What is the difference between interface and type?

Q: What is the difference between interface and type?

**Target Audience:** Beginners who have seen both keywords in the same codebase and want to know when the choice matters. **Difficulty:** Easy **TypeScript version:** Every error and output below comes from TypeScript 5.4.5 with `--strict`; release and guidance facts are cited. > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Every compiler message and output was produced by actually running the code; the **Verified** section quotes it in full. ## 1. Why This Even Matters — A Story First A city has two kinds of paperwork for buildings. A **registered blueprint** (an `interface`) is filed under a name at the city office. Anyone can later file an *amendment* under the same name ("add a balcony") and the office staples it to the original. A **sticky label** (a `type` alias) is a nickname you write on any drawing at all: a single house, a choice between "house or f

The problem

Question presented to candidate: "In TypeScript you can write interface User { ... } or type User = { ... }. What is actually different between the two, and how do you decide which one to use?"

What a strong answer should cover:

For a plain object shape the two are interchangeable: same checking, both erased at run time, a class can implements either
Only type can name a union, a primitive, a tuple, a mapped or a conditional type; interface only describes object (and function or constructor) shapes
Extending: interface X extends Y reports a conflicting member as an error (TS2430), while Y & { ... } silently produces never for that member
Declaration merging: two interface declarations with the same name merge (used for module augmentation and global types); two type aliases with the same name are a duplicate identifier error (TS2300)
The subtle one: an object type alias is assignable to Record<string, T> (implicit index signature), an interface is not
A sensible team rule (interfaces for object shapes you may extend, type for unions and computed types) and the performance note: the TypeScript wiki prefers extends over large intersections

Code / implementation expected: Optional — the same shape written both ways, plus a union that only type can express.

types

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

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A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners who have seen both keywords in the same codebase and want to know when the choice matters. Difficulty: Easy TypeScript version: Every error and output below comes from TypeScript 5.4.5 with --strict; release and guidance facts are cited.

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. Every compiler message and output was produced by actually running the code; the Verified section quotes it in full.

1. Why This Even Matters — A Story First

A city has two kinds of paperwork for buildings. A registered blueprint (an interface) is filed under a name at the city office. Anyone can later file an amendment under the same name ("add a balcony") and the office staples it to the original. A sticky label (a `type

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9 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

TypeScriptThe same shape both ways, unions, merging and conflicts

Practice

TypeScriptImplicit index signatures: why an interface does not fit Record

Practice

Discussion

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