Objects and Type Aliases

Learning objectives#

• Write inline object types
• Name shapes with type aliases
• Use optional (?) and readonly properties
• Describe dictionaries with index signatures and Record

Explanation#

An object type lists properties and their types: { name: string; age: number }.

type User = { … } creates a reusable name (an alias). Aliases can name any type, not just objects: type ID = string | number.

Optional properties end with ?: bio?: string means bio can be missing (its type becomes string | undefined).

readonly id: number forbids changing that property after creation.

For objects with unknown keys use an index signature { [key: string]: number } or the simpler Record<string, number>.

TypeScript is structural: any object with the right shape fits the type, regardless of how it was created. Extra properties are flagged only on fresh object literals.

Syntax#

1type Product = {
2  readonly id: number;
3  name: string;
4  description?: string;
5};

Code example: Modelling a product catalogue#

1type ID = number;
2

3type Price = {
4  amount: number;
5  currency: 'INR' | 'USD';
6};
7

8type Product = {
9  readonly id: ID;
10  name: string;
11  price: Price;            // nested type
12  description?: string;    // optional
13  tags: string[];
14};
15

16const kurta: Product = {
17  id: 1,
18  name: 'Cotton Kurta',
19  price: { amount: 799, currency: 'INR' },
20  tags: ['clothing'],
21};
22

23kurta.id = 2;                         // ✗ Cannot assign to 'id' because it is a read-only property
24kurta.description?.toUpperCase();    // ✓ safe: description may be undefined
25

26const stock: Record<string, number> = { kurta: 12, saree: 4 };
27stock['dupatta'] = 7;                 // ✓ any string key
28

29const bad: Product = { id: 3, name: 'Cap', price: { amount: 99, currency: 'INR' }, tags: [], colour: 'red' };
30//                                                                                           ~~~~~~
31// ✗ Object literal may only specify known properties, and 'colour' does not exist in type 'Product'

Two errors: changing the read-only id and the unknown colour property. The optional description is handled safely with ?.

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

1. type ID = number is an alias for a primitive, making intent clear.
2. Price is used inside Product — types compose.
3. readonly id cannot be reassigned.
4. description? may be missing, so we access it with ?.
5. Record<string, number> describes a dictionary of counts.
6. A typo-like extra property on a literal is reported.

Real-world use case#

Common mistakes#

Best practices#

• Name every shape you use more than once
• Use readonly for ids and data you must not change
• Prefer Record<K, V> over hand-written index signatures
• Keep types next to the code that owns them, or in a types.ts file

Remember this#

Practice exercise#

1type Address = {
2  street: string;
3  city: string;
4  pin: string;
5  landmark?: string;
6};
7

8type Customer = {
9  readonly id: number;
10  name: string;
11  addresses: Address[];
12};

Mini quiz#

Want more? Practise all TypeScript MCQs · TypeScript interview questions

Related topics#

My notes#

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