract.

What does Extract<'a' | 'b' | 'c', 'b' | 'd'> evaluate to?

A never

B 'a' | 'b' | 'c' | 'd'

C 'd'

D 'b'

Only 'b' is common and assignable to both unions.

Can Extract<T, U> be used with non-union types?

A No, it causes a compile error

B Yes, it works the same way

C Yes, but it always returns never

D No, it only works with unions

Extract works with any types, but is most useful with unions to filter types.

Explain in your own words what the Extract<T, U> type does and give a simple example.

Think about keeping only the parts of T that fit inside U.

How can Extract<T, U> help when working with union types in TypeScript?

Consider when you want to keep only some options from a bigger set.