What is structural typing? | Interview Question

Q: What is structural typing?

**Target Audience:** Beginners, and developers coming from Java or C# who expect class names to matter. **Difficulty:** Medium **TypeScript version:** Every error and output below comes from TypeScript 5.4.5 with `--strict`. > **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 power socket does not ask who made your plug. It does not check a brand logo or a certificate that says "I am a Type G plug". It checks the **shape**: three prongs, the right size, the right distance apart. Any plug with that shape fits — even one made by a company the socket designer never heard of. A plug with *extra* bits (a built-in night light) still fits, as long as the prongs are right. That is how TypeScript checks type

The problem

Question presented to candidate: "TypeScript is often described as structurally typed. What does that mean, how is it different from Java or C#, and where can it surprise you?"

What a strong answer should cover:

Structural typing: a value is assignable to a type when it has the required members with compatible types; names and implements clauses do not matter
Contrast with nominal typing (Java, C#), where a class must explicitly declare that it implements an interface
Extra members are allowed (width subtyping), except for the excess property check on fresh object literals
The surprises: two same-shaped types such as Celsius and Fahrenheit or UserId and OrderId are interchangeable; {} accepts almost anything
How to get nominal behaviour when you need it: branded types, or classes with private/#private members, which compare by declaration
Why TypeScript chose it: JavaScript code is duck-typed and objects are often plain literals, so shape is what really matters

Code / implementation expected: Optional — a class and an object literal that both satisfy an interface they never mention.

structural-typing

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Solution

Target Audience: Beginners, and developers coming from Java or C# who expect class names to matter. Difficulty: Medium TypeScript version: Every error and output below comes from TypeScript 5.4.5 with --strict.

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 power socket does not ask who made your plug. It does not check a brand logo or a certificate that says "I am a Type G plug". It checks the shape: three prongs, the right size, the right distance apart. Any plug with that shape fits — even one made by a company the socket designer never heard of. A plug with extra bits (a bu

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9 min read · 10 sections

Run the code

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

TypeScriptCompatibility by shape, excess property checks and {}

Practice

TypeScriptWhere it bites, and three ways to get nominal behaviour

Practice

Discussion

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