Answer.

Both mechanisms — interface and type alias — allow you to describe data structures (objects, functions, complex types).

• Interface — classic interface declaration. Allows for extending interfaces, merging (declaration merging), and is easy to inherit.
• Type alias — creates a new type alias. Can be combined using union / intersection, and assigned to any entity: primitives, functions, tuples, etc.

Differences:

• Extensibility: interface is better for object structures, supports declarative merging.
• Flexibility: type alias is versatile, suitable for everything: union (|) and intersection (&).

Example:

interface Animal {
  name: string
}
interface Dog extends Animal {
  bark(): void
}

// type alias for combining types:
type MyType = string | number

type Cat = Animal & { purr(): void }

Recommended:

• For public APIs — use interface.
• For complex unions and aliases — use type.

Trick question.

Question: Can a new field be added to a type defined via type alias in another file?

Answer:

No. Type alias cannot be declaratively extended from another file, unlike interface.

Example:

// main.ts
type User = { name: string }

// another.ts
type User = { age: number } // Error: Duplicate identifier

// interface:
// main.ts
interface User { name: string }
// another.ts
interface User { age: number } // Ok, User = { name: string, age: number }

Examples of real errors due to misunderstandings about the topic.

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Story

In a large project, common data structures were defined via type alias. When it became necessary to add a field to an existing type from another package, it turned out that type alias does not support declaration merging — had to rewrite to interface, causing delays.

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Story

One of the developers described functions via interface, and then tried to add a Union type (a string or a function) — it turned out that interface is not suitable for this, and all definitions had to be changed to type alias with intersection/union.

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Story

After changing the type from interface to type alias, they confused the extension syntax: tried to use extends, instead of combining via &. The error manifested late and the reason was not immediately understood.