Chapter 6
The Secret Life of Objects

The problem with object-oriented languages is they’ve got all this implicit environment that they carry around with them. You wanted a banana but what you got was a gorilla holding the banana and the entire jungle.

Joe Armstrong, interviewed in Coders at Work

When a programmer says “object”, this is a loaded term. In my profession, objects are a way of life, the subject of holy wars, and a beloved buzzword that still hasn’t quite lost its power.

To an outsider, this is probably a little confusing. Let’s start with a brief history of objects as a programming construct.

History

This story, like most programming stories, starts with the problem of complexity. One philosophy is that complexity can be made manageable by separating it into small compartments that are isolated from each other. These compartments have ended up with the name “objects”.

An object is a hard shell that hides the gooey complexity inside of it, and instead offers us a few knobs and connectors (like methods) that present an interface through which the object is to be used. The idea is that the interface is relatively simple, and all the complex things going on inside of the object can be ignored when working with it.

A simple interface can hide a lot of complexity

As an example, you can imagine an object that provides an interface to an area on your screen. It provides a way to draw shapes or text onto this area, but hides all the details of how these shapes are converted to the actual pixels that make up the screen. You’d have a set of methods, for example drawCircle, and those are the only things you need to know in order to use such an object.

These ideas were initially worked out in the 1970s and 80s, and, in the 90s, were carried up by a huge wave of hype—the object-oriented programming revolution. Suddenly, there was a large tribe of people declaring that objects were the right way to program, and that anything that did not involve objects was outdated nonsense.

That kind of zealotry always produces a lot of impractical silliness, and there has been a sort of counter-revolution since then. In some circles, objects have a rather bad reputation nowadays.

I prefer to look at the issue from a practical, rather than ideological angle. There are several very useful concepts, most importantly that of encapsulation (distinguishing between internal complexity and external interface), that the object-oriented culture has popularized. These are worth studying.

This chapter describes JavaScript’s rather eccentric take on objects, and the way they relate to some classical object-oriented techniques.

Methods

Methods are simply properties that hold function values. This is a very simple method:

var rabbit = {};
rabbit.speak = function(line) {
  console.log("The rabbit says '" + line + "'");
};

rabbit.speak("I'm alive.");
// → The rabbit says 'I'm alive.'

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 special variable this in its body will point to the object that it was called on.

function speak(line) {
  console.log("The " + this.type + " rabbit says '" +
              line + "'");
}
var whiteRabbit = {type: "white", speak: speak};
var fatRabbit = {type: "fat", speak: speak};

whiteRabbit.speak("Oh my ears and whiskers, " +
                  "how late it's getting!");
// → The white rabbit says 'Oh my ears and whiskers, how
//   late it's getting!'
fatRabbit.speak("I could sure use a carrot right now.");
// → The fat rabbit says 'I could sure use a carrot
//   right now.'

The code uses the this keyword to output the type of rabbit that is speaking.

Recall that the apply and bind methods both take a first argument that can be used to simulate method calls. This first argument is in fact used to give a value to this.

There is a method similar to apply, called call. It also calls the function it is a method of, but takes its arguments normally, rather than as an array. Like apply and bind, call can be passed a specific this value.

speak.apply(fatRabbit, ["Burp!"]);
// → The fat rabbit says 'Burp!'
speak.call({type: "old"}, "Oh my.");
// → The old rabbit says 'Oh my.'

Prototypes

Watch closely.

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

I just pulled a property out of an empty object. Magic!

Well, not really. I have simply been withholding information about the way JavaScript objects work. In addition to their set of properties, almost all 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, and 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

As you might expect, the Object.getPrototypeOf function 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, which provides its own default properties. Functions derive from Function.prototype, arrays from Array.prototype.

console.log(Object.getPrototypeOf(isNaN) ==
            Function.prototype);
// → true
console.log(Object.getPrototypeOf([]) ==
            Array.prototype);
// → true

Such a prototype object will itself have a prototype, often Object.prototype so that it still indirectly provides methods like toString.

The Object.getPrototypeOf function obviously returns the prototype of an object. You can use Object.create to create an object with a specific prototype.

var protoRabbit = {
  speak: function(line) {
    console.log("The " + this.type + " rabbit says '" +
                line + "'");
  }
};
var killerRabbit = Object.create(protoRabbit);
killerRabbit.type = "killer";
killerRabbit.speak("SKREEEE!");
// → The killer rabbit says 'SKREEEE!'

