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TypeScript Object Types

TypeScript has a specific syntax for typing objects

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.

TypeScript Exercises

Test Yourself With Exercises

Exercise:

Add the correct types for the object below:

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

Start the Exercise

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