Typescript: valueof field of an array of objects

Question

Is there a way to define a type by the values of a field in an array?

To simplify my question... Given that I have a static array:

const arr = [
  {name: 'cat', language: 'meow'},
  {name: 'dog', language: 'bark'},
  {name: 'cow', language: 'moo'}
]

And I want to be able to create a function that gets the language based on the name field defined like so

function getLanguage(name: X) {

What can I define the type X to be so that I can limit X (the parameter of that function) to be any of the name fields in that list of objects?

Does this answer your question? Is it possible to have dynamic typing based on object properties? — bugs
– bugs, Commented Jan 30, 2020 at 15:51

Maciej Sikora · Accepted Answer · 2020-01-30 16:03:24Z

9

const arr = [
  {name: 'cat', language: 'meow'},
  {name: 'dog', language: 'bark'},
  {name: 'cow', language: 'moo'}
] as const // pay attention here

type Name = (typeof arr)[number]['name']; // evaluates into 'cat' | 'dog' | 'cow'

function getLanguage(name: Name) { }

Few things to understand:

as const in order to say to TS that type should be narrowed to exact values
(typeof arr)[number] - it gives as array element type, as number is a key of array mapped type
(typeof arr)[number]['name'] - it gives type of name field of array element type

If you don't need to narrow the type and string satisfy you for the name field then simply not append as const, in result (typeof arr)[number]['name'] will be evaluated to string type.

answered Jan 30, 2020 at 16:03

Maciej Sikora

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