Typescript intersection and union types

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Intersection and union types allow flexible use of types, especially if the properties change. An intersection type combines different types and properties into one type, while a union type represents a type that can be one of multiple types.

If a function is using a union type and you want to ensure you’re working with a particular type, you can use type guards. Type guards ensure type safety by telling the compiler if an object is of a particular type.

interface EmployeeBasic {
    name: string;
    age: number;
}

const emp: EmployeeBasic = {
    name: "Buzz",
    age: 40,
};

// Intersection type
// GovernmentEmployee combines the properties of EmployeeBasic and another GovtBranchNo property
type GovernmentEmployee = EmployeeBasic & { GovtBranchNo: string };
const govtEmp: GovernmentEmployee = {
    ...emp,
    GovtBranchNo: "123",
};

// PrivateEmployee combines the properties of EmployeeBasic and another CompanyNo property
type PrivateEmployee = EmployeeBasic & { CompanyNo: string };
const privEmp: PrivateEmployee = {
    ...emp,
    CompanyNo: "456",
};

// Union type
// Employee can be of type GovernmentEmployee or PrivateEmployee
type Employee = GovernmentEmployee | PrivateEmployee;

function processEmployee(emp: Employee) {
    if (isGovernmentEmployee(emp)) {
        console.log(emp.GovtBranchNo);
    } else if (isPrivateEmployee(emp)) {
        console.log(emp.CompanyNo);
    }
}

function isGovernmentEmployee(emp: Employee): emp is GovernmentEmployee {
    return (emp as GovernmentEmployee).GovtBranchNo !== undefined;
}

function isPrivateEmployee(emp: Employee): emp is PrivateEmployee {
    return (emp as PrivateEmployee).CompanyNo !== undefined;
}

processEmployee(govtEmp); // 123
processEmployee(privEmp); // 456