[EloquentJS] Ch6. The Secret Life of Objects

Encapsulation

• The core idea in object-oriented programming is to divide programs into smaller pieces and make each piece responsible for managing its own state.
• Different pieces of such a program interact with each other through interfaces, limited sets of functions or bindings that provide useful functionality at a more abstract level, hiding their precise implementation.
• Such program pieces are modeled using objects. Their interface consists of a specific set of methods and properties. Properties that are part of the interface are called public. The others, which outside code should not be touching, are called private.
• Many languages provide a way to distinguish public and private properties and prevent outside code from accessing the private ones altogether. JavaScript, once again taking the minimalist approach, does not—not yet at least. There is work underway to add this to the language.
• Even though the language doesn’t have this distinction built in, JavaScript programmers are successfully using this idea. Typically, the available interface is described in documentation or comments. It is also common to put an underscore (_) character at the start of property names to indicate that those properties are private.
• Separating interface from implementation is a great idea. It is usually called encapsulation.

Methods

• Methods are nothing more than properties that hold function values.

• let rabbit = {};
  rabbit.speak = function(line) {
      console.log(`The rabbit says '${line}'`);
  }

• Usually a method needs to do something with the object it was called on. When a function is called as a method—looked up as a property and immediately called, as in object.method()—the binding called this in its body automatically points at the object that it was called on.

• function speak(line) {
      console.log(`The ${this.type} rabbit says '${line}'`);
  }
  let whiteRabbit = {type: "white", speak};
  let hungryRabbit = {type: "hungry", speak};
  whiteRabbit.speak("blahblah...");

• You can think of this as an extra parameter that is passed in a different way. If you want to pass it explicitly, you can use a function’s call method, which takes the this value as its first argument and treats further arguments as normal parameters.

• speak.call(hungryRabbit, "BlahBlah...");

• Since each function has its own this binding, whose value depends on the way it is called, you cannot refer to the this of the wrapping scope in a regular function defined with the function keyword. (아래 내용과 비교)
• Arrow functions are different—they do not bind their own this but can see the this binding of the scope around them. Thus, you can do something like the following code, which references this from inside a local function:

• function normalize() {
      console.log(this.coords.map(n => n / this.length));  
  }
  normalize.call({coords: [0, 2, 3], length: 5});

• If I had written the argument to map using the function keyword, the code wouldn’t work.

Prototypes

• let empty = {};
  console.log(empty.toString);
  // → function toString(){…}
  console.log(empty.toString());
  // → [object Object]

• In addition to their set of properties, most objects also have a prototype. A prototype is another object that is used as a fallback source of properties.
• When an object gets a request for a property that it does not have, its prototype will be searched for the property, then the prototype’s prototype, and so on.
• So who is the prototype of that empty object? It is the great ancestral prototype, the entity behind almost all objects, Object.prototype.

• console.log(Object.getPrototypeOf({}) == Object.prototype);
  // true
  console.log(Object.getPrototypeOf(Object.prototype));
  // null

• Object.getPrototypeOf returns the prototype of an object.
• The prototype relations of JavaScript objects form a tree-shaped structure, and at the root of this structure sits Object.prototype. It provides a few methods that show up in all objects, such as toString, which converts an object to a string representation.
• Many objects don’t directly have Object.prototype as their prototype but instead have another object that provides a different set of default properties. Functions derive from Function.prototype, and arrays derive from Array.prototype.
• Such a prototype object will itself have a prototype, often Object.prototype, so that it still indirectly provides methods like toString.
• You can use Object.create to create an object with a specific prototype.

• let protoRabbit = {
      speak(line) {
          console.log(`The ${this.type} rabbit says '${line}'`);
      }
  };
  let killerRabbit = Object.create(protoRabbit);
  killerRabbit.type = "killer";
  killerRabbit.speak("SKREEEE!");
  // → The killer rabbit says 'SKREEEE!'

• A property like speak(line) in an object expression is a shorthand way of defining a method. It creates a property called speak and gives it a function as its value.
• The “proto” rabbit acts as a container for the properties that are shared by all rabbits. An individual rabbit object, like the killer rabbit, contains properties that apply only to itself—in this case its type—and derives shared properties from its prototype.

Classes

• JavaScript’s prototype system can be interpreted as a somewhat informal take on an object-oriented concept called classes.

• A class defines the shape of a type of object—what methods and properties it has. Such an object is called an instance of the class.

• Prototypes are useful for defining properties for which all instances of a class share the same value, such as methods. Properties that differ per instance, such as our rabbits’ type property, need to be stored directly in the objects themselves.