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.

Constructors

A more convenient way to create objects that derive from some shared prototype is to use a constructor. In JavaScript, calling a function with the new keyword in front of it causes it to be treated as a constructor. The constructor will have its this variable bound to a fresh object, and, unless it explicitly returns another object value, this new object will be returned from the call.

An object created with new is said to be an instance of its constructor.

Here is a simple constructor for rabbits. It is a convention to capitalize the names of constructors, so that they are easily distinguished from other functions.

function Rabbit(type) {
  this.type = type;
}

var killerRabbit = new Rabbit("killer");
var blackRabbit = new Rabbit("black");
console.log(blackRabbit.type);
// → black

Constructors (in fact, all functions) automatically get a property named prototype, which by default holds a plain, empty object that derives from Object.prototype. Every instance created with this constructor will have this object as its prototype. So to add a speak method to rabbits created with the Rabbit constructor, we can simply do this:

Rabbit.prototype.speak = function(line) {
  console.log("The " + this.type + " rabbit says '" +
              line + "'");
};
blackRabbit.speak("Doom...");
// → The black rabbit says 'Doom...'

It is important to note the distinction between the way a prototype is associated with a constuctor (through its prototype property), and the way objects have a prototype (which can be retrieved with Object.getPrototypeOf). The actual prototype of a constructor is Function.prototype, since constructors are functions. Its prototype property will be the prototype of instances created though it, but is not its own prototype.

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, which will henceforth have it as its own property. If there is a property by the same name in the prototype, this property will no longer affect the object. The prototype itself is not changed.

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.

Rabbit object prototype schema

Overriding properties that exist in a prototype is often a useful thing to do. As the rabbit teeth example shows, it can be used to express exceptional properties in instances of a more generic class of objects, while letting the non-exceptional objects simply take a standard value from their prototype.

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

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]

Prototype interference

A prototype can be used at any time to add new properties and methods to all objects based on it. For example, it might become necessary for our rabbits to dance.

Rabbit.prototype.dance = function() {
  console.log("The " + this.type + " rabbit dances a jig.");
};
killerRabbit.dance();
// → The killer rabbit dances a jig.

That’s very convenient. But there are situations where it causes problems. In previous chapters, we used an object as a way to associate values with names by creating properties for the names and giving them the corresponding value as their value. Here’s an example from Chapter 4:

var map = {};
function storePhi(event, phi) {
  map[event] = phi;
}

storePhi("pizza", 0.069);
storePhi("touched tree", -0.081);

We can iterate over all phi values in the object using a for/in loop, and test whether a name is in there using the regular in operator. But unfortunately, the object’s prototype gets in the way.

Object.prototype.nonsense = "hi";
for (var name in map)
  console.log(name);
// → pizza
// → touched tree
// → nonsense
console.log("nonsense" in map);
// → true
console.log("toString" in map);
// → true

// Delete the problematic property again
delete Object.prototype.nonsense;

That’s all wrong. There is no event called “nonsense” in our data set. And there definitely is no event called “toString”.

Oddly, toString did not show up in the for/in loop, but the in operator did return true for it. This is because JavaScript distinguishes between enumerable and non-enumerable properties.

All properties that we create by simply assigning to them are enumerable. The standard properties in Object.prototype are all non-enumerable, which is why they do not show up in such a for/in loop.

It is possible to define our own non-enumerable properties by using the Object.defineProperty function, which allows us to control the type of property we are creating.

Object.defineProperty(Object.prototype, "hiddenNonsense",
                      {enumerable: false, value: "hi"});
for (var name in map)
  console.log(name);
// → pizza
// → touched tree
console.log(map.hiddenNonsense);
// → hi

So now the property is there, but it won’t show up in a loop. That’s good. But we still have the problem with the regular in operator claiming that the Object.prototype properties exist in our object. For that, we can use the object’s hasOwnProperty method.

console.log(map.hasOwnProperty("toString"));
// → false

This method tells us whether the object itself has the property, without looking at its prototypes. This is often a more useful piece of information than what the in operator gives us.

When you are worried that someone (some other code you loaded into your program) might have messed with the base object prototype, I recommend to write your for/in loops like this:

for (var name in map) {
  if (map.hasOwnProperty(name)) {
    // ... this is an own property
  }
}

Prototype-less objects

But the rabbit hole doesn’t end there. What if someone registered the name hasOwnProperty in our map object and set it to the value 42? Now the call to map.hasOwnProperty will try to call the local property, which holds a number, not a function.

