Mutation
Allows for declarative mutation of JavaScript objects. Is extremely useful for mutating things like default values and configuration objects. Works by recursing over a target object and applying any mutation directives it finds in the source object.

Source

Read the annotated source here

API

Here's a list of the available advice API methods:

Type Behavior
extendWith Mutates a target object using a source object. Keys with no mutation operators are simply replaced.
deepExtendWith Mutates a target object using a source object. Keys with no mutation operators are recursively mutated.
mutateWith Mutates a source object given a target object and some mutation settings

The following is a list of all the valid mutation operators:

Operator Behavior
extend Extends an object given another one
deepExtend Extends an object given another to an infinite depth
delete Deletes the given value
push Pushes into an array
insertAt Inserts into an array at the given index
callWith Calls a function with the given source value
extendFunction Calls the source function after the target function and returns the return value of the target extended with the return value of the source function
deepExtendFunction Same as extendFunction except deep extends the return value

Usage

The simplest possible example might be a delete operation on a target object. 
var foo = {
    a: 1,
    b: 2
};
var bar = {
    'delete.b': true
}
Mutation.extendWith(foo, bar);
// console.log(foo.b) is undefined

Let's take an example of where this might be useful. Suppose we have some instance where we are defining some default configuration. However, we would like the user to be able to change the behaviour of the instance be passing in some options. Mutation allows us to setup a format for such configuration without explicitly needing to define such override behaviour.

Check out the example for this here