Flash cards
Review the key moves
What is the main idea behind TypeScript Object Types?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
___ car: { type: string, model: string, year: number } = {Put the learning moves in the order that makes the concept easiest to apply.
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.