In Typescript, what is the difference between type and interface?
Differences between these too are already in this thread.
type Foo = {
foo: string
};
interface Foo {
foo: string;
}
Here type Foo
andinterface Foo
looks almost similar so its confusing.
interface
is contract that the following properties (herefoo:string
) should be there in a object.
interface
is not class
. It is used when language does not support Multiple Inheritance. So interface
can be a common structure between different classes.
class Bar implements Foo {
foo: string;
}
let p: Foo = { foo: 'a string' };
Buttype
and interface
are used in very different context.
let foo: Foo;
let today: Date = new Date();
Here type
of foo
is Foo
and today
is Date
.
Its like a variable decleration which holds the information of typeof other variable.
type
is like a superset of interfaces, classes, function signature, other types or even values (like type mood = 'Good' | 'Bad'
).
At the end type
describes the possible structure or value of a variable.
It is wrong to say "Interfaces can be implemented" since types can also be implemented
type A = { a: string };
class Test implements A {
a: string;
}
Although you can do this, you can't implement a type that is a Union of types, which makes totally sense honestly :)
Interfaces can be extended
interface A {
x: number;
}
interface B extends A {
y: string;
}
and also augmented
interface C {
m: boolean;
}
// ... later ...
interface C {
n: number;
}
Type aliases, however, can represent some things interfaces can't
type NumOrStr = number | string;
type NeatAndCool = Neat & Cool;
type JustSomeOtherName = SomeType;
So in general if you just have a plain object type, as shown in your question, an interface is usually a better approach. If you find yourself wanting to write something that can't be written as an interface, or want to just give something a different name, a type alias is better.