:chap_num: 2

:prev_link: 01_values

:next_link: 03_functions

= Program Structure =

[chapterquote="true"]

[quote, _why, Why's (Poignant) Guide to Ruby]

____

And my heart glows bright red under my

filmy, translucent skin and they have to administer 10cc of JavaScript

to get me to come back. (I respond well to toxins in the blood.) Man,

that stuff will kick the peaches right out your gills!

____

(((why)))(((Poignant Guide)))In this chapter, we will start to do

things that can actually be called _programming_. We will expand our

command of the JavaScript language beyond the nouns and sentence

fragments we've seen so far, to the point where we can

express some meaningful prose.

== Expressions and statements ==

(((grammar)))(((syntax)))(((code,structure

of)))(((grammar)))(((JavaScript,syntax)))In

link:01_values.html#values[Chapter 1], we made some values and then

applied operators to them to get new values. Creating values like this

is an essential part of every JavaScript program, but it is only

a part.

(((literal expression)))A fragment of code that produces a value is

called an _((expression))_. Every value that is written literally

(such as `22` or `"psychoanalysis"`) is an expression. An expression

between ((parentheses)) is also an expression, as is a ((binary

operator)) applied to two expressions or a unary operator applied to

one.

(((nesting,of expressions)))(((human language)))This shows part of the

beauty of a language-based interface. Expressions can nest in a way

very similar to the way subsentences in human languages are nested—a

subsentence can contain its own subsentences, and so on. This allows

us to combine expressions to express arbitrarily complex computations.

(((statement)))(((semicolon)))(((program)))If an expression

corresponds to a sentence fragment, a JavaScript _statement_

corresponds to a full sentence in a human language. A program is

simply a list of statements.

(((syntax)))The simplest kind of statement is an expression with a

semicolon after it. This is a program:

[source,javascript]

----

1;

!false;

----

It is a useless program, though. An ((expression)) can be content to

just produce a value, which can then be used by the enclosing

expression. A ((statement)) stands on its own and amounts to something

only if it affects the world. It could display something on the

screen—that counts as changing the world—or it could change the

internal state of the machine in a way that will affect the statements

that come after it. These changes are called _((side effect))s_. The

statements in the previous example just produce the values `1` and

`true` and then immediately throw them away. This leaves no impression

on the world at all. When executing the program, nothing observable

happens.

(((programming style)))(((automatic semicolon

insertion)))(((semicolon)))In some cases, JavaScript allows you to

omit the semicolon at the end of a statement. In other cases, it has

to be there, or the next ((line)) will be treated as part of the same

statement. The rules for when it can be safely omitted are somewhat

complex and error-prone. In this book, every statement that needs a

semicolon will always be terminated by one. I recommend you do the

same in your own programs, at least until you've learned more about

subtleties involved in leaving out semicolons.

== Variables ==

(((syntax)))(((variable,definition)))(((side effect)))(((memory)))How

does a program keep an internal ((state))? How does it remember

things? We have seen how to produce new values from old values, but

this does not change the old values, and the new value has to be

immediately used or it will dissipate again. To catch and hold values,

JavaScript provides a thing called a _variable_.

[source,javascript]

----

var caught = 5 * 5;

----

(((var keyword)))And that gives us our second kind of ((statement)).

The special word (_((keyword))_) `var` indicates that this sentence is

going to define a variable. It is followed by the name of the variable

and, if we want to immediately give it a value, by an `=` operator and

an expression.

The previous statement creates a variable called `caught` and uses it

to grab hold of the number that is produced by multiplying 5 by 5.

After a variable has been defined, its name can be used as an

((expression)). The value of such an expression is the value the

variable currently holds. Here's an example:

[source,javascript]

----

var ten = 10;

console.log(ten * ten);

// → 100

----

(((underscore character)))(((dollar

sign)))(((variable,naming)))Variable names can be any word that isn't

a reserved word (such as `var`). They may not include spaces.

Digits can also be part of variable names—`catch22` is a valid name,

for example—but the name must not start with a digit. A variable name

cannot include punctuation, except for the characters `$` and `_`.

(((= operator)))(((assignment)))(((variable,assignment)))When a

variable points at a value, that does not mean it is tied to that

value forever. The `=` operator can be used at any time on existing

variables to disconnect them from their current value and have them

point to a new one.

