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JS 101 – Week 3

February 15, 2011 Leave a comment

In Javascript, functions are first class objects. What does it mean to be a first class object?

A function can be used anywhere an object can be used; in particular, you can pass functions as method arguments. (We did this in last week’s homework by passing the print function into the iterateAndOperate method). This is in contrast to a language like Java where functions are not first class objects, and the programmer must use interfaces and anonymous classes to get around this problem. For instance, in Java, to delay the invocation of a function, you might do the following:

SwingUtilities.invokeLater(new Runnable() {
    @Override
    public void run() {
        System.out.println("I was run!");
    }
})

whereas in Javascript, you could do something like

invokeLater(function() { print("I was run") });

assuming there was an invokeLater function.

Functions and variables share the same namespace. What does this mean and what important implication does this have?

This means that you must take care in not defining variables in the global namespace that have the same name as a function. For instance,

// global variable
x = 5;

function x(args) {
    alert(args);   
}

// The global variable is shadowing the function name;
// the function is not called
x(5);

Douglas Crockford equates Javascript functions with Lambdas, and also mentions that they are a secure construct. Can you do some research and reflect on what he means by ‘secure construct’?

A lambda basically means that you can pass a function as an argument to another function; this is due to the aforementioned first class nature of functions. They are a secure construct because the scope of the functions is such that private variables in function A cannot be accessed by function B when A is passed into B. In other words,

function outer(func) {
  // "undefined", since the ‘privateVariable’
  // field is scoped only to the inner function.
  return func.privateVariable;
}

function inner(a,b,c) {
  var privateVariable = 25;
  return a + ", " + b + ", " + c;
}

alert(outer(inner("cat","dog","bear")));

f

Can you explain the concept of a closure.

A closure means that an inner function continues to have access to variables defined inside of an outer function, even after the outer function has finished. I wrote an example you can view on jsfiddle.

What is the difference between a function and a method?

A method is a function that is bound to an object, and thus it has an implicit “self” reference.

In Javascript there is no implicit type checking of arguments passed to functions. This could lead to bugs if programmers are not careful about what they pass to functions. If you create a function, how would you protect it from well meaning but careless programmers?

You can do defensive checks within the function. For instance, if a method is supposed to take in a number, you could do the following:

function numericalFunction(x) {
    if (typeof(x) != "number") {
        throw("Expected numerical value for numericalFunction; got " + x);
    }
    // Proceed as normal
}

You can also use the arguments implicit variable in the function to check that the number of arguments the user passed in is equal to the number of arguments that the method expects.

Javascript functions have implicit access to something called this. this points to different things depending on how the function was created. Can you explain why we need this, and what it represents in different type of functions.

If a function is created globally, its this pointer will be to the DOMObject. If it’s created and bound to an object, the this pointer points to that object. The following illustrates this:

function f() {
  return "f's this: " + this;
}

function nested() {
  function inner() {
    return "inner's this:" + this;
  }
  return inner();
}

// o is an object; add a method do it
var o = {};
o.f = f;
o.nested = nested;

o.newFunc = function() { return "newFunc's this: " + this; };
o.nestedFunc = function() { 
  var inner = function() {
    return "nestedFunc's inner this: " + this;
  }
  return inner();
}



print(f());
print(nested());
print(o.f());
print(o.nested());
print(o.newFunc());
print(o.nestedFunc());


// prints
// f's this: [object DOMWindow]
// inner's this:[object DOMWindow]
// f's this: [object Object]
// inner's this:[object DOMWindow]
// newFunc's this: [object Object]
// nestedFunc's inner this: [object DOMWindow]
//

Note that only the non nested functions that have been bound to object o point to that object as opposed to the DOMWindow.

The reason we want the this pointer is to be able to introspect on the type and contents of the object that is calling the function. For instance, in my last assignment, I used the this variable to produce a nice representation of an object:

nick = {name:"Nick",age:23};

// This function returns a string representation of whatever
// is invoking it
function selfStringRep() {
  var str = "";
  for (var v in this) {
    // This is not an inherited property
    if (this.hasOwnProperty(v)) {
      str += "’" + v + "’: " + this[v] +"\n";
    }
  }
  return str;
}
Object.prototype.toString = selfStringRep;
print(nick.toString());
// prints
//’name’: Nick
//’age’: 23

4.1

The solution for the cat problem talks about a ‘set’ of names. A set is a collection of values in which no value may occur more than once. If names are strings, can you think of a way to use an object to represent a set of names?