• So to create an instance of a given class, you have to make an object that derives from the proper prototype, but you also have to make sure it, itself, has the properties that instances of this class are supposed to have. This is what a constructor function does.

• function makeRabbit(type) {
      let rabbit = Object.create(protoRabbit);
      rabbit.type = type;
      return rabbit;
  }

• JavaScript provides a way to make defining this type of function easier. If you put the keyword new in front of a function call, the function is treated as a constructor. This means that an object with the right prototype is automatically created, bound to this in the function, and returned at the end of the function.

• The prototype object used when constructing objects is found by taking the prototype property of the constructor function.

• function Rabbit(type) {
      this.type = type;
  }
  Rabbit.prototype.speak = function(line) {
      console.log(`The ${this.type} rabbit says '${line}'`);
  }
  
  let weirdRabbit = new Rabbit("weird");

• Constructors (all functions, in fact) automatically get a property named prototype, which by default holds a plain, empty object that derives from Object.prototype. You can overwrite it with a new object if you want. Or you can add properties to the existing object, as the example does.

• By convention, the names of constructors are capitalized so that they can easily be distinguished from other functions.

• It is important to understand the distinction between the way a prototype is associated with a constructor (through its prototype property) and the way objects have a prototype (which can be found with Object.getPrototypeOf).

• The actual prototype of a constructor is Function.prototype since constructors are functions. Its prototype property holds the prototype used for instances created through it.

• console.log(Object.getPrototypeOf(Rabbit) == Function.prototype);
  // true
  console.log(Object.getPrototypeOf(weirdRabbit) == Rabbit.prototype);
  // true
  console.log(Object.getPrototypeOf(Rabbit.prototype) == Object.prototype);
  // true

Class notation

• So JavaScript classes are constructor functions with a prototype property.

• That is how they work, and until 2015, that was how you had to write them. These days, we have a less awkward notation.

• class Rabbit {
      constructor(type) {
          this.type = type;
      }
      speak(line) {
          console.log(`The ${this.type} rabbit says '${line}'`);
      }
  }
  
  let killerRabbit = new Rabbit("killer");
  let blackRabbit = new Rabbit("black");

• The class keyword starts a class declaration, which allows us to define a constructor and a set of methods all in a single place. Any number of methods may be written inside the declaration’s braces.

• The one named constructor is treated specially. It provides the actual constructor function, which will be bound to the name Rabbit. The others are packaged into that constructor’s prototype.

• Thus, the earlier class declaration is equivalent to the constructor definition from the previous section. It just looks nicer.

• Class declarations currently allow only methods to be added to the prototype. This can be somewhat inconvenient when you want to save a non-function value in there.

• For now, you can create such properties by directly manipulating the prototype after you’ve defined the class.

• Like function, class can be used both in statements and in expressions. When used as an expression, it doesn’t define a binding but just produces the constructor as a value. You are allowed to omit the class name in a class expression.

• let object = new class { getWord() { return "hello"; } };
  console.log(object.getWord());

Overriding derived properties

• When you add a property to an object, whether it is present in the prototype or not, the property is added to the object itself. If there was already a property with the same name in the prototype, this property will no longer affect the object, as it is now hidden behind the object’s own property.

• Rabbit.prototype.teeth = "small";
  console.log(killerRabbit.teeth);
  // small
  killerRabbit.teeth = "long, sharp, and bloody";
  console.log(killerRabbit.teeth);
  // long, sharp, and bloody
  console.log(blackRabbit.teeth);
  // small
  console.log(Rabbit.prototype.teeth);
  // small

• The following diagram sketches the situation after this code has run. The Rabbit and Object prototypes lie behind killerRabbit as a kind of backdrop, where properties that are not found in the object itself can be looked up.
• 
• Overriding properties that exist in a prototype can be a useful thing to do. As the rabbit teeth example shows, overriding can be used to express exceptional properties in instances of a more generic class of objects, while letting the nonexceptional objects take a standard value from their prototype.
• Overriding is also used to give the standard function and array prototypes a different toString method than the basic object prototype.

• console.log(Array.prototype.toString ==
              Object.prototype.toString);
  // → false
  console.log([1, 2].toString());
  // → 1,2

• Calling toString on an array gives a result similar to calling .join(",") on it—it puts commas between the values in the array. Directly calling Object.prototype.toString with an array produces a different string. That function doesn’t know about arrays, so it simply puts the word object and the name of the type between square brackets.

• console.log(Object.prototype.toString.call([1, 2]));
  // → [object Array]

Maps

• A map (noun) is a data structure that associates values (the keys) with other values. It is possible to use objects for this.

