Typescript's concept of extension is akin to C++'s inheritance.
Intersection in Typescript involves creating a new object with
all the properties from the intersected classes.
Why utilize intersection when extends keyword can already
merge all members from both classes into the derived class?
When would using intersection be more advantageous over
extension in certain scenarios?
https://www.typescriptlang.org/docs/handbook/2/objects.html#intersection-types
While they may seem similar, there are distinct differences between them:
extends is only applicable to types with statically known members:type SimpleType = {x: string};
interface SimpleInterface extends SimpleType {} // valid
type ComplexType = {x: string} | {x: number};
// error: An interface can only extend an object type or intersection of object types with statically known members.
interface ComplexInterface extends ComplexType {}
Type intersection is a more general type operation that can be applied to any two types and provide a result; extends is specifically for interfaces and interface-like types (as well as classes).
interface Parent {
x: string | number;
}
interface Child1 extends Parent {
x: string; // valid
}
interface Child2 extends Parent {
x: string | number | boolean; // error
}
However, type intersection does not have this restriction:
type IntersectedChild = Parent & {x: string | boolean}; // valid
// IntersectedChild will have property 'x' as an intersection
// of 'string | number' and 'string | boolean', resulting in a 'string':
type IntersectedChildX = IntersectedChild['x']; // string
extends with classes, the child class inherits all implementations from its parent; whereas type intersection operates solely at the type level, so if you define classes A and B with type C = A & B, C remains a type rather than a class, requiring manual construction of an object that meets the constraints of C (containing all members of A and B with matching visibility levels).For further clarification, refer to this playground link with the examples showcased above.
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