Today I Learned

Tricky difference between interfaces and types

types are indexed by the default whereas interfaces are not. It makes interfaces more safe.

Consider this example:

type Admin = {
    name: string,
    privileges: string[]
}

type Employee = {
    name: string,
    startDate: Date
}

type CombinedType = Admin & Employee;

interface CombinedInterface extends Admin, Employee { }

const interfaceData: CombinedInterface = { name: '', privileges: [], startDate: new Date() }
const typeData: CombinedType = { name: '', privileges: [], startDate: new Date() }


const handleType = (obj: Record<string, unknown>) => {
    obj['name'] = 42 // unsafe
}


handleType(interfaceData) // error
handleType(typeData) // ok

Source: https://stackoverflow.com/questions/72598769/intersection-types-vs-interfaces-in-typescript