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How to invoke a thunk for sub-object property access in JS?

There is a class of objects to which I need to lazy add a few functions as properties after the fact. Because these functions must reference instance variables in order to configure the function attached as a property, I've used thunks to create the function and forward the first function call. for a simplified example:
function Foo(val) {
this.val = val;
}
// simulates a library call which returns a complex object with methods
// this library call requires access to some of the object information,
// simulated by passing in `type`
function buildWorkFunction(type) {
var f = function() { return this.val; };
if (type === 'string')
f = function() { return this.val.length; };
f.getType = function() { return type; };
return f;
}
function addProperty(Base, prop) {
var thunk = function() {
// typeof is just a simple example of how buildWorkFunction could
// change based on this.val
this[prop] = buildWorkFunction(typeof this.val);
return this[prop].apply(this, arguments);
};
thunk.getType = function() {
return 'TODO'; // <-- this is the part I need help with
};
Base.prototype[prop] = thunk;
}
// simulates instances existing before the module load
var foo = new Foo('foo');
// runs after user requests a module load, runs from the module
setTimeout(function() {
// adding a module specific property to the non-module model
addProperty(Foo, 'getVal');
console.log(foo.getVal());
console.log(foo.getVal.getType());
// simulates instances created after the module load
var bar = new Foo(17);
console.log(bar.getVal.getType()); // called before thunk - fails
console.log(bar.getVal());
console.log(bar.getVal.getType()); // called after thunk - works
}, 0);
One problem I have left is that the property's value itself has properties which the application code sometimes references without calling the property itself, as with f.getType above. How can I properly catch/proxy/forward calls such as foo.getVal.getType() without first calling foo.getVal()? The names of the attached properties are well defined and can be shared - but I can't see how to access the correct this or the data from them.

How can I properly catch/proxy calls such as foo.getVal.getType()? If foo.getVal() isn't called first, I can't see how to access the correct this or the data from them.
You can use the same approach as you already did, but on the property getter level instead of the method call level. This makes it possible to lazily create arbitrary instance properties:
function addProperty(Base, prop) {
Object.defineProperty(Base.prototype, prop, {
get: function thunk() {
var propertyValue = buildWorkFunction(typeof this.val);
// ^^^^^^^^^^^^^^^ access the instance here
// overwrite (shadow) the prototype getter:
Object.defineProperty(this, prop, {
value: propertyValue,
writable: true,
enumerable: true,
configurable: true
});
return this[prop];
},
enumerable: true, // dunno
configurable: true
});
}

Add multiple interface to prototype javascript [duplicate]

I've come to a point where I need to have some sort of rudimentary multiple inheritance happening in JavaScript. (I'm not here to discuss whether this is a good idea or not, so please kindly keep those comments to yourself.)
I just want to know if anyone's attempted this with any (or not) success, and how they went about it.
To boil it down, what I really need is to be able to have an object capable of inheriting a property from more than one prototype chain (i.e. each prototype could have its own proper chain), but in a given order of precedence (it will search the chains in order for the first definition).
To demonstrate how this is theoretically possible, it could be achieved by attaching the secondary chain onto the end of the primary chain, but this would affect all instances of any of those previous prototypes and that's not what I want.
Thoughts?

Multiple inheritance can be achieved in ECMAScript 6 by using Proxy objects.
Implementation
function getDesc (obj, prop) {
var desc = Object.getOwnPropertyDescriptor(obj, prop);
return desc || (obj=Object.getPrototypeOf(obj) ? getDesc(obj, prop) : void 0);
}
function multiInherit (...protos) {
return Object.create(new Proxy(Object.create(null), {
has: (target, prop) => protos.some(obj => prop in obj),
get (target, prop, receiver) {
var obj = protos.find(obj => prop in obj);
return obj ? Reflect.get(obj, prop, receiver) : void 0;
},
set (target, prop, value, receiver) {
var obj = protos.find(obj => prop in obj);
return Reflect.set(obj || Object.create(null), prop, value, receiver);
},
*enumerate (target) { yield* this.ownKeys(target); },
ownKeys(target) {
var hash = Object.create(null);
for(var obj of protos) for(var p in obj) if(!hash[p]) hash[p] = true;
return Object.getOwnPropertyNames(hash);
},
getOwnPropertyDescriptor(target, prop) {
var obj = protos.find(obj => prop in obj);
var desc = obj ? getDesc(obj, prop) : void 0;
if(desc) desc.configurable = true;
return desc;
},
preventExtensions: (target) => false,
defineProperty: (target, prop, desc) => false,
}));
}
Explanation
A proxy object consists of a target object and some traps, which define custom behavior for fundamental operations.
When creating an object which inherits from another one, we use Object.create(obj). But in this case we want multiple inheritance, so instead of obj I use a proxy that will redirect fundamental operations to the appropriate object.
I use these traps:
The has trap is a trap for the in operator. I use some to check if at least one prototype contains the property.
The get trap is a trap for getting property values. I use find to find the first prototype which contains that property, and I return the value, or call the getter on the appropriate receiver. This is handled by Reflect.get. If no prototype contains the property, I return undefined.
The set trap is a trap for setting property values. I use find to find the first prototype which contains that property, and I call its setter on the appropriate receiver. If there is no setter or no prototype contains the property, the value is defined on the appropriate receiver. This is handled by Reflect.set.
The enumerate trap is a trap for for...in loops. I iterate the enumerable properties from the first prototype, then from the second, and so on. Once a property has been iterated, I store it in a hash table to avoid iterating it again.
Warning: This trap has been removed in ES7 draft and is deprecated in browsers.
The ownKeys trap is a trap for Object.getOwnPropertyNames(). Since ES7, for...in loops keep calling [[GetPrototypeOf]] and getting the own properties of each one. So in order to make it iterate the properties of all prototypes, I use this trap to make all enumerable inherited properties appear like own properties.
The getOwnPropertyDescriptor trap is a trap for Object.getOwnPropertyDescriptor(). Making all enumerable properties appear like own properties in the ownKeys trap is not enough, for...in loops will get the descriptor to check if they are enumerable. So I use find to find the first prototype which contains that property, and I iterate its prototypical chain until I find the property owner, and I return its descriptor. If no prototype contains the property, I return undefined. The descriptor is modified to make it configurable, otherwise we could break some proxy invariants.
The preventExtensions and defineProperty traps are only included to prevent these operations from modifying the proxy target. Otherwise we could end up breaking some proxy invariants.
There are more traps available, which I don't use
The getPrototypeOf trap could be added, but there is no proper way to return the multiple prototypes. This implies instanceof won't work neither. Therefore, I let it get the prototype of the target, which initially is null.
The setPrototypeOf trap could be added and accept an array of objects, which would replace the prototypes. This is left as an exercice for the reader. Here I just let it modify the prototype of the target, which is not much useful because no trap uses the target.
The deleteProperty trap is a trap for deleting own properties. The proxy represents the inheritance, so this wouldn't make much sense. I let it attempt the deletion on the target, which should have no property anyway.
The isExtensible trap is a trap for getting the extensibility. Not much useful, given that an invariant forces it to return the same extensibility as the target. So I just let it redirect the operation to the target, which will be extensible.
The apply and construct traps are traps for calling or instantiating. They are only useful when the target is a function or a constructor.
Example
// Creating objects
var o1, o2, o3,
obj = multiInherit(o1={a:1}, o2={b:2}, o3={a:3, b:3});
// Checking property existences
'a' in obj; // true (inherited from o1)
'b' in obj; // true (inherited from o2)
'c' in obj; // false (not found)
// Setting properties
obj.c = 3;
// Reading properties
obj.a; // 1 (inherited from o1)
obj.b; // 2 (inherited from o2)
obj.c; // 3 (own property)
obj.d; // undefined (not found)
// The inheritance is "live"
obj.a; // 1 (inherited from o1)
delete o1.a;
obj.a; // 3 (inherited from o3)
// Property enumeration
for(var p in obj) p; // "c", "b", "a"

