Key Inferences for TypeScript Objects

// A
export const config: Record<string, number> = {
  a: 1,
  b: 2,
} as const

export const test = (configKey: keyof typeof config) => {
  console.log(configKey)
}

test('') // key not deduced

// B
type A = 'a' | 'b'

export const config: Record<A, number> = {
  a: 1,
  b: 2,
} as const

export const test = (configKey: keyof typeof config) => {
  console.log(configKey)
}

test('') //Key inference ok

My obj has a type.
The current value is number for example, but in reality
It’s more complicated.

For A, key inference is not possible when you want to use a function that receives only obj’s key.
Of course, B works well.
However, I have no choice but to use A’s method considering the following.

1. When writing the inside of obj, get help from the value of the record type.
2. When fixing the inside of obj, it should not be fixed anywhere else.(Key must be fixed for b)

Therefore, I have to use A’s obj.
But in this case, I want to know how to infer the key of A’s obj.

>Solution :

When you add a type annotation to the config it ignores your const assertion, thus you need to remove the type, but in that case, the type-checking doesn’t work.

Fortunately, the satisfies operator, which was introduced in the typescript 4.9 assures the type safety:

export const config = {
  a: 1,
  b: 2,
} as const satisfies Record<string, number>;

Testing:

test(''); // error
test('a'); // no error
test('b'); // no error

playground