Compatibility primarily follows structure, not declared names. Excess-property checks catch some mistakes in fresh object literals but are not exact-object guarantees.
Before you start
You should be comfortable with JavaScript values, functions and objects. Use a TypeScript project with strict checking enabled when trying the examples. Separate what the compiler proves from what must still be checked when values arrive at runtime.
The practical goal is to reason through this situation: A variable with extra properties can often satisfy a smaller interface if required fields match. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Compare required structure
An interface is satisfied by compatible fields. The object’s declaration name is usually irrelevant to that compatibility decision.
Step 2: Observe fresh-literal checks
A fresh literal can trigger excess-property checking. An existing variable may still be assignable to a smaller shape with extra fields.
Step 3: Enforce external shape at runtime
If unknown properties are forbidden or sensitive, use validation or construct a whitelisted object. Assignment does not strip fields.
Worked scenario
A variable with extra properties can often satisfy a smaller interface if required fields match.
type PublicUser = { name: string };
const internal = { name: 'Ada', token: 'secret' };
const publicView: PublicUser = internal;
console.log(Object.keys(publicView)); // name and token
const sanitized: PublicUser = { name: internal.name };
console.log(Object.keys(sanitized)); // name onlyPublicView has a smaller static interface but still points at internal. Sanitized is a new runtime object containing only the intended field, which changes the data actually exposed.
Common mistake
Assignability does not strip extra runtime fields.
Verify the behavior
Compare a fresh object literal assignment with variable assignment. Serialize both views and confirm which fields leave the process; static typing alone is not sanitization.
Interview exercise
Enforce an exact external schema.
Answer and reasoning
Use runtime validation with an explicit unknown-property policy; structural typing is about compatibility, not sanitization.
Continue learning
Compare the scenario with the TypeScript interview questions and test your understanding with the TypeScript MCQs. For terminology and implementation details, consult the reference material.