• let ages = {
      Boris: 39,
      Liang: 22,
      Julia: 62
  };
  console.log(`Julia is ${ages["Julia"]}`);
  // Julia is 62
  console.log("Is Jack's age known?", "Jack" in ages);
  // Is Jack's age known? false
  console.log("Is toString's age known?", "toString" in ages);
  // Is toString's age known? true

• But we certainly didn’t list anybody named toString in our map. Yet, because plain objects derive from Object.prototype, it looks like the property is there.
• As such, using plain objects as maps is dangerous. There are several possible ways to avoid this problem.
• First, it is possible to create objects with no prototype. If you pass null to Object.create, the resulting object will not derive from Object.prototype and can safely be used as a map.

• console.log("toString" in Object.create(null));
  // false

• Object property names must be strings. If you need a map whose keys can’t easily be converted to strings—such as objects—you cannot use an object as your map.
• Fortunately, JavaScript comes with a class called Map that is written for this exact purpose. It stores a mapping and allows any type of keys.

• let ages = new Map();
  ages.set("Boris", 39);
  ages.set("Liang", 22);
  ages.set("Julia", 62);
  console.log(`Julia is ${ages.get("Julia")}`);
  console.log("Is Jack's age known?", ages.has("Jack"));
  // ... false
  console.log("Is toString's age known?", ages.has("toString"));
  // ... false

• The methods set, get, and has are part of the interface of the Map object.
• If you do have a plain object that you need to treat as a map for some reason, it is useful to know that Object.keys returns only an object’s own keys, not those in the prototype. As an alternative to the in operator, you can use the hasOwnProperty method, which ignores the object’s prototype.

• console.log({x: 1}.hasOwnProperty("x"));
  // true
  console.log({x: 1}.hasOwnProperty("toString"));
  // false

Polymorphism

• When you call the String function (which converts a value to a string) on an object, it will call the toString method on that object to try to create a meaningful string from it. I mentioned that some of the standard prototypes define their own version of toString so they can create a string that contains more useful information than "[object Object]".

• Rabbit.prototype.toString = function() {
      return `a ${this.type} rabbit`;
  };
  console.log(String(blackRabbit));
  // a black rabbit

• When a piece of code is written to work with objects that have a certain interface—in this case, a toString method—any kind of object that happens to support this interface can be plugged into the code, and it will just work.
• This technique is called polymorphism. Polymorphic code can work with values of different shapes, as long as they support the interface it expects.
• A for/of loop can loop over several kinds of data structures. This is another case of polymorphism—such loops expect the data structure to expose a specific interface, which arrays and strings do. And we can also add this interface to your own objects.

Symbols

• It is possible for multiple interfaces to use the same property name for different things. (That would be a bad idea, and this problem isn’t that common.)
• When I claimed that property names are strings, that wasn’t entirely accurate. They usually are, but they can also be symbols.
• Symbols are values created with the Symbol function. Unlike strings, newly created symbols are unique—you cannot create the same symbol twice.

• let sym = Symbol("name");
  console.log(sym == Symbol("name"));
  // false
  Rabbit.prototype[sym] = 55;
  console.log(blackRabbit[sym]);
  // 55

• The string you pass to Symbol is included when you convert it to a string and can make it easier to recognize a symbol when, for example, showing it in the console. But it has no meaning beyond that—multiple symbols may have the same name.
• Being both unique and usable as property names makes symbols suitable for defining interfaces that can peacefully live alongside other properties, no matter what their names are.

• const toStringSymbol = Symbol("toString");
  Array.prototype[toStringSymbol] = function() {
      return `${this.length} cm of blue yarn`;
  };
  console.log([1, 2].toString());
  // 1,2
  console.log([1, 2][toStringSymbol]());
  // 2 cm of blue yarn

• It is possible to include symbol properties in object expressions and classes by using square brackets around the property name. That causes the property name to be evaluated, much like the square bracket property access notation, which allows us to refer to a binding that holds the symbol.

• let stringObject = {
      [toStringObject]() {
          return "a jute rope";
      }
  };
  console.log(stringObject[toStringObject]());
  // a jute rope

The iterator interface

• The object given to a for/of loop is expected to be iterable. This means it has a method named with the Symbol.iterator symbol (a symbol value defined by the language, stored as a property of the Symbol function).

• When called, that method should return an object that provides a second interface, iterator. This is the actual thing that iterates. It has a next method that returns the next result. That result should be an object with a value property that provides the next value, if there is one, and a done property, which should be true when there are no more results and false otherwise.

