Optional properties can be left out. If so, their type (if checked) is undefined.
function printCar(car: {
make: string;
model: string;
year: number;
chargeVoltage?: number; // 'number | undefined'
}) {
// ...
}
// Works
printCar({
make: 'Honda',
model: 'Accord',
year: 2017,
});
// Also works
printCar({
make: 'Tesla',
model: 'Model 3',
year: 2020,
chargeVoltage: 220,
});However, just b/c the type of chargeVoltage is number | undefined doesn't mean you can directly type it that way and get the same result. The property is only optional if you use the ?
function printCar(car: {
make: string;
model: string;
year: number;
chargeVoltage: number | undefined; // <- no longer optional
}) {
// ...
}
// ERRORS !!
printCar({
make: 'Honda',
model: 'Accord',
year: 2017,
});
// Error: Property 'chargeVoltage' is missing in type ...
// You would have to include it as 'undefined'
printCar({
make: 'Tesla',
model: 'Model 3',
year: 2020,
chargeVoltage: undefined, // <- always needs to be included
});From the TypeScript Fundamentals, v3↗ course on FEM↗ taught by Mike North↗.