标签:TypeScript sheet type interfaces share extending interface data se
Difference between extending and intersecting interfaces in TypeScript?
Ask Question Asked 3 years, 7 months ago Modified 10 months ago Viewed 18k times 88Let's say the following type is defined:
interface Shape {
color: string;
}
Now, consider the following ways to add additional properties to this type:
Extension
interface Square extends Shape {
sideLength: number;
}
Intersection
type Square = Shape & {
sideLength: number;
}
What is the difference between both approaches?
And, for sake of completeness and out of curiosity, are there other ways to yield comparable results?
typescripttypesintersectionextends Share Improve this question edited Oct 6, 2018 at 17:01 asked Oct 6, 2018 at 16:56 Willem-Aart 1,87011 gold badge1717 silver badges2323 bronze badges- 2 See Also: Interfaces vs. Intersections – KyleMit♦ Dec 20, 2021 at 19:39
1 Answer
98Yes there are differences which may or may not be relevant in your scenario.
Perhaps the most significant is the difference in how members with the same property key are handled when present in both types.
Consider:
interface NumberToStringConverter {
convert: (value: number) => string;
}
interface BidirectionalStringNumberConverter extends NumberToStringConverter {
convert: (value: string) => number;
}
The extends
above results in an error because the derriving interface declares a property with the same key as one in the derived interface but with an incompatible signature.
error TS2430: Interface 'BidirectionalStringNumberConverter' incorrectly extends interface 'NumberToStringConverter'.
Types of property 'convert' are incompatible.
Type '(value: string) => number' is not assignable to type '(value: number) => string'.
Types of parameters 'value' and 'value' are incompatible.
Type 'number' is not assignable to type 'string'.
However, if we employ intersection types
type NumberToStringConverter = {
convert: (value: number) => string;
}
type BidirectionalStringNumberConverter = NumberToStringConverter & {
convert: (value: string) => number;
}
There is no error whatsoever and further given
// And this is a good thing indeed as a value conforming to the type is easily conceived
const converter: BidirectionalStringNumberConverter = {
convert: (value: string | number) => {
return (typeof value === 'string' ? Number(value) : String(value)) as string & number; // type assertion is an unfortunately necessary hack.
}
}
const s: string = converter.convert(0); // `convert`'s call signature comes from `NumberToStringConverter`
const n: number = converter.convert('a'); // `convert`'s call signature comes from `BidirectionalStringNumberConverter`
This leads to another interesting difference, interface
declarations are open ended. New members can be added anywhere because multiple interface
declarations with same name in the same declaration space are merged.
Here is a common use for merging behavior
lib.d.ts
interface Array<T> {
// map, filter, etc.
}
array-flat-map-polyfill.ts
interface Array<T> {
flatMap<R>(f: (x: T) => R[]): R[];
}
if (typeof Array.prototype.flatMap !== 'function') {
Array.prototype.flatMap = function (f) {
// Implementation simplified for exposition.
return this.map(f).reduce((xs, ys) => [...xs, ...ys], []);
}
}
Notice how no extends
clause is present, although specified in separate files the interfaces are both in the global scope and are merged by name into a single logical interface declaration that has both sets of members. (the same can be done for module scoped declarations with slightly different syntax)
By contrast, intersection types, as stored in a type
declaration, are closed, not subject to merging.
There are many, many differences. You can read more about both constructs in the TypeScript Handbook. The Interfaces and Advanced Types section are particularly relevant.
Share Improve this answer edited Jul 18, 2021 at 19:26 thanosades 322 silver badges33 bronze badges answered Oct 6, 2018 at 17:58 Aluan Haddad 26.3k66 gold badges6464 silver badges7777 bronze badges- 2 Great answer. Thanks for pointing out the difference in behaviour when 'overriding' properties, didn't know about that. That alone is a good reason to use types in certain use cases. Can you point out situations where interface merging is useful? Are there valid use cases when building applications (in other words: not libraries)? – Willem-Aart Oct 6, 2018 at 18:42
-
Willem Aart as you suggest, it is most useful for writing libraries, but what is an application if not a collection of libraries (including your own app). It can be extremely useful for applications as well. Ex:
interface Object {hasOwnProperty<T, K extends string>(this: T, key: K): this is {[P in K]?}}
which turnsObject.prototype.hasOwnProperty
into a type guard by introducing an additional, more specific signature for it. . – Aluan Haddad Oct 6, 2018 at 19:12 -
1
@AluanHaddad the
StringToNumberConverter
type should be instead namedBidirectionalStringNumberConverter
, correct? It seems like the other instances were possibly renamed... – Karl Horky Jul 4, 2019 at 19:56 - 1 @NathanChappell thank you for catching that. I don't know when that broke. I've updated the example to make it compile, but it now requires a type assertion. I will look into this more. – Aluan Haddad Sep 22, 2020 at 8:37
- 1 @AluanHaddad thanks. TS seems to be changing quite fast, so it's probably impossible to keep up with it (especially since they seem to have abandoned maintaining a specification...) – Nathan Chappell Sep 22, 2020 at 8:47
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