[source,javascript]

----

var mood = "light";

console.log(mood);

// → light

mood = "dark";

console.log(mood);

// → dark

----

(((variable,model of)))(((tentacle (analogy))))You should

imagine variables as tentacles, rather than boxes. They do not

_contain_ values; they _grasp_ them—two variables can refer to the

same value. A program can access only the values that it still has a

hold on. When you need to remember something, you grow a tentacle to

hold on to it or you reattach one of your existing tentacles to it.

image::img/octopus.jpg[alt="Variables as tentacles"]

Let's look at an example. To remember the number of dollars that Luigi

still owes you, you create a variable. And then when he pays back $35,

you give this variable a new value.

[source,javascript]

----

var luigisDebt = 140;

luigisDebt = luigisDebt - 35;

console.log(luigisDebt);

// → 105

----

(((undefined)))When you define a variable without giving it a value,

the tentacle has nothing to grasp, so it ends in thin air. If you ask

for the value of an empty variable, you'll get the value `undefined`.

(((var keyword)))A single `var` statement may define multiple

variables. The definitions must be separated by commas.

[source,javascript]

----

var one = 1, two = 2;

console.log(one + two);

// → 3

----

== Keywords and reserved words ==

(((syntax)))(((implements (reserved word))))(((interface (reserved

word))))(((let keyword)))(((package (reserved word))))(((private

(reserved word))))(((protected (reserved word))))(((public (reserved

word))))(((static (reserved word))))(((void operator)))(((yield

(reserved word))))(((reserved word)))(((variable,naming)))Words with

a special meaning, such as `var`, are _((keyword))s_, and they may not

be used as variable names. There are also a number of words that are

“reserved for use” in ((future)) versions of JavaScript. These are also

officially not allowed to be used as variable names, though some

JavaScript environments do allow them. The full list of keywords and

reserved words is rather long.

[source,text/plain]

----

break case catch class const continue debugger

default delete do else enum export extends false

finally for function if implements import in

instanceof interface let new null package private

protected public return static super switch this

throw true try typeof var void while with yield

----

Don't worry about memorizing these, but remember that this might be

the problem when a variable definition does not work as expected.

== The environment ==

(((standard environment)))The collection of variables and their values

that exist at a given time is called the _((environment))_. When a

program starts up, this environment is not empty. It always contains

variables that are part of the language ((standard)), and most of the

time, it has variables that provide ways to interact with the

surrounding system. For example, in a ((browser)), there are variables

and functions to inspect and influence the currently loaded website

and to read ((mouse)) and ((keyboard)) input.

== Functions ==

indexsee:[application (of functions),function application]

indexsee:[invoking (of functions),function application]

indexsee:[calling (of functions),function application]

(((output)))(((function)))(((function,application)))(((alert

function)))(((message box)))A lot of the values provided in the

default environment have the type _((function))_. A function is a

piece of program wrapped in a value. Such values can be _applied_ in

order to run the wrapped program. For example, in a ((browser))

environment, the variable `alert` holds a function that shows a little

((dialog box)) with a message. It is used like this:

[source,javascript]

----

alert("Good morning!");

----

image::img/alert.png[alt="An alert dialog",width="8cm"]

(((parameter)))(((function,application)))Executing a function is

called _invoking_, _calling_, or _applying_ it. You can call a

function by putting ((parentheses)) after an expression that produces a

function value. Usually you'll directly use the name of the variable

that holds the function. The values between the parentheses are given to

the program inside the function. In the example, the `alert` function

uses the string that we give it as the text to show in the dialog box.

Values given to functions are called _((argument))s_. The `alert`

function needs only one of them, but other functions might need a

different number or different types of arguments.