Update (2019): The original post is getting pretty outdated. This article (now internet archive link, since domain went away) and its associated GitHub library are a good modern approach.
Original post:
Multiple inheritance [edit, not proper inheritance of type, but of properties; mixins] in Javascript is pretty straightforward if you use constructed prototypes rather than generic-object ones. Here are two parent classes to inherit from:
function FoodPrototype() {
this.eat = function () {
console.log("Eating", this.name);
};
}
function Food(name) {
this.name = name;
}
Food.prototype = new FoodPrototype();
function PlantPrototype() {
this.grow = function () {
console.log("Growing", this.name);
};
}
function Plant(name) {
this.name = name;
}
Plant.prototype = new PlantPrototype();
Note that I have used the same "name" member in each case, which could be a problem if the parents did not agree about how "name" should be handled. But they're compatible (redundant, really) in this case.
Now we just need a class that inherits from both. Inheritance is done by calling the constructor function (without using the new keyword) for the prototypes and the object constructors. First, the prototype has to inherit from the parent prototypes
function FoodPlantPrototype() {
FoodPrototype.call(this);
PlantPrototype.call(this);
// plus a function of its own
this.harvest = function () {
console.log("harvest at", this.maturity);
};
}
And the constructor has to inherit from the parent constructors:
function FoodPlant(name, maturity) {
Food.call(this, name);
Plant.call(this, name);
// plus a property of its own
this.maturity = maturity;
}
FoodPlant.prototype = new FoodPlantPrototype();
Now you can grow, eat, and harvest different instances:
var fp1 = new FoodPlant('Radish', 28);
var fp2 = new FoodPlant('Corn', 90);
fp1.grow();
fp2.grow();
fp1.harvest();
fp1.eat();
fp2.harvest();
fp2.eat();

This one uses Object.create to make a real prototype chain:
function makeChain(chains) {
var c = Object.prototype;
while(chains.length) {
c = Object.create(c);
$.extend(c, chains.pop()); // some function that does mixin
}
return c;
}
For example:
var obj = makeChain([{a:1}, {a: 2, b: 3}, {c: 4}]);
will return:
a: 1
a: 2
b: 3
c: 4
<Object.prototype stuff>
so that obj.a === 1, obj.b === 3, etc.

I like John Resig's implementation of a class structure: http://ejohn.org/blog/simple-javascript-inheritance/
This can be simply extended to something like:
Class.extend = function(prop /*, prop, prop, prop */) {
for( var i=1, l=arguments.length; i<l; i++ ){
prop = $.extend( prop, arguments[i] );
}
// same code
}
which will allow you to pass in multiple objects of which to inherit. You're going to lose instanceOf capability here, but that's a given if you want multiple inheritance.
my rather convoluted example of the above is available at https://github.com/cwolves/Fetch/blob/master/support/plugins/klass/klass.js
Note that there is some dead code in that file, but it allows multiple inheritance if you want to take a look.
If you want chained inheritance (NOT multiple inheritance, but for most people it's the same thing), it can be accomplished with Class like:
var newClass = Class.extend( cls1 ).extend( cls2 ).extend( cls3 )
which will preserve the original prototype chain, but you'll also have a lot of pointless code running.