In such a case, prototypes just get in the way, and we would actually prefer to have objects without prototype. We saw the Object.create function, which allows us to create an object with a specific prototype. You are allowed to pass null as the prototype to create a fresh object with no prototype. For objects like map, where the properties could be anything, this is exactly what we want.

var map = Object.create(null);
map["pizza"] = 0.069;
console.log("toString" in map);
// → false
console.log("pizza" in map);
// → true

Much better! We no longer need the hasOwnProperty kludge, because all the properties the object has are its own properties. Now we can safely use for/in loops, no matter what people have been doing to Object.prototype.

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 and create a meaningful string to return. I mentioned that some of the standard prototypes define their own version of toString, in order to be able to create a string that contains more useful information than "[object Object]".

This is a simple instance of a very powerful idea. 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—though no actual shape-shifting is involved. Polymorphic code can work with values of different shapes, as long as they support the interface it expects.

Laying out a table

I am going to work through a slightly more involved example in an attempt to give you a better idea what polymorphism, as well as object-oriented programming in general, looks like. The project is this: we will write a program that, given an array of arrays of table cells, builds up a string that contains a nicely laid out table—meaning that the columns are straight and the rows are aligned. Something like this:

name         height country
------------ ------ -------------
Kilimanjaro    5895 Tanzania
Everest        8848 Nepal
Mount Fuji     3776 Japan
Mont Blanc     4808 Italy/France
Vaalserberg     323 Netherlands
Denali         6168 United States
Popocatepetl   5465 Mexico

The way our table-building system will work is that the builder function will ask each cell how wide and high it wants to be, and then use this information to determine the width of the columns and the height of the rows. The builder function will then ask the cells to draw themselves at the correct size, and assemble the results into a single string.

The layout program will communicate with the cell objects through a well-defined interface. That way, the types of cells that the program supports is not fixed in advance. We can add new cell styles later— for example, underlined cells for table headers—and if they support our interface, they will just work, without requiring changes to the layout program.

This is the interface:

I’m going to make heavy use of higher-order array methods in this example, since it lends itself well to that approach.

The first part of the program computes arrays of minimum column widths and row heights for a grid of cells. The rows variable will hold an array of arrays, each inner array representing a row of cells.

function rowHeights(rows) {
  return rows.map(function(row) {
    return row.reduce(function(max, cell) {
      return Math.max(max, cell.minHeight());
    }, 0);
  });
}

function colWidths(rows) {
  return rows[0].map(function(_, i) {
    return rows.reduce(function(max, row) {
      return Math.max(max, row[i].minWidth());
    }, 0);
  });
}

Using a variable name starting with an underscore (“_”), or consisting entirely of a single underscore, is a way to indicate (to human readers) that this argument is not going to be used.

The rowHeights function shouldn’t be too hard to follow. It uses reduce to compute the maximum height of an array of cells, and wraps that in map in order to do it for all rows in the rows array.

Things are slightly harder for the colWidths function, because the outer array is an array of rows, not of columns. I have failed to mention so far that map (as well as forEach, filter, and similar array methods) passes a second argument to the function it is given: the index of the current element. By mapping over the elements of the first row and only using the mapping function’s second argument, colWidths builds up an array with one element for every column index. The call to reduce runs over the outer rows array for each index, and picks out the width of the widest cell at that index.

Here’s the code to draw a table:

function drawTable(rows) {
  var heights = rowHeights(rows);
  var widths = colWidths(rows);

  function drawLine(blocks, lineNo) {
    return blocks.map(function(block) {
      return block[lineNo];
    }).join(" ");
  }

  function drawRow(row, rowNum) {
    var blocks = row.map(function(cell, colNum) {
      return cell.draw(widths[colNum], heights[rowNum]);
    });
    return blocks[0].map(function(_, lineNo) {
      return drawLine(blocks, lineNo);
    }).join("\n");
  }

  return rows.map(drawRow).join("\n");
}

The drawTable function uses the internal helper function drawRow to draw all rows, then joins them together with newline characters.

The drawRow function itself first converts the cell objects in the row to blocks, which are arrays of strings representing the content of the cells, split by line. A single cell containing simply the number 3776 might be represented by a single-element array like ["3776"], whereas an underlined cell might take up two lines, and be represented by the array ["name", "----"].

