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TypeScript has a specific syntax for typing objects.
JavaScript Objects chapter.
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.
TypeScript can infer the types of properties based on their values.
const car = {
type: "Toyota",
};
car.type = "Ford"; // no error car.type = 2; // Error: Type 'number' is not assignable to type 'string'.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 can be used for objects without a defined list of properties.
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
Formula
Record < string, number >.Learn more about utility types like this in our TypeScript Utility Types chapter.