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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 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).

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/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).

• javascript
• types
• typescript

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1 Answer 1

Sorted by: Reset to default 8

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

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