The blocks for a row, which all have the same height, should appear next to each other in the final output. The second call to map in drawRow builds up this output line by line, by mapping over the lines in the leftmost block, and for each of those, collecting a line that spans the full width of the table. These lines are then joined with newline characters to provide the whole row as drawRow’s return value.

The function drawLine extracts lines that should appear next to each other from an array of blocks, and joins them with a space character, to create a one-character gap between the table’s columns.

Now let’s write a constructor for cells that contain text, which implements the interface for table cells. The constructor splits a string into an array of lines, using the string method split, which cuts up a string at every occurrence of its argument, and returns an array of the pieces. The minWidth method finds the maximum line width in this array.

function repeat(string, times) {
  var result = "";
  for (var i = 0; i < times; i++)
    result += string;
  return result;
}

function TextCell(text) {
  this.text = text.split("\n");
}
TextCell.prototype.minWidth = function() {
  return this.text.reduce(function(width, line) {
    return Math.max(width, line.length);
  }, 0);
};
TextCell.prototype.minHeight = function() {
  return this.text.length;
};
TextCell.prototype.draw = function(width, height) {
  var result = [];
  for (var i = 0; i < height; i++) {
    var line = this.text[i] || "";
    result.push(line + repeat(" ", width - line.length));
  }
  return result;
};

The code uses a helper function called repeat, which builds a string whose value is the string argument repeated times number of times. The draw method uses it to add “padding” to lines, so that they all have the required length.

Let’s try out everything we’ve written so far by building up a 5 × 5 checkerboard.

var rows = [];
for (var i = 0; i < 5; i++) {
   var row = [];
   for (var j = 0; j < 5; j++) {
     if ((j + i) % 2 == 0)
       row.push(new TextCell("##"));
     else
       row.push(new TextCell("  "));
   }
   rows.push(row);
}
console.log(drawTable(rows));
// → ##    ##    ##
//      ##    ##
//   ##    ##    ##
//      ##    ##
//   ##    ##    ##

It works! But since all cells have the same size, the table-layout code doesn’t really do anything interesting.

The source data for the table of mountains that we are trying to build is available in the MOUNTAINS variable in the sandbox, and also downloadable from the list of data sets on the website.

We will want to highlight the top row, which contains the column names, by underlining the cells with a series of dash characters. No problem—we simply write a cell type that handles underlining.

function UnderlinedCell(inner) {
  this.inner = inner;
};
UnderlinedCell.prototype.minWidth = function() {
  return this.inner.minWidth();
};
UnderlinedCell.prototype.minHeight = function() {
  return this.inner.minHeight() + 1;
};
UnderlinedCell.prototype.draw = function(width, height) {
  return this.inner.draw(width, height - 1)
    .concat([repeat("-", width)]);
};

An underlined cell contains another cell. It reports its minimum size as being the same as that of its inner cell (by calling through to that cell’s minWidth and minHeight methods), but adds one to the height, to account for the space taken up by the underline.

Drawing such a cell is quite simple—we take the content of the inner cell, and concatenate a single line full of dashes to it.

Having an underlining mechanism, we can now write a function that builds up a grid of cells from our data set.

function dataTable(data) {
  var keys = Object.keys(data[0]);
  var headers = keys.map(function(name) {
    return new UnderlinedCell(new TextCell(name));
  });
  var body = data.map(function(row) {
    return keys.map(function(name) {
      return new TextCell(String(row[name]));
    });
  });
  return [headers].concat(body);
}

console.log(drawTable(dataTable(MOUNTAINS)));
// → name         height country
//   ------------ ------ -------------
//   Kilimanjaro  5895   Tanzania
//   … etcetera

The standard Object.keys function returns an array of property names in an object. The top row of the table must contain underlined cells that give the names of the columns. Below that, the values of all the objects in the data set appear as normal cells—we extract them by mapping over the keys array, so that we are sure that the order of the cells is the same in every row.

The resulting table resembles the example shown before, except that it does not right-align the numbers in the height colomn. We will get to that in a moment.

Getters and setters

When specifying an interface, it is possible to include properties that are not methods. We could have defined minHeight and minWidth to simply hold numbers. But that’d have required us to compute them in the constructor, which adds code there that isn’t strictly relevant to constructing the object. It would cause problems if, for example, the inner cell of an underlined cell was changed, at which point the size of the underlined cell should also change.

This has led some people to adopt a principle of never including non-method properties in interfaces. Rather than directly access a simple value property, they’d use getSomething and setSomething methods to read and write the property. This approach has the downside that you will end up writing—and reading—a lot of additional methods.

