Union and UnionType

[stanislav-zavistnov]

Is there a difference between Union and UnionType in typescript documentation? For example: https://www.typescriptlang.org/docs/handbook/utility-types.html#excludeuniontype-excludedmembers https://www.typescriptlang.org/docs/handbook/utility-types.html#extracttype-union I'm new to Typescript and in the context of these examples I didn't see any difference, but I might have missed something.

[lucaimbalzano]

I'll provide a detailed explanation highlighting the key differences between Union and UnionType in TypeScript.

Union

A Union in TypeScript refers to the practical use of combining multiple types into one. When you define a union type (like string | number), you're creating a type that can hold values from either of those types. This is a feature that's commonly used in day-to-day TypeScript coding.

type MyUnion = string | number;

UnionType

UnionType refers to an internal concept in TypeScript's type system. It is not something you directly interact with when writing normal TypeScript code. Instead, UnionType comes into play when TypeScript handles union types behind the scenes, especially in more advanced or internal utilities like Exclude or Extract.

For example, let’s look at the source code for Exclude and Extract:

/**
 * Exclude from T those types that are assignable to U
 */
type Exclude<T, U> = T extends U ? never : T;

/**
 * Extract from T those types that are assignable to U
 */
type Extract<T, U> = T extends U ? T : never;

In both Exclude and Extract, the generic types T and U can be union types. The TypeScript compiler uses UnionType internally to evaluate how to exclude or extract specific types within those unions.

In Few Words

• Union: A union type is what you use in everyday programming to combine different types.
• UnionType: UnionType is the abstract, internal mechanism that TypeScript uses to handle unions in operations like conditional types.

Example

type MyUnion = string | number;
type ExcludeNumber = Exclude<MyUnion, number>; // Result: string
type ExtractString = Extract<MyUnion, string>; // Result: string

Let me know if is more clear now 👋

[jakebailey]

To be clear, the names given on the linked doc pages are just the names of type variables. They don't have anything to do with the internal concepts of the compiler itself, they're just there to tell you what the type variables of the helper type mean.