Show how a name can be added to this set, how one can be removed, and how you can check whether a name occurs in it.

var cat_set = {};
// Add a cat
cat_set["Tigger"] = true;
// remove a cat
delete cat_set["Tigger"];

// Check if cat exists
cat_set["Frog"] != undefined

// Wrap it all up nicely: in an object with functions:
var cat_set = {};
cat_set.add_cat = function(cat) {
    this[cat] = true;
}
cat_set.remove_cat = function(cat) {
    delete this[cat];
}
cat_set.cat_exists = function(cat) {
    return this[cat] != undefined;
}

cat_set.add_cat("Frisky");
// prints "true"
show(cat_set.cat_exists("Frisky"))
cat_set.remove_cat("Frisky");
// "false"
show(cat_set.cat_exists("Frisky"))

4.2

Write a function range that takes one argument, a positive number, and returns an array containing all numbers from 0 up to and including the given number.

An empty array can be created by simply typing []. Also remember that adding properties to an object, and thus also to an array, can be done by assigning them a value with the = operator. The length property is automatically updated when elements are added.

// Assumes n >= 0
function range(n) {
    var array = [];
    for (var i = 0; i <= n; i++) {
        array[i] = i;
    }
    return array;
}

4.3

split and join are not precisely each other’s inverse. string.split(x).join(x) always produces the original value, but array.join(x).split(x) does not. Can you give an example of an array where .join(" ").split(" ") produces a different value?

[" ", " ", " "].join(" ").split(" ");
// equals ["", "", "", "", "", ""]

4.4

Write a function called startsWith that takes two arguments, both strings. It returns true when the first argument starts with the characters in the second argument, and false otherwise.

function startsWith(string, pattern) {
    return string.slice(0,pattern.length) === pattern;
}

4.5

Can you write a function catNames that takes a paragraph as an argument and returns an array of names?

Strings have an indexOf method that can be used to find the (first) position of a character or sub-string within that string. Also, when slice is given only one argument, it will return the part of the string from the given position all the way to the end.

It can be helpful to use the console to ‘explore’ functions. For example, type "foo: bar".indexOf(":") and see what you get.

// The sentence is in the form "Blah blah: cat1, cat2, …, catN."
function catNames(paragraph) {
    var colonIndex = paragraph.indexOf(":");
    // Skip the colon and the following space
    var catListString = paragraph.slice(colonIndex + 2);
    return catString.split(", ");
}

/*#p2pu-Jan2011-javascript101*/

Javascript 101 Week 2: Functions, Encapsulation, Augmentation

February 5, 2011 4 comments

Here’s the second week of homework/ reflection for the JS 101 course offered through Peer 2 Peer University.  You can read my week one homework as well.

Why do languages provide the switch statement, when we can achieve the same thing with multiple if… elseif statements? Show one example of how you might use the switch statement.

Languages provide the switch statement for two main reasons. The first is that they are arguably more readable than multiple if/else if statements. Secondly, in some languages the compiler can optimize switch statements to be more efficient than the equivalent if/else if/else if construct. See the wikipedia article on Switch statements and Branch tables if you’re interested in the details.

Unfortunately, switch statements can be more error prone due to the ability to ‘fall through’ switch statements. Consider the following:

var BOY = 1;
var GIRL = 2;
var sentence = "Congratulations, it's a ";
var gender = GIRL;
switch (gender) {
    case BOY:
        sentence += "boy!";
    // ERROR: no break statement; will fall through to girl case, ending
    // with the sentence "Congratulations, it's a boy!girl!" if gender == BOY
    case GIRL:
        sentence += "girl!";
}

(Note: the above example is somewhat contrived, but forgetting to put a break at the end of a case is a very common error. Please be cognizant of it when using switch.)

