Prototype-based programming

Prototype-based programming is a style of object-oriented programming in which behaviour reuse (known as inheritance) is performed via a process of reusing existing objects via delegation that serve as prototypes. This model can also be known as prototypal, prototype-oriented, classless, or instance-based programming. Delegation is the language feature that supports prototype-based programming.

Prototype object oriented programming uses generalized objects, which can then be cloned and extended. Using fruit as an example, a "fruit" object would represent the properties and functionality of fruit in general. A "banana" object would be cloned from the "fruit" object, and would also be extended to include general properties specific to bananas. Each individual "banana" object would be cloned from the generic "banana" object. Compare to the class-based paradigm, where a "fruit" class (not object) would be extended by a "banana" class.

Almost all prototype-based systems are based on interpreted and dynamically typed languages.

Object construction[edit]

In prototype-based languages there are no explicit classes. Objects inherit directly from other objects through a prototype property. The prototype property is called prototype in Self and JavaScript, or proto in Io. There are two methods of constructing new objects: ex nihilo ("from nothing") object creation or through cloning an existing object. The former is supported through some form of object literal, declarations where objects can be defined at runtime through special syntax such as {...} and passed directly to a variable. While most systems support a variety of cloning, ex nihilo object creation is not as prominent.^[4]

In class-based languages, a new instance is constructed through a class's constructor function, a special function that reserves a block of memory for the object's members (properties and methods) and returns a reference to that block. An optional set of constructor arguments can be passed to the function and are usually held in properties. The resulting instance will inherit all the methods and properties that were defined in the class, which acts as a kind of template from which similar typed objects can be constructed.

Delegation[edit]

In prototype-based languages that use delegation, the language runtime is capable of dispatching the correct method or finding the right piece of data simply by following a series of delegation pointers (from object to its prototype) until a match is found. All that is required to establish this behavior-sharing between objects is the delegation pointer. Unlike the relationship between class and instance in class-based object-oriented languages, the relationship between the prototype and its offshoots does not require that the child object have a memory or structural similarity to the prototype beyond this link. As such, the child object can continue to be modified and amended over time without rearranging the structure of its associated prototype as in class-based systems. It is also important to note that not only data, but also methods can be added or changed. For this reason, some prototype-based languages refer to both data and methods as "slots" or "members".^{[citation needed]}

// Example of true prototypal inheritance style 
// in JavaScript.

// object creation using the literal 
// object notation {}.
var foo = {name: "foo", one: 1, two: 2};

// Another object.
var bar = {two: "two", three: 3};

// Object.setPrototypeOf() is a method introduced in ECMAScript 2015.
// For the sake of simplicity, let us pretend 
// that the following line works regardless of the 
// engine used:
Object.setPrototypeOf(bar, foo); // foo is now the prototype of bar.

// If we try to access foo's properties from bar 
// from now on, we'll succeed. 
bar.one // Resolves to 1.

// The child object's properties are also accessible.
bar.three // Resolves to 3.

// Own properties shadow prototype properties
bar.two; // Resolves to "two"
bar.name; // unaffected, resolves to "foo"
foo.name; // Resolves to "foo"

var foo = {one: 1, two: 2};

// bar.[[prototype]] = foo
var bar = Object.create(foo);

bar.three = 3;

bar.one; // 1
bar.two; // 2
bar.three; // 3

在使用委托的基于原型的语言中，运行时语言可以仅仅通过循着一个序列的指针直到找到匹配这样的方式来定位属性或者寻找寻找正确的数据。所有这些建立行为共享的行为 需要的是委托指针。
不像是基于类的面向对象语言中类和借口的关系，原型和他的分支之间的关系并不要求子对象有相似的内存结构，因为如此，子对象可以继续修改和...而无须像基于类的系统那样整理结构。
还有一个要提到的地方是，不仅仅是数据，方法也能被修改。因为这个原因，大多数基于类型的语言把数据和方法提作“slots”。