Extract type in Typescript - Step-by-Step Execution

Concept Flow - Extract type

Define union type

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Use Extract<Type, Union>

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Check each member of Type

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Keep members assignable to Union

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Result: new union with extracted members

Extract type takes two types and keeps only those members from the first type that are assignable to the second.

Execution Sample

Typescript

type Fruit = "apple" | "banana" | "orange";
type Citrus = Extract<Fruit, "orange" | "lemon">;
// Citrus is "orange"

Extracts only the members from Fruit that are also in the union "orange" | "lemon".

Execution Table

Step	Member Checked	Is Member in Extract Type?	Reason	Resulting Extract Type
1	"apple"	No	"apple" is not assignable to "orange" | "lemon"	"" (empty)
2	"banana"	No	"banana" is not assignable to "orange" | "lemon"	"" (empty)
3	"orange"	Yes	"orange" is assignable to "orange" | "lemon"	"orange"
4	End	N/A	All members checked	"orange"

💡 All union members checked; only "orange" is assignable to "orange" | "lemon".

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
ExtractedType	"" (empty)	"" (empty)	"" (empty)	"orange"	"orange"

Key Moments - 3 Insights

Why is "apple" not included in the Extract type?

"apple" is not assignable to the union "orange" | "lemon", so it is excluded as shown in execution_table row 1.

What happens if none of the members match the Extract condition?

The resulting Extract type becomes empty (never), as shown after checking "apple" and "banana" in rows 1 and 2.

Can Extract be used with non-union types?

Extract works by checking assignability; if the second type is not a union, it returns the type if assignable, else never.

Visual Quiz - 3 Questions

Look at the execution_table, what is the Extract type after checking "banana"?

A "" (empty)

B "orange"

C "banana"

D "apple" | "banana"

💡 Hint

Check execution_table row 2 under 'Resulting Extract Type' column.

At which step does the Extract type become "orange"?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

See execution_table row 3 where "orange" is included.

If we change the Extract to Extract<Fruit, "banana" | "lemon">, what would the Extract type be?

A "lemon"

B "orange"

C "banana"

D "banana" | "lemon"

💡 Hint

Only members from Fruit assignable to "banana" | "lemon" are kept; "lemon" is not in Fruit.

Concept Snapshot

Extract<Type, Union>
- Takes two types
- Keeps members of Type assignable to Union
- Result is a new union type
- Useful to filter union members
- If no match, result is never

Full Transcript

The Extract type in TypeScript takes two types: a type to extract and a union type. It checks each member of the union and keeps only those assignable to the second type. For example, given Fruit as "apple" | "banana" | "orange" and Extract<Fruit, "orange" | "lemon">, it checks each Fruit member. "apple" and "banana" are not assignable to "orange" | "lemon", so they are excluded. "orange" is assignable, so it is included. The final Extract type is "orange". This process is shown step-by-step in the execution table and variable tracker. Common confusions include why some members are excluded and what happens if no members match. Extract is useful to filter union types based on assignability.