Function property

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
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However, it also works correctly with dictionary types:

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
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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
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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
Copy

However, it also works correctly with dictionary types:

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
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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
Copy