I offer a function to allow classes to be defined with multiple inheritance. It allows for code like the following:
let human = new Running({ name: 'human', numLegs: 2 });
human.run();
let airplane = new Flying({ name: 'airplane', numWings: 2 });
airplane.fly();
let dragon = new RunningFlying({ name: 'dragon', numLegs: 4, numWings: 6 });
dragon.takeFlight();
to produce output like this:
human runs with 2 legs.
airplane flies away with 2 wings!
dragon runs with 4 legs.
dragon flies away with 6 wings!
Here are what the class definitions look like:
let Named = makeClass('Named', {}, () => ({
init: function({ name }) {
this.name = name;
}
}));
let Running = makeClass('Running', { Named }, protos => ({
init: function({ name, numLegs }) {
protos.Named.init.call(this, { name });
this.numLegs = numLegs;
},
run: function() {
console.log(`${this.name} runs with ${this.numLegs} legs.`);
}
}));
let Flying = makeClass('Flying', { Named }, protos => ({
init: function({ name, numWings }) {
protos.Named.init.call(this, { name });
this.numWings = numWings;
},
fly: function( ){
console.log(`${this.name} flies away with ${this.numWings} wings!`);
}
}));
let RunningFlying = makeClass('RunningFlying', { Running, Flying }, protos => ({
init: function({ name, numLegs, numWings }) {
protos.Running.init.call(this, { name, numLegs });
protos.Flying.init.call(this, { name, numWings });
},
takeFlight: function() {
this.run();
this.fly();
}
}));
We can see that each class definition using the makeClass function accepts an Object of parent-class names mapped to parent-classes. It also accepts a function that returns an Object containing properties for the class being defined. This function has a parameter protos, which contains enough information to access any property defined by any of the parent-classes.
The final piece required is the makeClass function itself, which does quite a bit of work. Here it is, along with the rest of the code. I've commented makeClass quite heavily:
let makeClass = (name, parents={}, propertiesFn=()=>({})) => {
// The constructor just curries to a Function named "init"
let Class = function(...args) { this.init(...args); };
// This allows instances to be named properly in the terminal
Object.defineProperty(Class, 'name', { value: name });
// Tracking parents of `Class` allows for inheritance queries later
Class.parents = parents;
// Initialize prototype
Class.prototype = Object.create(null);
// Collect all parent-class prototypes. `Object.getOwnPropertyNames`
// will get us the best results. Finally, we'll be able to reference
// a property like "usefulMethod" of Class "ParentClass3" with:
// `parProtos.ParentClass3.usefulMethod`
let parProtos = {};
for (let parName in parents) {
let proto = parents[parName].prototype;
parProtos[parName] = {};
for (let k of Object.getOwnPropertyNames(proto)) {
parProtos[parName][k] = proto[k];
}
}
// Resolve `properties` as the result of calling `propertiesFn`. Pass
// `parProtos`, so a child-class can access parent-class methods, and
// pass `Class` so methods of the child-class have a reference to it
let properties = propertiesFn(parProtos, Class);
properties.constructor = Class; // Ensure "constructor" prop exists
// If two parent-classes define a property under the same name, we
// have a "collision". In cases of collisions, the child-class *must*
// define a method (and within that method it can decide how to call
// the parent-class methods of the same name). For every named
// property of every parent-class, we'll track a `Set` containing all
// the methods that fall under that name. Any `Set` of size greater
// than one indicates a collision.
let propsByName = {}; // Will map property names to `Set`s
for (let parName in parProtos) {
for (let propName in parProtos[parName]) {
// Now track the property `parProtos[parName][propName]` under the
// label of `propName`
if (!propsByName.hasOwnProperty(propName))
propsByName[propName] = new Set();
propsByName[propName].add(parProtos[parName][propName]);
}
}
// For all methods defined by the child-class, create or replace the
// entry in `propsByName` with a Set containing a single item; the
// child-class' property at that property name (this also guarantees
// there is no collision at this property name). Note property names
// prefixed with "$" will be considered class properties (and the "$"
// will be removed).
for (let propName in properties) {
if (propName[0] === '$') {
// The "$" indicates a class property; attach to `Class`:
Class[propName.slice(1)] = properties[propName];
} else {
// No "$" indicates an instance property; attach to `propsByName`:
propsByName[propName] = new Set([ properties[propName] ]);
}
}
// Ensure that "init" is defined by a parent-class or by the child:
if (!propsByName.hasOwnProperty('init'))
throw Error(`Class "${name}" is missing an "init" method`);
// For each property name in `propsByName`, ensure that there is no
// collision at that property name, and if there isn't, attach it to
// the prototype! `Object.defineProperty` can ensure that prototype
// properties won't appear during iteration with `in` keyword:
for (let propName in propsByName) {
let propsAtName = propsByName[propName];
if (propsAtName.size > 1)
throw new Error(`Class "${name}" has conflict at "${propName}"`);
Object.defineProperty(Class.prototype, propName, {
enumerable: false,
writable: true,
value: propsAtName.values().next().value // Get 1st item in Set
});
}
return Class;
};
let Named = makeClass('Named', {}, () => ({
init: function({ name }) {
this.name = name;
}
}));
let Running = makeClass('Running', { Named }, protos => ({
init: function({ name, numLegs }) {
protos.Named.init.call(this, { name });
this.numLegs = numLegs;
},
run: function() {
console.log(`${this.name} runs with ${this.numLegs} legs.`);
}
}));
let Flying = makeClass('Flying', { Named }, protos => ({
init: function({ name, numWings }) {
protos.Named.init.call(this, { name });
this.numWings = numWings;
},
fly: function( ){
console.log(`${this.name} flies away with ${this.numWings} wings!`);
}
}));
let RunningFlying = makeClass('RunningFlying', { Running, Flying }, protos => ({
init: function({ name, numLegs, numWings }) {
protos.Running.init.call(this, { name, numLegs });
protos.Flying.init.call(this, { name, numWings });
},
takeFlight: function() {
this.run();
this.fly();
}
}));
let human = new Running({ name: 'human', numLegs: 2 });
human.run();
let airplane = new Flying({ name: 'airplane', numWings: 2 });
airplane.fly();
let dragon = new RunningFlying({ name: 'dragon', numLegs: 4, numWings: 6 });
dragon.takeFlight();
The makeClass function also supports class properties; these are defined by prefixing property names with the $ symbol (note that the final property name that results will have the $ removed). With this in mind, we could write a specialized Dragon class that models the "type" of the Dragon, where the list of available Dragon types is stored on the Class itself, as opposed to on the instances:
let Dragon = makeClass('Dragon', { RunningFlying }, protos => ({
$types: {
wyvern: 'wyvern',
drake: 'drake',
hydra: 'hydra'
},
init: function({ name, numLegs, numWings, type }) {
protos.RunningFlying.init.call(this, { name, numLegs, numWings });
this.type = type;
},
description: function() {
return `A ${this.type}-type dragon with ${this.numLegs} legs and ${this.numWings} wings`;
}
}));
let dragon1 = new Dragon({ name: 'dragon1', numLegs: 2, numWings: 4, type: Dragon.types.drake });
let dragon2 = new Dragon({ name: 'dragon2', numLegs: 4, numWings: 2, type: Dragon.types.hydra });
The Challenges of Multiple Inheritance
Anyone who followed the code for makeClass closely will note a rather significant undesirable phenomenon occurring silently when the above code runs: instantiating a RunningFlying will result in TWO calls to the Named constructor!
