Add P.object, P.object.empty and P.object.exact()

[gvergnaud]

TODO

• add unit tests and doc for P.object.exact({...})
• Make sure P.object.exact({ prop: P.select() }) works, both for type inference and runtime.

P.object predicates

P.object is itself a pattern, but also a module containing predicates related to object types.

P.object

P.object matches any value assignable to the object TypeScript type. This includes all object literals, but also arrays and functions!

P.object does not match primitive types, like strings or numbers.

import { match, P } from 'ts-pattern';

const fn = (input: unknown) =>
  match(input)
    .with(P.object, () => '✅')
    .otherwise(() => '❌');

console.log(fn({})); // ✅
console.log(fn({ hello: 'world!' })); // ✅
console.log(fn([])); // ✅
console.log(fn(() => {})); // ✅

console.log(fn(1, true, 'hi')); // ❌ ❌ ❌

P.object.empty()

P.object.empty() matches the empty object {}:

import { isMatching, P } from 'ts-pattern';

console.log(isMatching(P.object.empty(), {})); // true

P.object.empty() does not match empty arrays, 0 values or nullish values:

import { isMatching, P } from 'ts-pattern';

console.log(isMatching(P.object.empty(), [])); // false
console.log(isMatching(P.object.empty(), 0)); // false
console.log(isMatching(P.object.empty(), null)); // false
console.log(isMatching(P.object.empty(), undefined)); // false

[Lezzio]

Very nice! Looking forward to using this new pattern

[yunsii]

Any plan for this PR?

[nigeltroy]

I know this MR has been open for a while, but I was wondering - would users of this pattern expect P.object to match with anything that matches a JS object, or expect it to match with a variable that is a Record type, i.e. always a key-value map? I use the following predicate extensively for example:

/** Check if the value provided is a key-value object (Record, not array, string, etc.) */
export const isRecord = (
  value: unknown,
): value is Record<string | number | symbol, unknown> => {
  return Boolean(value?.constructor.name === 'Object');
};

Here, arrays and nulls return false while Records return true. Personally, my mind gravitates toward using P.array as an Array matcher and a potential P.object as a Record matcher. What are your thoughts? Maybe something like this would be more suited towards a potential P.record matcher (which I could open a PR for if needed).