• Note that the next, value, and done property names are plain strings, not symbols. Only Symbol.iterator, which is likely to be added to a lot of different objects, is an actual symbol. (Why?)

• let okIterator = "OK"[Symbol.iterator]();
  console.log(okIterator.next());
  // → {value: "O", done: false}
  console.log(okIterator.next());
  // → {value: "K", done: false}
  console.log(okIterator.next());
  // → {value: undefined, done: true}

• Let’s implement an iterable data structure. We’ll build a matrix class, acting as a two-dimensional array.

• class Matrix {
      constructor(width, height, element = (x, y) => undefined) {
          this.width = width;
          this.height = height;
          this.content = [];
  
          for (let y = 0; y < height; y++) {
              for (let x = 0; x < width; x++) {
                  this.content[y * width + x] = element(x, y);
              }
          } 
      }
  
      get(x, y) {
          return this.content[y * this.width + x];
      }
      set(x, y, value) {
          this.content[y * this.width + x] = value;
      }
  }

• When looping over a matrix, you are usually interested in the position of the elements as well as the elements themselves, so we’ll have our iterator produce objects with x, y, and value properties.

• class MatrixIterator {
      constructor(matrix) {
          this.x = 0;
          this.y = 0;
          this.matrix = matrix;
      }
  
      next() {
          if (y == this.matrix.heigth) return {done: true};
  
          let value = {x: this.x,
                       y: this.y,
                       value: this.matrix.get(this.x, this.y)};
          this.x++;
          if (this.x == this.matrix.width) {
              this.x = 0;
              this.y++;
          }
          return {value, done: false};
      }
  }

• Let’s set up the Matrix class to be iterable.

• (Throughout this book, I’ll occasionally use after-the-fact prototype manipulation to add methods to classes so that the individual pieces of code remain small and self-contained. In a regular program, where there is no need to split the code into small pieces, you’d declare these methods directly in the class instead.)

• Matrix.prototype[Symbol.iterator] = function() {
      return new MatrixIterator(this);
  };

• let matrix = new Matrix(2, 2, (x, y) => `value ${x},${y}`);
  for (let {x, y, value} of matrix) {
      console.log(x, y, value);
  }
  // → 0 0 value 0,0
  // → 1 0 value 1,0
  // → 0 1 value 0,1
  // → 1 1 value 1,1

Getters, setters, and statics

• Interfaces often consist mostly of methods, but it is also okay to include properties that hold non-function values. For example, Map objects have a size property that tells you how many keys are stored in them.

• It is not even necessary for such an object to compute and store such a property directly in the instance. Even properties that are accessed directly may hide a method call. Such methods are called getters, and they are defined by writing get in front of the method name in an object expression or class declaration.

• let varyingSize = {
      get size() {
          return Math.floor(Math.random() * 100);
      }
  };
  console.log(varyingSize.size);
  console.log(varyingSize.size);

• Whenever someone reads from this object’s size property, the associated method is called.

• You can do a similar thing when a property is written to, using a setter.

• class Temperature {
      constructor(celsius) {
          this.celsius = celsius;
      }
  
      get fahrenheit() {
          return this.celsius * 1.8 + 32;
      }
      set fahrenheit(value) {
          this.celsius = (value - 32) / 1.8;
      }
  
      static fromFahrenheit(value) {
          return new Temperature((value - 32) / 1.8);
      }
  }
  
  let temp = new Temperature(22);
  console.log(temp.fahrenheit);
  // 71.6
  temp.fahrenheit = 86;
  console.log(temp.celsius);
  // 30

• The Temperature class allows you to read and write the temperature in either degrees Celsius or degrees Fahrenheit, but internally it stores only Celsius and automatically converts to and from Celsius in the fahrenheit getter and setter.

• Sometimes you want to attach some properties directly to your constructor function, rather than to the prototype. Such methods won’t have access to a class instance but can, for example, be used to provide additional ways to create instances.

• Inside a class declaration, methods that have static written before their name are stored on the constructor. So the Temperature class allows you to write Temperature.fromFahrenheit(100) to create a temperature using degrees Fahrenheit.

Inheritance

• JavaScript’s prototype system makes it possible to create a new class, much like the old class, but with new definitions for some of its properties. The prototype for the new class derives from the old prototype but adds a new definition for, say, the set method.

• In object-oriented programming terms, this is called inheritance. The new class inherits properties and behavior from the old class.