Fortunately, JavaScript provides a technique that gets us the best of both worlds. We can specify properties that, from the outside, look like normal properties, but secretly have methods associated with them.

var pile = {
  elements: ["eggshell", "orange peel", "worm"],
  get height() {
    return this.elements.length;
  },
  set height(value) {
    console.log("Ignoring attempt to set height to", value);
  }
};

console.log(pile.height);
// → 3
pile.height = 100;
// → Ignoring attempt to set height to 100

In object literal, the get or set notation for properties allows you to specify a function to be run when the property is read or written. You can also add such a property to an existing object, for example a prototype, using the Object.defineProperty function (which we previously used to create non-enumerable properties).

Object.defineProperty(TextCell.prototype, "heightProp", {
  get: function() { return this.text.length; }
});

var cell = new TextCell("no\nway");
console.log(cell.heightProp);
// → 2
cell.heightProp = 100;
console.log(cell.heightProp);
// → 2

You can use a similar set property, in the object passed to defineProperty, to specify a setter method. When a getter but no setter is defined, writing to the property is simply ignored.

Inheritance

We are not quite done yet with our table layout exercise. It helps readability to right-align columns of numbers. We should create another cell type that is like TextCell, but rather than padding the lines on the right side, pads them on the left side, so that they align to the right.

We could simply write a whole new constructor, with all three methods in its prototype. But prototypes may themselves have prototypes, and this allows us to do something clever:

function RTextCell(text) {
  TextCell.call(this, text);
}
RTextCell.prototype = Object.create(TextCell.prototype);
RTextCell.prototype.draw = function(width, height) {
  var result = [];
  for (var i = 0; i < height; i++) {
    var line = this.text[i] || "";
    result.push(repeat(" ", width - line.length) + line);
  }
  return result;
};

We reuse the constructor and the minHeight and minWidth methods from the regular TextCell. An RTextCell is now basically equivalent to a TextCell, except that its draw method contains a different function.

This pattern is called inheritance. It allows us to build slightly different data types from existing datatypes with relatively little work. Typically, the new constructor will call the old constructor (using the call method in order to be able to give it the new object as its this value). Once this constructor has been called, we can assume that all the fields that the old object type is supposed to contain have been added. We arrange for the constructor’s prototype to derive from the old prototype, so that instances of this type will also have access to the properties in that prototype. Finally, we can override some of these properties by adding them to our new prototype.

Now if we slightly adjust the dataTable function to use RTextCell's for cells whose value is a number, we get the table we were aiming for:

function dataTable(data) {
  var keys = Object.keys(data[0]);
  var headers = keys.map(function(name) {
    return new UnderlinedCell(new TextCell(name));
  });
  var body = data.map(function(row) {
    return keys.map(function(name) {
      var value = row[name];
      // This was changed:
      if (typeof value == "number")
        return new RTextCell(String(value));
      else
        return new TextCell(String(value));
    });
  });
  return [headers].concat(body);
}

console.log(drawTable(dataTable(MOUNTAINS)));
// → … beautifully aligned table

Inheritance 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 somewhat controversial.

The main reason for this is that it is often confused with polymorphism, sold as a more powerful tool than it really is—and subsequently overused in all kinds of ugly ways. 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 types together, creating more tangle.

You can have polymorphism without inheritance, as we saw. I am not going to tell you to avoid inheritance entirely—I use it regularly in my own programs. But see it as a slightly dodgy trick that can help you define new types with little code, not as a grand principle of code organization. A preferable way to extend types is through composition, such as how UnderlinedCell builds on another cell object by simply storing it in a property and forwarding method calls to it in its own methods.

The instanceof operator

It is occasionally useful to know whether an object was derived from a specific constructor. For this, JavaScript provides a binary operator called instanceof:

console.log(new RTextCell("A") instanceof RTextCell);
// → true
console.log(new RTextCell("A") instanceof TextCell);
// → true
console.log(new TextCell("A") instanceof RTextCell);
// → false
console.log([1] instanceof Array);
// → true

The operator will also see through inherited types. An RTextCell is an instance of TextCell, because RTextCell.prototype derives from TextCell.prototype. The operator can be applied to standard constructors like Array. Almost every object is an instance of Object.

Summary

So objects are more complicated than I initially portrayed them. They have prototypes, which are other objects, and will act as if they have properties they don’t have as long as the prototype has that property. Simple objects have Object.prototype as their prototype.