This is because the inheritance graph looks like this:
(^^ More Specialized ^^)
RunningFlying
/ \
/ \
Running Flying
\ /
\ /
Named
(vv More Abstract vv)
When there are multiple paths to the same parent-class in a sub-class' inheritance graph, instantiations of the sub-class will invoke that parent-class' constructor multiple times.
Combatting this is non-trivial. Let's look at some examples with simplified classnames. We'll consider class A, the most abstract parent-class, classes B and C, which both inherit from A, and class BC which inherits from B and C (and hence conceptually "double-inherits" from A):
let A = makeClass('A', {}, () => ({
init: function() {
console.log('Construct A');
}
}));
let B = makeClass('B', { A }, protos => ({
init: function() {
protos.A.init.call(this);
console.log('Construct B');
}
}));
let C = makeClass('C', { A }, protos => ({
init: function() {
protos.A.init.call(this);
console.log('Construct C');
}
}));
let BC = makeClass('BC', { B, C }, protos => ({
init: function() {
// Overall "Construct A" is logged twice:
protos.B.init.call(this); // -> console.log('Construct A'); console.log('Construct B');
protos.C.init.call(this); // -> console.log('Construct A'); console.log('Construct C');
console.log('Construct BC');
}
}));
If we want to prevent BC from double-invoking A.prototype.init we may need to abandon the style of directly calling inherited constructors. We will need some level of indirection to check whether duplicate calls are occurring, and short-circuit before they happen.
We could consider changing the parameters supplied to the properties function: alongside protos, an Object containing raw data describing inherited properties, we could also include a utility function for calling an instance method in such a way that parent methods are also called, but duplicate calls are detected and prevented. Let's take a look at where we establish the parameters for the propertiesFn Function:
let makeClass = (name, parents, propertiesFn) => {
/* ... a bunch of makeClass logic ... */
// Allows referencing inherited functions; e.g. `parProtos.ParentClass3.usefulMethod`
let parProtos = {};
/* ... collect all parent methods in `parProtos` ... */
// Utility functions for calling inherited methods:
let util = {};
util.invokeNoDuplicates = (instance, fnName, args, dups=new Set()) => {
// Invoke every parent method of name `fnName` first...
for (let parName of parProtos) {
if (parProtos[parName].hasOwnProperty(fnName)) {
// Our parent named `parName` defines the function named `fnName`
let fn = parProtos[parName][fnName];
// Check if this function has already been encountered.
// This solves our duplicate-invocation problem!!
if (dups.has(fn)) continue;
dups.add(fn);
// This is the first time this Function has been encountered.
// Call it on `instance`, with the desired args. Make sure we
// include `dups`, so that if the parent method invokes further
// inherited methods we don't lose track of what functions have
// have already been called.
fn.call(instance, ...args, dups);
}
}
};
// Now we can call `propertiesFn` with an additional `util` param:
// Resolve `properties` as the result of calling `propertiesFn`:
let properties = propertiesFn(parProtos, util, Class);
/* ... a bunch more makeClass logic ... */
};
The whole purpose of the above change to makeClass is so that we have an additional argument supplied to our propertiesFn when we invoke makeClass. We should also be aware that every function defined in any class may now receive a parameter after all its others, named dup, which is a Set that holds all functions that have already been called as a result of calling the inherited method:
let A = makeClass('A', {}, () => ({
init: function() {
console.log('Construct A');
}
}));
let B = makeClass('B', { A }, (protos, util) => ({
init: function(dups) {
util.invokeNoDuplicates(this, 'init', [ /* no args */ ], dups);
console.log('Construct B');
}
}));
let C = makeClass('C', { A }, (protos, util) => ({
init: function(dups) {
util.invokeNoDuplicates(this, 'init', [ /* no args */ ], dups);
console.log('Construct C');
}
}));
let BC = makeClass('BC', { B, C }, (protos, util) => ({
init: function(dups) {
util.invokeNoDuplicates(this, 'init', [ /* no args */ ], dups);
console.log('Construct BC');
}
}));
This new style actually succeeds in ensuring "Construct A" is only logged once when an instance of BC is initialized. But there are three downsides, the third of which is very critical:
This code has become less readable and maintainable. A lot of complexity hides behind the util.invokeNoDuplicates function, and thinking about how this style avoids multi-invocation is non-intuitive and headache inducing. We also have that pesky dups parameter, which really needs to be defined on every single function in the class. Ouch.
This code is slower - quite a bit more indirection and computation is required to achieve desirable results with multiple inheritance. Unfortunately this is likely to be the case with any solution to our multiple-invocation problem.
Most significantly, the structure of functions which rely on inheritance has become very rigid. If a sub-class NiftyClass overrides a function niftyFunction, and uses util.invokeNoDuplicates(this, 'niftyFunction', ...) to run it without duplicate-invocation, NiftyClass.prototype.niftyFunction will call the function named niftyFunction of every parent class that defines it, ignore any return values from those classes, and finally perform the specialized logic of NiftyClass.prototype.niftyFunction. This is the only possible structure. If NiftyClass inherits CoolClass and GoodClass, and both these parent-classes provide niftyFunction definitions of their own, NiftyClass.prototype.niftyFunction will never (without risking multiple-invocation) be able to:
A. Run the specialized logic of NiftyClass first, then the specialized logic of parent-classes
B. Run the specialized logic of NiftyClass at any point other than after all specialized parent logic has completed
C. Behave conditionally depending on the return values of its parent's specialized logic
D. Avoid running a particular parent's specialized niftyFunction altogether
Of course, we could solve each lettered problem above by defining specialized functions under util:
A. define util.invokeNoDuplicatesSubClassLogicFirst(instance, fnName, ...)
B. define util.invokeNoDuplicatesSubClassAfterParent(parentName, instance, fnName, ...) (Where parentName is the name of the parent whose specialized logic will be immediately followed by the child-classes' specialized logic)
C. define util.invokeNoDuplicatesCanShortCircuitOnParent(parentName, testFn, instance, fnName, ...) (In this case testFn would receive the result of the specialized logic for the parent named parentName, and would return a true/false value indicating whether the short-circuit should happen)
D. define util.invokeNoDuplicatesBlackListedParents(blackList, instance, fnName, ...) (In this case blackList would be an Array of parent names whose specialized logic should be skipped altogether)
These solutions are all available, but this is total mayhem! For every unique structure that an inherited function call can take, we would need a specialized method defined under util. What an absolute disaster.
With this in mind we can start to see the challenges of implementing good multiple inheritance. The full implementation of makeClass I provided in this answer does not even consider the multiple-invocation problem, or many other problems which arise regarding multiple inheritance.
This answer is getting very long. I hope the makeClass implementation I included is still useful, even if it isn't perfect. I also hope anyone interested in this topic has gained more context to keep in mind as they do further reading!

Don't get confused with JavaScript framework implementations of multiple inheritance.
All you need to do is use Object.create() to create a new object each time with the specified prototype object and properties, then be sure to change the Object.prototype.constructor each step of the way if you plan on instantiating B in the future.
To inherit instance properties thisA and thisB we use Function.prototype.call() at the end of each object function. This is optional if you only care about inheriting the prototype.
Run the following code somewhere and observe objC:
function A() {
this.thisA = 4; // objC will contain this property
}
A.prototype.a = 2; // objC will contain this property
B.prototype = Object.create(A.prototype);
B.prototype.constructor = B;
function B() {
this.thisB = 55; // objC will contain this property
A.call(this);
}
B.prototype.b = 3; // objC will contain this property
C.prototype = Object.create(B.prototype);
C.prototype.constructor = C;
function C() {
this.thisC = 123; // objC will contain this property
B.call(this);
}
C.prototype.c = 2; // objC will contain this property
var objC = new C();
B inherits the prototype from A
C inherits the prototype from B
objC is an instance of C
This is a good explanation of the steps above:
OOP In JavaScript: What You NEED to Know

I’m in no way an expert on javascript OOP, but if I understand you correctly you want something like (pseudo-code):
Earth.shape = 'round';
Animal.shape = 'random';
Cat inherit from (Earth, Animal);
Cat.shape = 'random' or 'round' depending on inheritance order;
In that case, I’d try something like:
var Earth = function(){};
Earth.prototype.shape = 'round';
var Animal = function(){};
Animal.prototype.shape = 'random';
Animal.prototype.head = true;
var Cat = function(){};
MultiInherit(Cat, Earth, Animal);
console.log(new Cat().shape); // yields "round", since I reversed the inheritance order
console.log(new Cat().head); // true
function MultiInherit() {
var c = [].shift.call(arguments),
len = arguments.length
while(len--) {
$.extend(c.prototype, new arguments[len]());
}
}

It's possible to implement multiple inheritance in JavaScript, although very few libraries does it.
I could point Ring.js, the only example I know.

I was working on this a lot today and trying to achieve this myself in ES6. The way I did it was using Browserify, Babel and then I tested it with Wallaby and it seemed to work. My goal is to extend the current Array, include ES6, ES7 and add some additional custom features I need in the prototype for dealing with audio data.
Wallaby passes 4 of my tests. The example.js file can be pasted in the console and you can see that the 'includes' property is in the prototype of the class. I still want to test this more tomorrow.
Here's my method: (I will most likely refactor and repackage as a module after some sleep!)
var includes = require('./polyfills/includes');
var keys = Object.getOwnPropertyNames(includes.prototype);
keys.shift();
class ArrayIncludesPollyfills extends Array {}
function inherit (...keys) {
keys.map(function(key){
ArrayIncludesPollyfills.prototype[key]= includes.prototype[key];
});
}
inherit(keys);
module.exports = ArrayIncludesPollyfills
Github Repo:
https://github.com/danieldram/array-includes-polyfill

I think it is ridiculously simple. The issue here is that the child class will only refer to instanceof for the first class you call
https://jsfiddle.net/1033xzyt/19/
function Foo() {
this.bar = 'bar';
return this;
}
Foo.prototype.test = function(){return 1;}
function Bar() {
this.bro = 'bro';
return this;
}
Bar.prototype.test2 = function(){return 2;}
function Cool() {
Foo.call(this);
Bar.call(this);
return this;
}
var combine = Object.create(Foo.prototype);
$.extend(combine, Object.create(Bar.prototype));
Cool.prototype = Object.create(combine);
Cool.prototype.constructor = Cool;
var cool = new Cool();
console.log(cool.test()); // 1
console.log(cool.test2()); //2
console.log(cool.bro) //bro
console.log(cool.bar) //bar
console.log(cool instanceof Foo); //true
console.log(cool instanceof Bar); //false

Check the code below which IS showing support for multiple inheritance. Done by using PROTOTYPAL INHERITANCE
function A(name) {
this.name = name;
}
A.prototype.setName = function (name) {
this.name = name;
}
function B(age) {
this.age = age;
}
B.prototype.setAge = function (age) {
this.age = age;
}
function AB(name, age) {
A.prototype.setName.call(this, name);
B.prototype.setAge.call(this, age);
}
AB.prototype = Object.assign({}, Object.create(A.prototype), Object.create(B.prototype));
AB.prototype.toString = function () {
return `Name: ${this.name} has age: ${this.age}`
}
const a = new A("shivang");
const b = new B(32);
console.log(a.name);
console.log(b.age);
const ab = new AB("indu", 27);
console.log(ab.toString());

Take a look of the package IeUnit.
The concept assimilation implemented in IeUnit seems to offers what you are looking for in a quite dynamical way.

Here is an example of prototype chaining using constructor functions:
function Lifeform () { // 1st Constructor function
this.isLifeform = true;
}
function Animal () { // 2nd Constructor function
this.isAnimal = true;
}
Animal.prototype = new Lifeform(); // Animal is a lifeform
function Mammal () { // 3rd Constructor function
this.isMammal = true;
}
Mammal.prototype = new Animal(); // Mammal is an animal
function Cat (species) { // 4th Constructor function
this.isCat = true;
this.species = species
}
Cat.prototype = new Mammal(); // Cat is a mammal
This concept uses Yehuda Katz's definition of a "class" for JavaScript:
...a JavaScript "class" is just a Function object that serves as a constructor plus an attached prototype object. (Source: Guru Katz)
Unlike the Object.create approach, when the classes are built in this way and we want to create instances of a "class", we don't need to know what each "class" is inheriting from. We just use new.
// Make an instance object of the Cat "Class"
var tiger = new Cat("tiger");
console.log(tiger.isCat, tiger.isMammal, tiger.isAnimal, tiger.isLifeform);
// Outputs: true true true true
The order of precendence should make sense. First it looks in the instance object, then it's prototype, then the next prototype, etc.
// Let's say we have another instance, a special alien cat
var alienCat = new Cat("alien");
// We can define a property for the instance object and that will take
// precendence over the value in the Mammal class (down the chain)
alienCat.isMammal = false;
// OR maybe all cats are mutated to be non-mammals
Cat.prototype.isMammal = false;
console.log(alienCat);
We can also modify the prototypes which will effect all objects built on the class.
// All cats are mutated to be non-mammals
Cat.prototype.isMammal = false;
console.log(tiger, alienCat);
I originally wrote some of this up with this answer.

A latecomer in the scene is SimpleDeclare. However, when dealing with multiple inheritance, you will still end up with copies of the original constructors. That's a necessity in Javascript...
Merc.

