Object Types and Type Aliases

Introduction

Most real data is an object: a user, a product, a lesson. TypeScript lets you describe the shape of an object, which properties it has and what type each one is. Then typos like user.nmae become errors instead of silent undefined values.

Object types

TypeScript checks every property you read or write. Misspell one and you find out immediately.

Optional and readonly properties

• A ? after a property name makes it optional.
• readonly means it can be set when the object is made, but not changed afterwards.

Type aliases

Writing the same object type again and again is tiring. A type alias gives a type a name, using type. By convention names start with a capital letter.

An alias can name any type, not just objects:

Interfaces

An interface is another way to name an object shape. For simple objects it works the same:

Which should you use? For now, either is fine. A common guideline: use interface for object shapes that other code might extend, and type for everything else, such as unions. The next course covers the differences.

Arrays of objects

Combine an alias with [] to type a list of objects. The array methods then know what is inside.

Hover-style help in editors shows names is string[]. TypeScript worked that out from item.name.

Try it yourself

Create a type alias Book with a title (string), an author (string), a pages (number) and an optional isbn (string). Then fix the three books so that they match, and print the total number of pages.

Summary

• An object type lists property names and types: { name: string; age: number }.
• ? marks a property optional, and readonly stops it being changed.
• type and interface both give a shape a reusable name.
• Product[] types a list of objects, and array methods then know their contents.