learn-typescript

TypeScript Object Types

TypeScript has a specific syntax for typing objects.

Read more about objects in our JavaScript Objects chapter.

Example:

const car: { type: string; model: string; year: number } = {
  type: "Toyota",
  model: "Corolla",
  year: 2009,
};

Object types like this can also be written separately, and even be reused, look at interfaces for more details.

Type Inference

TypeScript can infer the types of properties based on their values.

Example:

const car = {
  type: "Toyota",
};
car.type = "Ford"; // no error
car.type = 2; // Error: Type 'number' is not assignable to type 'string'.

Optional Properties

Optional properties are properties that don’t have to be defined in the object definition.

Example without an optional property:

const car: { type: string; mileage: number } = {
  // Error: Property 'mileage' is missing in type '{ type: string; }' but required in type '{ type: string; mileage: number; }'.
  type: "Toyota",
};
car.mileage = 2000;

Example with an optional property:

const car: { type: string; mileage?: number } = {
  // no error
  type: "Toyota",
};
car.mileage = 2000;

Index Signatures

Index signatures can be used for objects without a defined list of properties.

Example:

const nameAgeMap: { [index: string]: number } = {};
nameAgeMap.Jack = 25; // no error
nameAgeMap.Mark = "Fifty"; // Error: Type 'string' is not assignable to type 'number'.

Index signatures like this one can also be expressed with utility types like Record<string, number>. Learn more about utility types like this in our TypeScript Utility Types chapter.