I would use ds.oop. Its similar to prototype.js and others. makes multiple inheritance very easy and its minimalist. (only 2 or 3 kb) Also supports some other neat features like interfaces and dependency injection
/*** multiple inheritance example ***********************************/
var Runner = ds.class({
run: function() { console.log('I am running...'); }
});
var Walker = ds.class({
walk: function() { console.log('I am walking...'); }
});
var Person = ds.class({
inherits: [Runner, Walker],
eat: function() { console.log('I am eating...'); }
});
var person = new Person();
person.run();
person.walk();
person.eat();

How about this, it implements multiple inheritance in JavaScript:
class Car {
constructor(brand) {
this.carname = brand;
}
show() {
return 'I have a ' + this.carname;
}
}
class Asset {
constructor(price) {
this.price = price;
}
show() {
return 'its estimated price is ' + this.price;
}
}
class Model_i1 { // extends Car and Asset (just a comment for ourselves)
//
constructor(brand, price, usefulness) {
specialize_with(this, new Car(brand));
specialize_with(this, new Asset(price));
this.usefulness = usefulness;
}
show() {
return Car.prototype.show.call(this) + ", " + Asset.prototype.show.call(this) + ", Model_i1";
}
}
mycar = new Model_i1("Ford Mustang", "$100K", 16);
document.getElementById("demo").innerHTML = mycar.show();
And here's the code for specialize_with() utility function:
function specialize_with(o, S) { for (var prop in S) { o[prop] = S[prop]; } }
This is real code that runs. You can copy-paste it in html file, and try it yourself. It does work.
That's the effort to implement MI in JavaScript. Not much of code, more of a know-how.
Please feel free to look at my complete article on this, https://github.com/latitov/OOP_MI_Ct_oPlus_in_JS

I just used to assign what classes I need in properties of others, and add a proxy to auto-point to them i like:
class A {
constructor()
{
this.test = "a test";
}
method()
{
console.log("in the method");
}
}
class B {
constructor()
{
this.extends = [new A()];
return new Proxy(this, {
get: function(obj, prop) {
if(prop in obj)
return obj[prop];
let response = obj.extends.find(function (extended) {
if(prop in extended)
return extended[prop];
});
return response ? response[prop] : Reflect.get(...arguments);
},
})
}
}
let b = new B();
b.test ;// "a test";
b.method(); // in the method

How can I create an object of fixed structure?

I have the following code inside my revealing module, but I am uncertain with how to declare/define imageListItem, which is strictly a DTO and doesn't really require any information hiding. Am I correctly defining this object?
var imageListItem = function() {
var _title;
Object.defineProperty(this, "title", {
get: function () { return _title; },
set: function (value) { _title = value; }
}
);
};
var imageList = (function () {
var buffer = new CBuffer();
return {
populate: function (listItems) {
buffer.push(listItems);
},
rotate: function() {
buffer.rotateLeft();
}
}
})();
With imageListItem, I want to declare an object structure for later use. That declaration should not logically be dependent on how that object will later be used. That is, I don't want to find myself dynamically assigning new properties to, or deleting properties from, imageListItem by accident. Any assignment to properties should strictly be only to properties that have already been declared on the object.
Object.freeze() almost accomplihses this, by preventing properties being added or removed, but it also prevents properties being changed.
E.g. I want this:
var obj = {
prop: function() {},
foo: 'bar'
};
// New properties may be added, existing properties may be changed or removed
obj.foo = 'baz';
obj.lumpy = 'woof';
var o = Object.freeze(obj);
// Now any changes will fail
function fail(){
'use strict';
obj.delete(foo); // throws a TypeError
obj.quaxxor = 'the friendly duck'; // throws a TypeError
}
I dont' want this:
// Now any changes will fail
function fail(){
'use strict';
obj.foo = 'sparky'; // throws a TypeError
}
You see? I want freeze to prevent quaxxor being added to obj, but I don't want it to prevent me changing the value of foo.

What you are looking for may be either Object.preventExtensions() or Object.seal().
Similarly to Object.freeze(), both methods prevent new properties from being added to the object, nevertheless allow changing values of existing properties.
The difference between seal and preventExtensions is that seal strictly disallows deletion and conversion of properties from/to data accessors, while preventExtensions doesn't actually prevent existing properties from being deleted: this behavior depends on the JS engine you're using (some engines may let you delete the property, other ones may not).
So basically, quoting from the MDN Documentation:
The Object.preventExtensions() method prevents new properties from ever being added to an object (i.e. prevents future extensions to the object). [...] Note that the properties of a non-extensible object, in general, may still be deleted.
The Object.seal() method seals an object, preventing new properties from being added to it and marking all existing properties as non-configurable. Values of present properties can still be changed as long as they are writable. [...] Attempting to delete or add properties to a sealed object, or to convert a data property to accessor or vice versa, will fail.
Here's an example to demonstrate the behavior of both methods:
var myFirstObj = { foo: 1 },
mySecondObj = { bar: "baz" };
Object.preventExtensions(myFirstObj);
Object.seal(mySecondObj);
myFirstObj.foo = false; // Works fine
mySecondObj.baz = "hello"; // Works fine
delete myFirstObj.foo; // May work fine depending on your JS engine
(function() {
'use strict';
myFirstObj.qux = 'something'; // Throws a TypeError
mySecondObj.qux = 'something'; // Throws a TypeError
delete mySecondObj.foo; // Throws a TypeError
})();
Now, talking about your ImageListItem Object, you can achieve what you want simply adding a line of code:
var ImageListItem = function() {
var _title;
Object.defineProperty(this, "title", {
get: function () { return _title; },
set: function (value) { _title = value; }
});
// Choose the one which fits your needs
Object.preventExtensions(this);
// or
Object.seal(this);
};

Javascript equivalent function to PHP __call [duplicate]

I am aware of how to create getters and setters for properties whose names one already knows, by doing something like this:
// A trivial example:
function MyObject(val){
this.count = 0;
this.value = val;
}
MyObject.prototype = {
get value(){
return this.count < 2 ? "Go away" : this._value;
},
set value(val){
this._value = val + (++this.count);
}
};
var a = new MyObject('foo');
alert(a.value); // --> "Go away"
a.value = 'bar';
alert(a.value); // --> "bar2"
Now, my question is, is it possible to define sort of catch-all getters and setters like these? I.e., create getters and setters for any property name which isn't already defined.
The concept is possible in PHP using the __get() and __set() magic methods (see the PHP documentation for information on these), so I'm really asking is there a JavaScript equivalent to these?
Needless to say, I'd ideally like a solution that is cross-browser compatible.

