typescript strong typing - specifying object value types

Question

In TypeScript, is it possible to specify allowed values in an Object? E.g. to specify that all keys should have numbers:

{
  'id': 1,
  'attr1': 124,
  'attr2': 4356,
  ...
}

?

I've searched through http://www.typescriptlang.org/Handbook and found out, that I can use Array typing (both keys and values) like this:

interface StringArray {
  [index: number]: string;
}

but actually, a Map (JS Object) and an Array is not the same, conceptually (in JavaScript, it behaves similarly, but in TypeScript, it should be treated separately because of the strong typing).

Answer 1

is it possible to specify allowed values in an Object? E.g. to specify that all keys should have numbers

Yes, this is possible.

In both JavaScript & TypeScript (which is a superset of JS) you can access properties via obj.prop or obj['prop'] which is what allows the syntax below to work.

// This defines an interface that only allows values to be numbers
interface INumbersOnly {
  [key: string]: number;
}

// when using it, it will check that all properties are numbers
var x: INumbersOnly = {
  num: 1, // works fine
  str: 'x' // will give a type error
};

Above example in TS Playground