Mixins

Heated debates of composition over inheritance made mixins appear to be the winner construct for composing objects. Therefore, libraries such as UnderscoreJs and LoDash created support for this construct with their methods _.extend or _.mixin.

In ES6, Object.assign does the same thing as _.extend or _.mixin.

Why are mixins important?

They are important because the alternative of establishing a class hierarchy using inheritance is inefficient and rigid.

Suppose you have a view object, which can be defined with or without the following extensions:

• validation
• tooltips
• abstractions for two-way data binding
• toolbar
• preloader animation

Assuming the order of the extensions does not matter, 32 different view types can be defined using the five enhancements above. To fight the combinatoric explosion, we take these extensions as mixins and extend our object prototypes with the extensions that we need.

For instance, a validating view with a preloader animation can be defined in the following way: