A mixin is a small, reusable piece of object behavior or data. Instead of relying on one prototype chain to supply an object’s API, you can copy properties from several independent objects into a target. Keep each mixin narrowly focused and self-sustaining, even if it looks incomplete on its own. section 3.4 section 3.0
Use `Object.assign(target, ...mixins)` to assemble an object, or spread mixin properties into an object literal. This is concatenative extension: the resulting object has its own assigned properties rather than inheriting those properties through links. A mixin can also extend a class prototype or one particular instance. section 3.4 section 3.5
const Sandwich = (size = '6', unit = 'in') =>
Object.assign({
size, unit
},
HasBread,
HasToppings,
HasTomatoes,
HasMeat
);
const footLong = Sandwich(1, 'ft');
footLong.tomatoe; // 'Cherry'
The sandwich factory joins small property sets with Object.assign, then reads a property from the assembled object. section 3.4
Copying trades the memory efficiency of shared prototype properties for independently assembled objects and less exposure to downstream changes through prototype links. When sources define the same property, Object.assign favors the last source. Assignment is not identical to definition, however: assigning an existing key can invoke a target setter. section 1.2 section 3.4.2 section 3.3.2
const TerrestrialAnimal = {
walk() {
...
},
breathe() {
return 'Using my lungs to breathe';
}
};
const AquaticAnimal = {
swim() {
...
},
breathe() {
return 'Using my gills to breathe';
}
};
const Amphibian = name => Object.assign(
{
name
},
AquaticAnimal,
TerrestrialAnimal
);
const frog = Amphibian('Frog');
frog.walk();
frog.swim();
Two behavior objects both define `breathe`; their order in Object.assign determines which method the assembled amphibian uses. section 3.4.2
A configurable mixin can be a function returning methods: `HasHash(['sender', 'recipient', 'funds'])` supplies `calculateHash()` for a suitable target. The same hashing and validation behaviors can be attached to different domain objects. Mixins can also supply interfaces such as a functor’s `map` or a monad’s `flatMap`. Watch for methods that silently depend on the target’s `this` and property names; extracting functions with explicit inputs makes those dependencies easier to see. section 3.4.1 section 3.5 section 5.4.1 section 5.4.2
