Deep dive into type manipulation in typescript?

Inline type parameter declaration

List pattern like

cs

Span<int> span = Enumerable.Range(1, 100).ToArray().AsSpan();
Random.Shared.Shuffle(span);
var condition = span is [var first, .. var _]; // always true

ts

type First<T extends any[]> = T extends [infer TFirst, ...infer _] ? TFirst : never;
type A = First<[1, 2, 3]>; // type A = 1;
type B = First<[null, 'a', 'b']>; // type B = null;
type C = First<[]>; // type C = never

Property pattern like

cs

var condition = new[] { 1, 2, 3 } is { Length: var length };

ts

type Length<T extends any[]> = T extends { length: infer TLength } ? TLength : never;

• typescript uses infer to declare a type parameter inline, while C# uses var to declare a inline variable.
• Two language both uses _ as discard symbol.

Loop over a type

Since a type is a set, we can loop over it in conditional clause, however, implicitly. Let's implement Exclude<T, U> from scratch.

type MyExclude<T, U> = T extends U ? never : T;
type Union = 'a' | 'b' | 'c';
type A = MyExclude<Union, 'c'>; // 'a' | 'b'
type B = MyExclude<Union, 'b' | 'c'>; // 'a'

If interpret it as a loop using collection to represent a type union, it should be equivalent to

const T = ['a', 'b', 'c'];
const U = ['b', 'c'];
const Excluded: string[] = [];
T.forEach(t => {
    if (!U.some(u => u === t)) {
        Excluded.push(t);
    }
});

Well, equality checked on type level though.