Furthermore, programmers frequently forget to provide a default case to handle the case when none of the alternative options match. This frequently happens in Java with enumerations, since the compiler won’t mark this as an error:

enum Foo {
    FOO_1,
    FOO_2
}

Foo x = …;
switch (x) {
    case FOO_1:
        …
        break;
    case FOO_2:
        …
        break;
}

// Everything is fine.  Later, the Foo enum is modified, and a new option is added

enum Foo {
    FOO_1,
    FOO_2,
    FOO_3
}

With the addition of the new enumerated value, we can inadvertently fall through all the options without having executed any code. It is for this reason that I suggest you always provide a default case, even if all it does is throw an exception indicating that this should never happen. Trust me, you’ll find your bugs much earlier/easier this way.

What is encapsulation, and what do functions encapsulate?

Encapsulation is a programming technique to manage complexity and reduce duplication. If I am writing a geometric library I might have code to calculate the area of a circle in a few different places. If this happens once or twice, that might not be the end of the world. The problem is that it becomes very difficult to keep the code in sync in multiple places. A better solution is to break out the logic for calculating the area into a separate function, and then call that function from various places.

This has three main benefits.

  1. This encapsulation of the logic reduces code duplication. Code duplication is bad, as it means that any time you find a bug or want to change something, you need to remember to change it in multiple places. This is a maintenance headache and is extremely error prone.
  2. It makes the code easier to read by better expressing the intent of what you are trying to do and by hiding the details. For instance, instead of having a few lines of code to calculate some number, you can move these lines into a small helper function whose name expresses what the calculation does.
  3. By encapsulating the implementation details within a function, you can change the underlying algorithm without necessitating any client code to change. For instance, you might change a recursive function to an iterative approach for efficiency purposes, but as long as the method name and parameter list stay constant, dependent code is unaffected.

Encapsulation is extremely useful for all the above reasons. It allows the programmer to work at higher levels of abstraction, by hiding implementation details and allowing complex functionality to be built from multiple small, simple function calls.

What is a pure function? Is the function show() provided in Eloquent Javascript a pure function?

A pure function is one without side effects, i.e. a function in the mathematical sense of the word. (It doesn’t make use of variables other than those passed in to the function, so that you are always guaranteed to get the same output for the same input) show is not a pure function, as it has the side effect of writing to the screen.

What do we mean when we say a variable in a function is shadowing a top level variable?

If two variables have the same name, with one being in the global scope and one being in the function scope, the function scoped variable shadows that of the global, top level variable. This means that when we refer to the variable within the function, we access the function scoped variable rather than the top level variable. For instance,


// No var declaration; this is global
name = "Nick";

function greet() {
    // This name variable shadows the global name declaration.
    var name = "Kitty";
    alert("Hi " + name);
}
// Says "Hi Kitty"
greet();

A recursive function, must have some sort of an end condition. Why would we get a “out of stack space” error message if a recursive function does not have an end condition?

A running computer program has memory space allocated both for a stack and for a heap. The heap consists of global or static variables and those things that must remain throughout the life of the program. The stack is where local variables reside. When we call a function, all of the local variables necessary for that function reside are pushed to the top of the stack. When a function exits, the space on the stack is able to be reclaimed.

When we have unbounded recursion, more and more variables are added to the stack. Since none of the functions are exiting, nothing is freed from the stack, and we soon run out of stack space. See the following screenshot and the accompanying article on Stack vs Heap allocation for more information

Organization of the stack

By the way, the fact that function calls are added to this stack is the reason that you can always replace a recursive function with an iterative solution using an explicit stack data structure. This is often not worth the added complexity, but it’s something to be aware of. (You can often replace a recursive algorithm with an iterative solution without using a stack, but the code can be much more complicated. Iterative solutions are often more efficient than recursive solutions, so as always it’s a tradeoff. See the recursion article on Wikipedia for more information.)

Reflect about the difference between object inheritance and class inheritance

Class inheritance, in a language like Java, means that an instance of a subclass inherits all of the members and methods of its superclass; in a sense it has its own ‘copies’. In object inheritance, we get the members and methods via a hidden pointer to another object. Thus changes in that linked object implicitly change the values in the other object.

What is object augmentation, and how do we do it?

Object augmentation is the act of adding fields or functions to an existing object. For instance,

var nick = {name:"Nick",age:23};
// Augment the nick object with a new field
nick.profession = "programmer";
// Remove that field
nick.profession = undefined

Why is this useful? Well it’s useful in the case I just presented, since you do not need to know the full set of fields in your object at initialization, but instead can add and remove them after the fact.

