Argument of type '"test"' is not assignable to parameter of type 'never'?

Question

typescript

export interface IAuth {
    roles: string[];
}

@Module({namespaced: true})
export default class Auth extends VuexModule implements IAuth {
    roles = [];

    public get addRole(role: string): void {
        this.roles.push('test')

it is solved if we add the data type again: string[]

export default class Auth extends VuexModule implements IAuth {
    roles: string[] = [];

but the meaning of the interface then disappears

Answer 1

Initially, type inference determines whether a field belongs roles = [];to a type, never[]which does not lead to an error associated with the implementation of the interface, since the never type is a subtype of all types in typescript. In simple words, in your case, the usual upcasting occurs. If this behavior were different then subtypes would not be compatible with their supertypes.