TypeScript Object Types

Objects are the fundamental way data is grouped in JavaScript. TypeScript provides several modifiers and patterns to describe them accurately.

Property Modifiers

Optional Properties (?): Properties that may or may not be present (e.g., { x?: number }). Always check for undefined when reading.
readonly Properties: Properties that cannot be re-assigned after initialization (e.g., { readonly id: string }). Note: This doesn't make the value itself immutable if it's an object.

Used when the names of properties aren't known ahead of time but their values' types are.

interface StringArray {
  [index: number]: string;
}

Constraint: All named properties must match the index signature's return type (or a union that includes it).

TypeScript triggers errors when an object literal is assigned to a type and contains properties not defined in that type.

Workarounds: Use a type assertion (as), add an index signature, or assign the literal to an intermediate variable first.

Extending Interfaces (extends): Copy members from other interfaces to build complex types. Supports multiple inheritance.
Intersection Types (&): Combine existing types. Conflicting properties result in a never type if they are incompatible.

Create reusable container types by using type parameters.

interface Box<Type> {
  contents: Type;
}

Array<T>: Standard array (shorthand T[]).
ReadonlyArray<T>: Arrays that cannot be mutated (shorthand readonly T[]).
Tuples: Arrays with fixed length and specific types at each position (e.g., [string, number]). Supports optional (?) and rest (...) elements.