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For an ordinary named object shape, either an interface or a type alias can work. Prefer an interface when the shape is meant to be extended or intentionally augmented; use a type alias when you need to describe a union, tuple, or other type expression that an interface cannot represent. For equivalent, closed object shapes, follow the style already used in your project.
Both can describe an object shape
Neither declaration creates a JavaScript object or changes runtime behavior. Both give TypeScript a name for a compile-time shape; the practical differences are what each form can express and how other declarations can build on it.
A type alias names a type rather than creating a distinct nominal type. It can name an object shape, but also a primitive, union, tuple, or another type expression. An interface is designed to describe an object shape and can be extended. The TypeScript Handbook recommends using an interface over a type alias when possible, noting that interfaces are open to extension.
Use interface for an extendable object contract
Use interface when you want to name an object shape that other interfaces or types can extend, especially when it represents a public contract. An extends clause expresses that one shape builds on another:
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interface Entity {
id: string;
}
interface User extends Entity {
email: string;
}User includes the members of Entity as well as its own. Interfaces can extend one or more types, allowing shared members to be declared once and making the relationship between the shapes explicit. The Handbook explains this pattern in its section on extending types.
Use a type alias for unions, tuples, and other type expressions
An interface cannot directly name a union or tuple. A type alias can, so it is the natural choice when a value may have one of several shapes or when the type is a tuple:
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type Result =
| { state: "success"; value: string }
| { state: "error"; message: string };
type Point = [number, number];In the Result union, a value is one alternative, not a value that must satisfy both. TypeScript can narrow the alternatives using their shared literal property:
function describe(result: Result) {
if (result.state === "success") {
return result.value;
}
return result.message;
}Before narrowing, code can safely use only members common to every union alternative. Checking state lets TypeScript identify which alternative is active. See the Handbook’s explanation of union types.
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A type alias can combine types with an intersection (&): the result must satisfy both component types. This is useful for composing reusable pieces, but it is not interchangeable with interface inheritance when the same property is declared incompatibly.
interface HasId {
id: string;
}
interface HasNumericId {
id: number;
}
interface Invalid extends HasId, HasNumericId {} // Error
type Both = HasId & HasNumericId;
// Both.id must satisfy string and number, so its type is never.The incompatible inherited property makes the interface declaration an error. With the intersection, the property must satisfy both declarations; because a value cannot be both a string and a number, the resulting property type is never. Check for property collisions when composing aliases with &. The Handbook describes intersection types alongside interface extension.
Interfaces can merge; type aliases cannot be reopened
Two interface declarations with the same name can merge into one interface, provided their members are compatible:
interface Settings {
theme: string;
}
interface Settings {
compact: boolean;
}
// The merged Settings has both theme and compact.Non-function members must have unique names, unless repeated declarations use the same type; incompatible repeated members produce an error. A same-named type alias does not offer this reopening mechanism. Declaration merging can be useful for intentional augmentation, including in ambient declarations, but it also means another declaration in scope can alter the final interface shape. See the Handbook’s rules for merging interfaces.
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Quick choice guide
| Need | Choose | Why |
|---|---|---|
| A named object shape intended for extension | interface |
Supports extends and declaration merging. |
| A union of alternatives, such as a result or state | type |
A type alias can name a union; an interface cannot directly represent one. |
| A tuple or another type expression beyond an object interface | type |
Aliases can name tuples, primitives, unions, and other type expressions. |
| Add members while expressing an inheritance relationship | interface extends |
Incompatible inherited properties are reported as errors. |
| Combine reusable types as an intersection | type A = B & C |
Combines requirements, but conflicting properties can become never. |
| Provide an intentional augmentation point | interface |
Same-name declarations can merge when compatible. |
| Describe an equivalent, closed object shape in a project with an established style | Follow the project | For this case, the choice is a convention rather than a universal capability requirement. |
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