isKeyOf

See source on Github 

Checks whether a given key exists directly on an object.

This only matches own properties and does not consider keys inherited through the prototype chain.

Use this when you have a computed key and need TypeScript to narrow that key to keyof T for safe indexed access. This is especially useful in cases where key in obj confirms the runtime check but does not narrow the key enough for obj[key] to type-check.

• @param obj - The object to check.
• @param key - The property key to look up.
• @returns true if key is an own key of obj, false otherwise.

Example

const reports = {
  reportDaily: { total: 1 },
  reportWeekly: { total: 7 },
}
 
const period = 'Daily' as 'Daily' | 'Weekly' | 'Monthly'
const reportKey = `report${period}` as const
 
if (isKeyOf(reports, reportKey)) {
  reports[reportKey] // type-safe
}