What's the difference between ?: and | undefined in TypeScript?
One difference is that cost: number | undefined; requires the property to exist, and have a value whose type is number or undefined. In contrast, cost?: number permits the property to not exist at all.
This fails to compile:
interface Fruit { cost: number | undefined;}const x: Fruit = {};For it to work, you'd have to do:
interface Fruit { cost: number | undefined;}const x: Fruit = { cost: undefined };But this succeeds:
interface Fruit { cost?: number;}const x: Fruit = {};Explicitly typing undefined can be tedious when an alternative is available, so you'll probably prefer the cost?: number option.