How can I ensure the types of an object while keeping the key as a constant in TypeScript?

I'm currently working on a project where I have an object that needs to meet specific typing requirements. Here is the initial code snippet:

export const dateFormat = {
  hourOnly: { hour: 'numeric' }
  …
}

To ensure that the values in this object adhere to Intl.DateTimeFormatOptions, I attempted the following modification:

export const dateFormat: {[key: string]: Intl.DateTimeFormatOptions} = {
  hourOnly: { hour: 'numeric' }
  …
}

While this adjustment achieved the desired outcome, it caused me to lose access to auto-complete functionality for this object elsewhere in the project. Adding as const did not resolve this issue.

Is there a method to enforce the values of an object while retaining auto-complete for the keys?

I also experimented with the following approach:

type dateFormatOptions = 'hourOnly'

export const dateFormat: {[key: dateFormatOptions]: Intl.DateTimeFormatOptions} = {
  hourOnly: { hour: 'numeric' }
  …
}

Unfortunately, TypeScript alerted me that the index signature should be a string or a number in this scenario.

typescript

Answer №1

If you have the keys at hand, simply utilize Record to construct a type containing those specific keys for the desired type (referred to as a mapped type).

type dateFormatOptions = 'hourOnly'

export const dateFormat: Record<dateFormatOptions, Intl.DateTimeFormatOptions> = {
  hourOnly: { hour: 'numeric' }
}

This method does necessitate maintaining property names in two locations. Another approach involves using a function with a restricted generic type parameter. The constraint on the type parameter ensures that all values are of the specified type, while the actual keys will be automatically determined based on the object being provided.

function createDateFormatOptions<K extends PropertyKey>(o: Record<K, Intl.DateTimeFormatOptions>): Record<K, Intl.DateTimeFormatOptions> {
    return o;
}
export const dateFormat = createDateFormatOptions({
  hourOnly: { hour: 'numeric' },
})

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