TS & Angular: Unlocking the Power of Conditional Interfaces

My user component includes a variable called user, which can be either an Employee or a Student. In my HTML, I have an element

{{ user.coure ?? user.department }}

I'm encountering an issue in my HTML because some properties in the Employee interface are not available in the Student interface. I want to avoid using the "any" type or having separate variables for student and employee. What is the recommended solution?

Here is an example of the interface I am working with:

// user.interface.ts
export BaseUser {
    id: number;
    firstName: string;
    middleName: string;
}

export Employee extends BaseUser {
    department: string;
}

export Student extends BaseUser {
    course: string;
}

Answer №1

Implementing a custom type guard function is a great solution

const isStudent = (user: Employee | Student): user is Student => {
  return "course" in user
}

const isEmployee = (user: Employee | Student): user is Employee => {
  return "department" in user
}

console.log(
  isStudent(user) ? user.course : user.department
)

Feel free to use either isStudent or isEmployee depending on your needs. The TypeScript compiler's control flow analysis will understand that user is of type Student when isStudent evaluates to true.

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