What are optional properties and parameters in TypeScript?

Optional properties in interfaces and type aliases are marked with a ? after the property name: interface User { name: string; email?: string; } — email may or may not be present. When accessing optional properties, TypeScript requires checking for undefined first (unless using optional chaining user.email?.toUpperCase()). Optional function parameters are also marked with ?: function greet(name: string, greeting?: string): string { return (greeting ?? "Hello") + ", " + name; }. Optional parameters must come after required parameters. Default parameters are similar but provide a fallback value: function greet(name: string, greeting = "Hello"): string { ... } — TypeScript infers the parameter is optional and of the default value's type. The difference: optional parameters can be explicitly passed as undefined; default parameters also handle undefined by substituting the default.