== The console.log function ==

(((JavaScript console)))(((developer

tools)))(((Node.js)))(((console.log)))(((output)))The `alert` function

can be useful as an output device when experimenting, but clicking

away all those little windows will get on your nerves. In past

examples, we've used `console.log` to output values. Most JavaScript

systems (including all modern web ((browser))s and Node.js) provide a

`console.log` function that writes out its arguments to _some_ text

output device. In browsers, the output lands in the ((JavaScript

console)). This part of the browser interface is hidden by default,

but most browsers open it when you press F12 or, on Mac, when you

press Command-Option-I. If that does not work, search through the

menus for an item named “web console” or “developer tools”.

ifdef::interactive_target[]

When running the examples, or your own code, on the pages of this

book, `console.log` output will be shown after the example, instead of

in the browser's JavaScript console.

endif::interactive_target[]

[source,javascript]

----

var x = 30;

console.log("the value of x is", x);

// → the value of x is 30

----

(((object)))Though ((variable)) names cannot contain ((period

character))s, `console.log` clearly has one. This is because

`console.log` isn't a simple variable. It is actually an expression

that retrieves the `log` ((property)) from the value held by the

`console` variable. We will find out exactly what this means in

link:04_data.html#properties[Chapter 4].

[[return_values]]

== Return values ==

(((comparison,of numbers)))(((return value)))(((Math.max

function)))(((maximum)))Showing a dialog box or writing text to

the screen is a _((side effect))_. A lot of functions are useful

because of the side effects they produce. Functions may also produce

values, and in that case, they don't need to have a side effect to be

useful. For example, the function `Math.max` takes any number of

number values and gives back the greatest.

[source,javascript]

----

console.log(Math.max(2, 4));

// → 4

----

(((function,application)))(((minimum)))(((Math.min

function)))When a function produces a value, it is said to _return_

that value. Anything that produces a value is an ((expression)) in

JavaScript, which means function calls can be used within larger

expressions. Here a call to `Math.min`, which is the opposite of

`Math.max`, is used as an input to the plus operator:

[source,javascript]

----

console.log(Math.min(2, 4) + 100);

// → 102

----

The link:03_functions.html#functions[next chapter] explains how to

write your own functions.

== prompt and confirm ==

(((dialog box)))(((input)))(((browser)))(((confirm function)))Browser

environments contain other functions besides `alert` for popping up

windows. You can ask the user an OK/Cancel question using

`confirm`. This returns a Boolean: `true` if the user clicks OK and

`false` if the user clicks Cancel.

[source,javascript]

----

confirm("Shall we, then?");

----

image::img/confirm.png[alt="A confirm dialog",width="8cm"]

(((input)))(((prompt function)))(((text input)))The `prompt` function

can be used to ask an “open” question. The first argument is the

question, the second one is the text that the user starts with. A line

of text can be typed into the dialog window, and the function will

return this text as a string.

[source,javascript]

----

prompt("Tell me everything you know.", "...");

----

image::img/prompt.png[alt="An prompt dialog",width="8cm"]

These two functions aren't used much in modern web programming, mostly

because you have no control over the way the resulting windows look,

but they are useful for toy programs and experiments.

== Control flow ==

(((execution order)))(((program)))(((control flow)))When your program

contains more than one ((statement)), the statements are executed,

predictably, from top to bottom. As a basic example, this program has

two statements. The first one asks the user for a number, and the

second, which is executed afterward, shows the ((square)) of that

number.

[source,javascript]

----

var theNumber = Number(prompt("Pick a number", ""));

alert("Your number is the square root of " +

theNumber * theNumber);

----

(((number,conversion to)))(((type coercion)))(((Number

function)))(((String function)))(((Boolean

function)))(((Boolean,conversion to)))The function `Number` converts a

value to a number. We need that conversion because the result of

`prompt` is a string value, and we want a number. There are similar

functions called `String` and `Boolean` that convert values to those

types.

Here is the rather trivial schematic representation of straight

control flow:

image::img/controlflow-straight.svg[alt="Trivial control flow",width="4cm"]

== Conditional execution ==

(((Boolean)))(((control flow)))Executing statements in straight-line

order isn't the only option we have. An alternative is _((conditional

execution))_, where we choose between two different routes based on a

Boolean value, like this:

image::img/controlflow-if.svg[alt="Conditional control flow",width="4cm"]

(((syntax)))(((Number function)))(((if keyword)))Conditional execution

is written with the `if` keyword in JavaScript. In the simple case, we

just want some code to be executed if, and only if, a certain

condition holds. For example, in the previous program, we might want

to show the square of the input only if the input is actually a

number.

[source,javascript]

----

var theNumber = Number(prompt("Pick a number", ""));

if (!isNaN(theNumber))

alert("Your number is the square root of " +

theNumber * theNumber);

----

With this modification, if you enter “cheese”, no output will be shown.

