A mixin is a class or interface in which some or all of its methods and/or properties are unimplemented, requiring that another class or interface provide the missing implementations. The new class or interface then includes both the properties and methods from the mixin as well as those it defines itself. All of the methods and properties are used exactly the same regardless of whether they're implemented in the mixin or the interface or class that implements the mixin.
The advantage to mixins is that they can be used to simplify the design of APIs in which multiple interfaces need to include the same methods and properties.
For example, the WindowOrWorkerGlobalScope mixin is used to provide methods and properties that need to be available on both the Window and WorkerGlobalScope interfaces. The mixin is implemented by both of those interfaces.
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General knowledge
- Mixin on Wikipedia