• class SymmetricMatrix extends Matrix {
      constructor(size, element = (x, y) => undefined) {
          super(size, size, (x, y) => {
              if (x < y) return element(y, x);
              else return element(x, y);
          });
      }
  
      set(x, y, value) {
          super.set(x, y, value);
          if (x != y) {
              super.set(y, x, value);
          }
      }
  }
  
  let matrix = new SymmetricMatrix(5, (x, y) => `${x},${y}`);
  console.log(matrix.get(2, 3));
  // 3,2
  
  console.log(Object.getPrototypeOf(SymmetricMatrix.prototype)
              == Matrix.prototype);
  // true

• The use of the word extends indicates that this class shouldn’t be directly based on the default Object prototype but on some other class. This is called the superclass. The derived class is the subclass.

• To initialize a SymmetricMatrix instance, the constructor calls its superclass’s constructor through the super keyword. This is necessary because if this new object is to behave (roughly) like a Matrix, it is going to need the instance properties that matrices have.

• We are redefining set but do want to use the original behavior. Because this.set refers to the new set method, calling that wouldn’t work. Inside class methods, super provides a way to call methods as they were defined in the superclass.

• Inheritance allows us to build slightly different data types from existing data types with relatively little work. It is a fundamental part of the object-oriented tradition, alongside encapsulation and polymorphism.

• But while the latter two are now generally regarded as wonderful ideas, inheritance is more controversial.

• Whereas encapsulation and polymorphism can be used to separate pieces of code from each other, reducing the tangledness of the overall program, inheritance fundamentally ties classes together, creating more tangle.

• When inheriting from a class, you usually have to know more about how it works than when simply using it.

• Inheritance can be a useful tool, and I use it now and then in my own programs, but it shouldn’t be the first tool you reach for, and you probably shouldn’t actively go looking for opportunities to construct class hierarchies (family trees of classes).

The instanceof operator

• It is occasionally useful to know whether an object was derived from a specific class.

• console.log(new SymmetricMatrix(2) instanceof SymmetricMatrix);
  // true
  console.log(new SymmetricMatrix(2) instanceof Matrix);
  // true
  console.log(new Matrix(2, 2) instanceof SymmetricMatrix);
  // false
  console.log([1] instanceof Array);
  // true

• The operator will see through inherited types, so a SymmetricMatrix is an instance of Matrix. The operator can also be applied to standard constructors like Array. Almost every object is an instance of Object.
• (생각해보니 Object.prototype도 특별한게 아니었음. Object 생성자의 prototype 속성이네)

Summary (헷갈릴 때마다 원문 다시 읽기)

• So objects do more than just hold their own properties. They have prototypes, which are other objects. Simple objects have Object.prototype as their prototype.
• Constructors, which are functions whose names usually start with a capital letter, can be used with the new operator to create new objects. The new object’s prototype will be the object found in the prototype property of the constructor. You can make good use of this by putting the properties that all values of a given type share into their prototype.
• Static methods are methods stored in a class’s constructor, rather than its prototype.

Exercises

A vector type

class Vec {
    constructor(x, y) {
        this.x = x;
        this.y = y;
    }

    plus(other) {
        return new Vec(this.x + other.x, this.y + other.y);
    }
    minus(other) {
        return new Vec(this.x - other.x, this.y - other.y);
    }
    get length() {
        return Math.sqrt(this.x * this.x + this.y * this.y);
    }
}

Groups

• The standard JavaScript environment provides another data structure called Set. Like an instance of Map, a set holds a collection of values. Unlike Map, it does not associate other values with those—it just tracks which values are part of the set. A value can be part of a set only once—adding it again doesn’t have any effect.

  class Group {
    constructor() { 
        this.elements = [];
    }
  
    add(e) {
        // Note: this.has()
        if (!this.has(e)) { 
            this.elements.push(e);
        }
    }
    delete(e) {
        // Note: !== 사용, 덮어 쓰기
        this.elements = this.elements.filter(elem => elem !== e);  
    }
    has(e) {
        return this.elements.includes(e);
    }
  
    static from(sequence) {
        let output = new Group();
        for (let e of sequence) output.add(e);
        return output;
    }
  }

Iterable groups

class GroupIterator {
    constructor(group) {
        this.group = group;
        this.index = -1;
    }

    next() {
        this.index++;
        if (this.index === this.group.elements.length) {
            return {done: true};
        }
        return {value: this.group.elements[this.index], done: false};
    }
}

Group.prototype[Symbol.iterator] = function() {
    return new GroupIterator(this);
}
/* Alternative:
(in class Group...)
    [Symbol.iterator]() {
        return new GroupIterator(this);
    }
*/

Borrowing a method (Fail to solve)

let map = {one: true, two: true, hasOwnProperty: true};

console.log(Object.prototype.hasOwnProperty.call(map, "one"));