This changed as of the ES2015 (aka "ES6") specification: JavaScript now has proxies. Proxies let you create objects that are true proxies for (facades on) other objects. Here's a simple example that turns any property values that are strings to all caps on retrieval, and returns "missing" instead of undefined for a property that doesn't exist:
"use strict";
if (typeof Proxy == "undefined") {
throw new Error("This browser doesn't support Proxy");
}
let original = {
example: "value",
};
let proxy = new Proxy(original, {
get(target, name, receiver) {
if (Reflect.has(target, name)) {
let rv = Reflect.get(target, name, receiver);
if (typeof rv === "string") {
rv = rv.toUpperCase();
}
return rv;
}
return "missing";
}
});
console.log(`original.example = ${original.example}`); // "original.example = value"
console.log(`proxy.example = ${proxy.example}`); // "proxy.example = VALUE"
console.log(`proxy.unknown = ${proxy.unknown}`); // "proxy.unknown = missing"
original.example = "updated";
console.log(`original.example = ${original.example}`); // "original.example = updated"
console.log(`proxy.example = ${proxy.example}`); // "proxy.example = UPDATED"
Operations you don't override have their default behavior. In the above, all we override is get, but there's a whole list of operations you can hook into.
In the get handler function's arguments list:
target is the object being proxied (original, in our case).
name is (of course) the name of the property being retrieved, which is usually a string but could also be a Symbol.
receiver is the object that should be used as this in the getter function if the property is an accessor rather than a data property. In the normal case this is the proxy or something that inherits from it, but it can be anything since the trap may be triggered by Reflect.get.
This lets you create an object with the catch-all getter and setter feature you want:
"use strict";
if (typeof Proxy == "undefined") {
throw new Error("This browser doesn't support Proxy");
}
let obj = new Proxy({}, {
get(target, name, receiver) {
if (!Reflect.has(target, name)) {
console.log("Getting non-existent property '" + name + "'");
return undefined;
}
return Reflect.get(target, name, receiver);
},
set(target, name, value, receiver) {
if (!Reflect.has(target, name)) {
console.log(`Setting non-existent property '${name}', initial value: ${value}`);
}
return Reflect.set(target, name, value, receiver);
}
});
console.log(`[before] obj.example = ${obj.example}`);
obj.example = "value";
console.log(`[after] obj.example = ${obj.example}`);
The output of the above is:
Getting non-existent property 'example'
[before] obj.example = undefined
Setting non-existent property 'example', initial value: value
[after] obj.example = value
Note how we get the "non-existent" message when we try to retrieve example when it doesn't yet exist, and again when we create it, but not after that.
Answer from 2011 (obsoleted by the above, still relevant to environments limited to ES5 features like Internet Explorer):
No, JavaScript doesn't have a catch-all property feature. The accessor syntax you're using is covered in Section 11.1.5 of the spec, and doesn't offer any wildcard or something like that.
You could, of course, implement a function to do it, but I'm guessing you probably don't want to use f = obj.prop("example"); rather than f = obj.example; and obj.prop("example", value); rather than obj.example = value; (which would be necessary for the function to handle unknown properties).
FWIW, the getter function (I didn't bother with setter logic) would look something like this:
MyObject.prototype.prop = function(propName) {
if (propName in this) {
// This object or its prototype already has this property,
// return the existing value.
return this[propName];
}
// ...Catch-all, deal with undefined property here...
};
But again, I can't imagine you'd really want to do that, because of how it changes how you use the object.

Preface:
T.J. Crowder's answer mentions a Proxy, which will be needed for a catch-all getter/setter for properties which don't exist, as the OP was asking for. Depending on what behavior is actually wanted with dynamic getters/setters, a Proxy may not actually be necessary though; or, potentially, you may want to use a combination of a Proxy with what I'll show you below.
(P.S. I have experimented with Proxy thoroughly in Firefox on Linux recently and have found it to be very capable, but also somewhat confusing/difficult to work with and get right. More importantly, I have also found it to be quite slow (at least in relation to how optimized JavaScript tends to be nowadays) - I'm talking in the realm of deca-multiples slower.)
To implement dynamically created getters and setters specifically, you can use Object.defineProperty() or Object.defineProperties(). This is also quite fast.
The gist is that you can define a getter and/or setter on an object like so:
let obj = {};
let val = 0;
Object.defineProperty(obj, 'prop', { //<- This object is called a "property descriptor".
//Alternatively, use: `get() {}`
get: function() {
return val;
},
//Alternatively, use: `set(newValue) {}`
set: function(newValue) {
val = newValue;
}
});
//Calls the getter function.
console.log(obj.prop);
let copy = obj.prop;
//Etc.
//Calls the setter function.
obj.prop = 10;
++obj.prop;
//Etc.
Several things to note here:
You cannot use the value property in the property descriptor (not shown above) simultaneously with get and/or set; from the docs:
Property descriptors present in objects come in two main flavors: data descriptors and accessor descriptors. A data descriptor is a property that has a value, which may or may not be writable. An accessor descriptor is a property described by a getter-setter pair of functions. A descriptor must be one of these two flavors; it cannot be both.
Thus, you'll note that I created a val property outside of the Object.defineProperty() call/property descriptor. This is standard behavior.
As per the error here, don't set writable to true in the property descriptor if you use get or set.
You might want to consider setting configurable and enumerable, however, depending on what you're after; from the docs:
configurable
true if and only if the type of this property descriptor may be changed and if the property may be deleted from the corresponding object.
Defaults to false.
enumerable
true if and only if this property shows up during enumeration of the properties on the corresponding object.
Defaults to false.
On this note, these may also be of interest:
Object.getOwnPropertyNames(obj): gets all properties of an object, even non-enumerable ones (AFAIK this is the only way to do so!).
Object.getOwnPropertyDescriptor(obj, prop): gets the property descriptor of an object, the object that was passed to Object.defineProperty() above.
obj.propertyIsEnumerable(prop);: for an individual property on a specific object instance, call this function on the object instance to determine whether the specific property is enumerable or not.

The following could be an original approach to this problem:
var obj = {
emptyValue: null,
get: function(prop){
if(typeof this[prop] == "undefined")
return this.emptyValue;
else
return this[prop];
},
set: function(prop,value){
this[prop] = value;
}
}
In order to use it the properties should be passed as strings.
So here is an example of how it works:
//To set a property
obj.set('myProperty','myValue');
//To get a property
var myVar = obj.get('myProperty');
Edit:
An improved, more object-oriented approach based on what I proposed is the following:
function MyObject() {
var emptyValue = null;
var obj = {};
this.get = function(prop){
return (typeof obj[prop] == "undefined") ? emptyValue : obj[prop];
};
this.set = function(prop,value){
obj[prop] = value;
};
}
var newObj = new MyObject();
newObj.set('myProperty','MyValue');
alert(newObj.get('myProperty'));
You can see it working here.

