Ensuring the type of a specific key in an object

Seeking a more stringent approach regarding object keys in TypeScript.

type UnionType = '1' | '2' | '3'
type TypeGuardedObject = { [key in UnionType]: number }

const exampleObject: TypeGuardedObject = {
  '1': 1,
  '2': 2,
  '3': 3
}

// This is acceptable since '1' is part of the type UnionType.
const test1 = exampleObject['1']

// This should be flagged as an error because the key is a string not included in UnionType,
// yet it passes TypeScript build successfully.
const wrongKey: string = 'bad string'
const test2 = exampleObject[wrongKey]

Is there a way to prevent the second example from passing the TypeScript build?

Thank you

Answer №1

One of the main reasons TypeScript supports indexing into objects is not related to unions. By default, TypeScript assumes that all objects have an index property unless you enable the compiler option noImplicitAny. Turning on this option will reveal potential errors in your code, which ultimately enhances type safety. Without this setting, you may not even realize the implicit typing issues present in your code.

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