Constructors, which are functions whose name usually starts 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 function. You can make good use of this by putting the properties that all values of a given type share into their prototype. The instanceof operator can, given an object and a constructor, tell you whether that object is an instance of that constructor.

One useful thing to do with objects is to specify an interface for them, and tell everybody that they are only supposed to talk to your object through that interface. The rest of the details that make up your object are now encapsulated, hidden behind the interface.

Once you are talking in terms of interfaces, who says that only one kind of object may implement this interface? Having different objects expose the same interface, and then writing code that works on any object with the interface, is called polymorphism. It is very useful.

When implementing multiple types that only differ in some details, it can be helpful to simply make the prototype of your new type derive from the prototype of your old type, and have your new constructor call the old one. This gives you an object type very similar to the old type, but for which you can add and override properties as you see fit.

Exercises

A vector type

Write a constructor Vector that represents a vector in two-dimensional space. It takes x and y parameters (numbers), which it should save to properties of the same name.

Give the Vector prototype two methods, plus and minus, which take another vector as a parameter, and return a new vector that has the sum or difference of the two vectors’ (the one in this and the parameter) x and y values.

Add a getter property length to the prototype that computes the length of the vector—that is, the distance of the point (x, y) from the origin (0, 0).

// Your code here.

console.log(new Vector(1, 2).plus(new Vector(2, 3)));
// → Vector{x: 3, y: 5}
console.log(new Vector(1, 2).minus(new Vector(2, 3)));
// → Vector{x: -1, y: -1}
console.log(new Vector(3, 4).length);
// → 5

Your solution can follow the pattern of the Rabbit constructor from this chapter quite closely.

Adding a getter property to the constructor can be done with the Object.defineProperty function. To compute the distance from (0, 0) to (x, y), you can use the Pythagorean theorem, which says that the square of the distance we are looking for is equal to the square of the x coordinate plus the square of the y coordinate. Thus √(x2 + y2) is the number you want, and Math.sqrt is the way you compute a square root in JavaScript.

Another cell

Implement a cell type named StretchCell(inner, width, height) that conforms to the table cell interface described earlier in the chapter. It should wrap another cell (like UnderlinedCell does), and ensure that the resulting cell has at least the given width and height, even if the inner cell would naturally be smaller.

// Your code here.

var sc = new StretchCell(new TextCell("abc"), 1, 2);
console.log(sc.minWidth());
// → 3
console.log(sc.minHeight());
// → 2
console.log(sc.draw(3, 2));
// → ["abc", "   "]

You’ll have to store all three constructor arguments in the instance object. The minWidth and minHeight methods should call through to the corresponding methods in the inner cell, but ensure that no number less than the given size is returned (possibly using Math.max).

Don’t forget to add a draw method that simply forwards the call to the inner cell.

Sequence interface

Design an interface that abstracts iteration over a collection of values. An object that provides this interface represents a sequence, and the interface must somehow make it possible for code that uses such an object to iterate over the sequence, looking at the element values it is made up of, and having some way to find out when the end of the sequence is reached.

When you have specified your interface, try to write a function logFive that takes a sequence object and calls console.log on its first five elements—or less, if the sequence has less than five elements.

Then implement an object type ArraySeq that wraps an array and allows iteration over the array using the interface you designed. Implement another object type RangeSeq that iterates over a range of integers (taking from and to arguments to its constructor) instead.

// Your code here.

logFive(new ArraySeq([1, 2]));
// → 1
// → 2
logFive(new RangeSeq(100, 1000));
// → 100
// → 101
// → 102
// → 103
// → 104

One way to solve this is to give the sequence objects state, meaning their properties are changed in the process of using them. You could store a counter that indicates how far the sequence object has advanced.

Your interface will need to expose at least a way to get the next element, and to find out whether the iteration has reached the end of the sequence yet. It is tempting to roll these into one method, next, which returns null or undefined when the sequence is at its end. But now you have a problem when a sequence actually contains null. So a separate method (or getter property) to find out whether the end has been reached is probably preferable.

Another solution is to avoid changing state in the object. You can expose a method for getting the current element (without advancing any counter), and another for getting a new sequence that represents the remaining elements after the current one (or a special value if the end of the sequence is reached). This is quite elegant—a sequence value will “stay itself” even after it is used, and can thus be shared with other code without worrying about what might happen to it. It is, unfortunately, also somewhat inefficient in a language like JavaScript, because it involves creating a lot of objects during iteration.