TypeScript · Lesson 3 of 7

Interfaces, type aliases & objects

Describing object shapes, optional and readonly properties, and interface vs type.

An interface or a type alias describes the shape of an object — which properties it has and their types.

? marks an optional property: remote?: boolean.
readonly stops a property being changed after creation.
Interfaces can extends other interfaces; type aliases combine with & (intersection).

interface vs type: both describe objects. Interfaces can be extended and merged; type aliases can also describe unions, tuples and primitives. Many teams use interfaces for objects and types for everything else — the important thing is to be consistent.

Example code

Try it in the TypeScript Playground or with tsc

interface Job {
  readonly id: number;
  title: string;
  city: string;
  salaryLpa?: number;          // optional
}

interface RemoteJob extends Job {
  timezone: string;
}

const job: Job = { id: 1, title: "Frontend Developer", city: "Kochi" };
job.title = "Senior Frontend Developer";  // OK
job.id = 2;                                // error

const remote: RemoteJob = { id: 3, title: "QA", city: "Remote", timezone: "IST" };
console.log(job.title, "|", remote.timezone);

Compiler / program output

error TS2540: Cannot assign to 'id' because it is a read-only property.

(after removing that line)
Senior Frontend Developer | IST

Paste the code into the official TypeScript Playground in your browser, or install TypeScript (npm install -D typescript) and run npx tsc --noEmit to type-check. Names are fictional.

Key points

Interfaces and type aliases describe object shapes.
? = optional, readonly = can't be reassigned.
Interfaces extend; types combine with &.
Pick one convention and use it consistently.

Quick check

What does salaryLpa?: number mean?

salaryLpa must be a number salaryLpa may be missing; if present it's a number salaryLpa is read-only salaryLpa can be any type

Practice task

Create an interface Candidate with name, email, skills (string array) and an optional portfolioUrl, then write a function that returns a one-line summary of a candidate.

Interview corner

What is the difference between an interface and a type alias?

Both describe object shapes. Interfaces support extends and declaration merging; type aliases can also name unions, tuples and primitives and combine with intersections. For plain objects they're largely interchangeable.

What does readonly do?

It prevents reassigning a property after the object is created, at compile time. It doesn't freeze the object at runtime.