I was looking for something and I figured out on my own.
/*
This function takes an object and converts to a proxy object.
It also takes care of proxying nested objectsa and array.
*/
let getProxy = (original) => {
return new Proxy(original, {
get(target, name, receiver) {
let rv = Reflect.get(target, name, receiver);
return rv;
},
set(target, name, value, receiver) {
// Proxies new objects
if(typeof value === "object"){
value = getProxy(value);
}
return Reflect.set(target, name, value, receiver);
}
})
}
let first = {};
let proxy = getProxy(first);
/*
Here are the tests
*/
proxy.name={} // object
proxy.name.first={} // nested object
proxy.name.first.names=[] // nested array
proxy.name.first.names[0]={first:"vetri"} // nested array with an object
/*
Here are the serialised values
*/
console.log(JSON.stringify(first)) // {"name":{"first":{"names":[{"first":"vetri"}]}}}
console.log(JSON.stringify(proxy)) // {"name":{"first":{"names":[{"first":"vetri"}]}}}

var x={}
var propName = 'value'
var get = Function("return this['" + propName + "']")
var set = Function("newValue", "this['" + propName + "'] = newValue")
var handler = { 'get': get, 'set': set, enumerable: true, configurable: true }
Object.defineProperty(x, propName, handler)
this works for me

Is there a tidy way to create 'internal' properties in JavaScript?

In JavaScript, I may begin writing a 'library' or collection of functionality using a top level object like this:
window.Lib = (function()
{
return {
// Define Lib here.
//
};
})();
I may also add some functions within Lib which serve to create objects related to it:
window.Lib = (function()
{
return {
ObjectA: function()
{
var _a = 5;
return {
getA: function(){ return _a; }
};
},
ObjectB: function()
{
var _b = 2;
var _c = 1;
return {
getB: function(){ return _b; }
};
}
};
})();
Which would be used like so:
var thing = Lib.ObjectA();
var thing2 = Lib.ObjectA();
var thing3 = Lib.ObjectB();
And I can use the methods within each of those created above to get the values of _a defined within ObjectA() or _b defined within ObjectB():
alert(thing.getA()); // 5
alert(thing3.getB()); // 2
What I want to achieve is this:
Say I want to access the property _c (defined within ObjectB()) but only within the scope of Lib. How could I go about that? By this I mean, I want to make the property readable within any function that I define within the object returned by Lib(), but I don't want to expose those values outside of that.
Code example:
window.Lib = (function()
{
return {
ObjectA: function(){ ... },
ObjectB: function(){ ... },
assess: function(obj)
{
// Somehow get _c here.
alert( obj.getInternalC() );
}
};
})();
Which would work like so:
var thing = Lib.ObjectB();
alert( thing.getInternalC() ) // error | null | no method named .getInternalC()
Lib.assess(thing); // 1
Hope this makes sense.

So you want per-instance protected properties? That is, properties on the instances created by ObjectA, ObjectB, etc., but which are only accessible to the code within your library, and not to code outside it?
You cannot currently do that properly in JavaScript, but you'll be able to in the next version using private name objects. (See "Almost doing it" below for something similar you can do now in ES5, though.)
It's easy to create data that's shared by all code within Lib, but not per-instance properties, like so:
window.Lib = (function()
{
var sharedData;
// ...
})();
All of the functions defined within there (your ObjectA, etc.) will have access to that one sharedData variable, which is completely inaccessible from outside. But it's not per-instance, each object created by ObjectA, ObjectB, etc. doesn't get its own copy.
Almost doing it
If your code will be running in an environment with ES5 (so, any modern browser, where "modern" does not include IE8 or earlier), you can have obscured but not actually private properties, via Object.defineProperty. This is similar to how private name objects will work in ES.next, but not genuinely private:
Live Example | Source
window.Lib = (function() {
// Get a random name for our "c" property
var c = "__c" + Math.round(Math.random() * 1000000);
// Return our library functions
return {
ObjectA: function() {
// Create an object with a couple of public proprties:
var obj = {
pub1: "I'm a public property",
pub2: "So am I"
};
// Add our obscured "c" property to it, make sure it's
// non-enumerable (doesn't show up in for-in loops)
Object.defineProperty(obj, c, {
enumerable: false, // false is actually the default value, just emphasizing
writable: true,
value: "I'm an obscured property"
});
// Return it
return obj;
},
ObjectB: function(){ /* ... */ },
assess: function(obj) {
// Here, we access the property using the `c` variable, which
// contains the property name. In JavaScript, you can access
// properties either using dotted notation and a literal
// (`foo.propName`), or using bracketed notation and a string
// (`foo["propName"]`). Here we're using bracketed notation,
// and our `c` string, which has the actual property name.
display( obj[c] );
},
alter: function(obj, value) {
// Similarly, we can change the value with code that has
// access to the `c` variable
obj[c] = value;
}
};
})();
And use it like this:
// Create our object
var o = Lib.ObjectA();
// Play with it
display("pub1: " + o.pub1); // displays "pub1: I'm a public property"
display("c: " + o.c); // displays "c: undefined" since `o` has no property called `c`
Lib.assess(o); // displays "I'm an obscured property"
// Note that our obscured property doesn't show up in for-in loops or Object.keys:
var propName, propNames = [];
for (propName in o) {
propNames.push(propName);
}
display("propNames: " + propNames.join(","));
display("Object.keys: " + Object.keys(o).join(","));
// Our Lib code can modify the property
Lib.alter(o, "Updated obscured property");
Lib.assess(o);
The object returned by Lib.ObjectA has a property whose name will change every time Lib is loaded, and which is not enumerable (doesn't show up in for-in loops). The only way to get at it is to know it's name (which, again, changes every time Lib is created — e.g., every page load). The code within Lib knows what the property name is, because it's in the c variable which is shared by all of the Lib code. Since you can access properties using bracketed notation and a string, we can use instance[c] to access the property.
You see how these are pretty well obscured. Code outside of Lib don't see the obscured property when enumerating the property in the object, and they don't know the semi-random name we assigned it, so can't find the property. Of course, you could find it via inspection using a debugger, but debuggers can do lots of things.
And in fact, this is how private properties will work in ES.next, except that c won't be a string, it'll be a private name object.

Well, you would "just" need to declare those variables within the Context of Lib
window.Lib = (function()
{
var _c = 42;
return {
};
});
Notice that I removed the automatic invocation of that pseudo constructor function. That means, you would need to create multiple calls to Lib() for multiple instances, each would have its own unique set of values.
var inst1 = Lib(),
inst2 = Lib();
If you only want to have shared access from all child-context's (functions), you can just use the same pattern you already do (only with moving the var declarations to the parent context like shown above).