True Myth / maybe / property

Function: property()

Call Signature

property<T, K>(key, obj): Maybe<NonNullable<T[K]>>

Safely extract a key from an object, returning Just if the key has a value on the object and Nothing if it does not.

The check is type-safe: you won't even be able to compile if you try to look up a property that TypeScript knows doesn't exist on the object.

type Person = { name?: string };

const me: Person = { name: 'Chris' };
console.log(Maybe.property('name', me)); // Just('Chris')

const nobody: Person = {};
console.log(Maybe.property('name', nobody)); // Nothing

However, it also works correctly with dictionary types:

import * as maybe from 'true-myth/maybe';

type Dict<T> = { [key: string]: T };

const score: Dict<number> = {
  player1: 0,
  player2: 1
};

console.log(maybe.property('player1', score)); // Just(0)
console.log(maybe.property('player2', score)); // Just(1)
console.log(maybe.property('player3', score)); // Nothing

The order of keys is so that it can be partially applied:

type Person = { name?: string };

const lookupName = maybe.property('name');

const me: Person = { name: 'Chris' };
console.log(lookupName(me)); // Just('Chris')

const nobody: Person = {};
console.log(lookupName(nobody)); // Nothing

Type Parameters

T

T

K

K extends string | number | symbol

Parameters

key

K

The key to pull out of the object.

obj

T

The object to look up the key from.

Returns

Maybe<NonNullable<T[K]>>

Call Signature

property<T, K>(key): (obj) => Maybe<NonNullable<T[K]>>

Safely extract a key from an object, returning Just if the key has a value on the object and Nothing if it does not.

The check is type-safe: you won't even be able to compile if you try to look up a property that TypeScript knows doesn't exist on the object.

type Person = { name?: string };

const me: Person = { name: 'Chris' };
console.log(Maybe.property('name', me)); // Just('Chris')

const nobody: Person = {};
console.log(Maybe.property('name', nobody)); // Nothing

However, it also works correctly with dictionary types:

import * as maybe from 'true-myth/maybe';

type Dict<T> = { [key: string]: T };

const score: Dict<number> = {
  player1: 0,
  player2: 1
};

console.log(maybe.property('player1', score)); // Just(0)
console.log(maybe.property('player2', score)); // Just(1)
console.log(maybe.property('player3', score)); // Nothing

The order of keys is so that it can be partially applied:

type Person = { name?: string };

const lookupName = maybe.property('name');

const me: Person = { name: 'Chris' };
console.log(lookupName(me)); // Just('Chris')

const nobody: Person = {};
console.log(lookupName(nobody)); // Nothing

Type Parameters

T

T

K

K extends string | number | symbol

Parameters

key

K

The key to pull out of the object.

Returns

(obj): Maybe<NonNullable<T[K]>>

Parameters

obj

T

Returns

Maybe<NonNullable<T[K]>>