Native Generic for converting from Union to Intersection

[SephReed]

Suggestion

🔍 Search Terms

Union to Intersection, Generic

https://stackoverflow.com/questions/50374908/transform-union-type-to-intersection-type

✅ Viability Checklist

My suggestion meets these guidelines:

• [ x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• [ x] This wouldn't change the runtime behavior of existing JavaScript code
• [ x] This could be implemented without emitting different JS based on the types of the expressions
• [ x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• [ x] This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

A new generic UnionToIntersection<> which converts from a union to an intersection

📃 Motivating Example

There are 20k views and 250 upvotes on a Stack Overflow question asking to convert from a union to an intersection.

The top rated answer? Check it out:

type UnionToIntersection<U> = 
  (U extends any ? (k: U)=>void : never) extends ((k: infer I)=>void) ? I : never

Nobody knows how it works, and it makes debugging a nightmare. But now there is a utility type for it!

(I guess this is how I could announce it on a blog post)

💻 Use Cases

In my case, I'm chewing up massive amounts of resources and getting a The inferred type of this node exceeds the maximum length the compiler will serialize. An explicit type annotation is needed. message because of it.

I don't know what the other ~20k people felt who read that answer. Many were probably curious, but I'm sure many used it for other things.

The most basic example I can imagine is some function that takes an array of objects, then flattens them like so:

function smoosh<T extends object>(objs: Array<T>): UnionToIntersection<T> {
  let out = {};
  objs.forEach(obj => _.extend(out, obj));
  return out;
}

const eg = smoosh([{a: 1}, {b: 2}, {c: 3}]);
eg.a = 1;
eg.b = 2;
eg.c = 3;

[MartinJohns]

Duplicate of #29594. Used search terms: union intersection type in:title.

[SephReed]

@MartinJohns thanks