It’s a more powerful concept when combined with the idea of object prototypes. Inheritance is achieved in Javascript through a hidden pointer to another object. All objects have their pointer set to the Object.prototype, so we can augment all objects with a new method, simply by augmenting the Object.prototype object:

nick = {name:"Nick",age:23};

// This function returns a string representation of whatever
// is invoking it
function selfStringRep() {
  var str = "";
  for (var v in this) {
    // This is not an inherited property
    if (this.hasOwnProperty(v)) {
      str += "'" + v + "': " + this[v] +"\n";
    }
  }
  return str;
}
Object.prototype.toString = selfStringRep;
print(nick.toString());
// prints
‘name': Nick
‘age': 23

This is an incredibly powerful feature, and one that I could see being extremely useful.

There is a way to add a method to String, such as any new String we create will have that augmented method (this is a bit different from object augmentation). How would you do this?

To augment all strings, we need to augment String.prototype, since all string instances inherit from this object. For instance, let’s add the ability to reverse any string.

function reverseString(str) {
  var result = "";
  for (var i = str.length - 1; i >= 0; i—) {
    result +=  str.charAt(i);
  }
  return result;
}

String.prototype.reversed = function() { return reverseString(this); }
print("Nick".reversed());
// prints "kciN"

What is garbage collection?

Garbage collection refers to the fact that objects that go out of scope and no longer are referenced are automatically found and their memory is restored. Garbage collection is a feature that makes programmers’ lives easier, as we do not have to manually keep track of freeing the memory of each and every object as it goes in and out of scope. Languages like C do not have garbage collection, introducing a whole potential of errors for programmers. The most common error is a memory leak which might not immediately crash a program, but leads to an increasing amount of memory usage over time.

What is the difference between an array and an object?

While arrays are very similar to objects, there are at least 3 differences. (See jsfiddle for an illustrative example)

  1. Literal construction syntax
    Arrays are formed by square brackets ([]), whereas general objects are formed with curly braces ({})
  2. “Secret link”
    Arrays are automatically linked with Array.prototype; Objects are automatically linked with Object.prototype. This affects the fields available to an array vs an object; for instance, an array will have a length field that automatically reflects the size of the array, whereas a general object will not. Arrays also have methods defined on them, such as concat(), join(), pop(), etc. (see w3schools for more)
  3. Objects’ entries can be accessed with dot syntax or using the bracket notation (e.g. o.name or o[“name”]); arrays can only be accessed with bracket notation (e.g. array[1])

Homework

3.1

Write a function called absolute, which returns the absolute value of the number it is given as its argument. The absolute value of a negative number is the positive version of that same number, and the absolute value of a positive number (or zero) is that number itself.

function abs(x) {
    if (x < 0) {
        return -x;
    }
    else {
        return x;
    }
}

3.2

Write a function greaterThan, which takes one argument, a number, and returns a function that represents a test. When this returned function is called with a single number as argument, it returns a boolean: true if the given number is greater than the number that was used to create the test function, and false otherwise.

function greaterThan(x) {
    return function(y) {
        return y>x;
    }
}

3

Shown below is some code which does something useful. The function ‘iterateAndOperate’ is the one which accomplishes something useful. The remaining code helps this function. Try to understand what the function accomplishes and solve the problems in part a, b, and c. The code can be done inside the console in Javascript, or in the web browser. Please see this comment, for hints on how you may do it inside a web page(remember, HTML has special codes for spaces and newlines).

var pictureArray = ["++++@++++", "+++@@@+++", "++@@@@@++", "+++@@@+++", "++++@++++"];
iterateAndOperate(pictureArray, print)
++++@++++
+++@@@+++
++@@@@@++
+++@@@+++
++++@++++

var triangleArray = ["*", "***", "*****", "***", "*"];
iterateAndOperate(triangleArray, print);
*
***
*****
***
*

try {
    iterateAndOperate();
}
catch (err) {
    alert("Error, you must provide an array and function argument to iterateAndOperate");
}

/*#p2pu-Jan2011-javascript101*/