Constraints and defaults in TypeScript

A type parameter can be bounded, and can have a default.

interface Paged<Item, Meta extends object = { page: number }> {
  rows: Item[];
  meta: Meta;
}

function longest<T extends { length: number }>(a: T, b: T): T {
  return a.length >= b.length ? a : b;
}

function makeStore<Value, Key extends string = string>() {
  const rows = new Map<Key, Value>();
  return {
    set(key: Key, value: Value) {
      rows.set(key, value);
      return rows.size;
    },
    get: (key: Key) => rows.get(key),
  };
}

const page: Paged<string> = { rows: ["a"], meta: { page: 1 } };
const store = makeStore<number, "a" | "b">();
store.set("a", 1);
console.log(longest([1, 2], [1]), page.meta.page, store.get("a"));

How it works

1. extends bounds what may be passed.
2. = T supplies a default when it cannot be inferred.
3. Constraints are what let the body use the parameter.

Keywords and builtins used here

• Item
• Key
• Map
• Meta
• Paged
• T
• Value
• b
• const
• extends
• function
• interface
• number
• return
• string

The run, in numbers

Lines

24

Characters to type

609

Tokens

216

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 130 seconds.

Type this snippet

Step 2 of 4 in Type parameters, step 2 of 20 in Generics.

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