You describe the shape of objects. A type alias lists each property and its type, and the compiler reports a missing property, a property that doesn't exist, and an extra property that is really a typo. You compare interfaces with type aliases, extend an interface and combine types with an intersection. Then you mark properties optional or read-only, and read values that might be missing with optional chaining and nullish coalescing.

You let a value be one of several types and learn how the compiler follows your checks. A union of string and number accepts both, but a string method fails on it until you narrow it. You narrow with typeof, with in and instanceof, and with truthiness and equality checks. Then you give each shape a shared tag field, switch on it, and use the never type so the compiler reports a case you forgot to handle.

You handle values that might not be there. Strict null checks report a lookup that can come back undefined, array indexing is unchecked by default, and checked index access closes that gap. You compare the any type, which switches checking off, with unknown, which must be narrowed before use, and use never for impossible states. Finally you model a typed shopping cart with unions and objects, and prove it with its contract check.