The keyword `if` executes or skips a statement depending on the value

of a Boolean expression. The deciding expression is written after the

keyword, between ((parentheses)), followed by the statement to execute.

(((isNaN function)))The `isNaN` function is a standard JavaScript

function that returns `true` only if the argument it is given is

`NaN`. The `Number` function happens to return `NaN` when you give it

a string that doesn't represent a valid number. Thus, the condition

translates to “unless `theNumber` is not-a-number, do this”.

(((else keyword)))You often won't just have code that executes when a

condition holds true, but also code that handles the other case. This

alternate path is represented by the second arrow in the

diagram. The `else` keyword can be used, together with `if`, to create

two separate, alternative execution paths.

[source,javascript]

----

var theNumber = Number(prompt("Pick a number", ""));

if (!isNaN(theNumber))

alert("Your number is the square root of " +

theNumber * theNumber);

else

alert("Hey. Why didn't you give me a number?");

----

(((if keyword,chaining)))If we have more than two paths to choose

from, multiple `if`/`else` pairs can be “chained” together. Here's an

example:

[source,javascript]

----

var num = Number(prompt("Pick a number", "0"));

if (num < 10)

alert("Small");

else if (num < 100)

alert("Medium");

else

alert("Large");

----

The program will first check whether `num` is less than 10. If it is,

it chooses that branch, shows `"Small"`, and is done. If it isn't, it

takes the `else` branch, which itself contains a second `if`. If the

second condition (`< 100`) holds, that means the number is between 10

and 100, and `"Medium"` is shown. If it doesn't, the second, and last,

`else` branch is chosen.

The flow chart for this program looks something like this:

image::img/controlflow-nested-if.svg[alt="Nested if control flow",width="4cm"]

[[loops]]

== while and do loops ==

(((even number)))Consider a program that prints all even numbers from

0 to 12. One way to write this is as follows:

[source,javascript]

----

console.log(0);

console.log(2);

console.log(4);

console.log(6);

console.log(8);

console.log(10);

console.log(12);

----

(((control flow)))That works, but the idea of writing a program is to

make something _less_ work, not more. If we needed all even numbers

less than 1,000, the previous would be unworkable. What we need is a

way to repeat some code. This form of control flow is called a

_((loop))_:

image::img/controlflow-loop.svg[alt="Loop control flow",width="4cm"]

(((syntax)))(((counter variable)))Looping control flow allows us to go

back to some point in the program where we were before and repeat it

with our current program state. If we combine this with a variable

that counts, we can do something like this:

[source,javascript]

----

var number = 0;

while (number <= 12) {

console.log(number);

number = number + 2;

}

// → 0

// → 2

// … etcetera

----

(((while loop)))(((Boolean)))A ((statement)) starting with the

keyword `while` creates a loop. The word `while` is followed by an

((expression)) in ((parentheses)) and then a statement, much like `if`.

The loop executes that statement as long as the expression produces a

value that is `true` when converted to Boolean type.

(((grouping)))((({} (block))))(((block)))In this loop, we want to both

print the current number and add two to our variable. Whenever we need

to execute multiple ((statement))s inside a loop, we wrap them in

((curly braces)) (`{` and `}`). Braces do for statements what

((parentheses)) do for expressions: they group them together, making

them count as a single statement. A sequence of statements wrapped in

braces is called a _block_.

(((programming style)))Many JavaScript programmers wrap every single

loop or `if` body in braces. They do this both for the sake of

consistency and to avoid having to add or remove braces when changing

the number of statements in the body later. In this book, I will write

most single-statement bodies without braces, since I value brevity.

You are free to go with whichever style you prefer.

(((comparison)))(((state)))The variable `number` demonstrates the way

a ((variable)) can track the progress of a program. Every time the

loop repeats, `number` is incremented by `2`. Then, at the beginning

of every repetition, it is compared with the number `12` to decide

whether the program has done all the work it intended to do.

(((exponentiation)))As an example that actually does something useful,

we can now write a program that calculates and shows the value of

2^10^ (2 to the 10th power). We use two variables: one to keep

track of our result and one to count how often we have multiplied this

result by 2. The loop tests whether the second variable has reached 10

yet and then updates both variables.

[source,javascript]

----

var result = 1;

var counter = 0;

while (counter < 10) {

result = result * 2;

counter = counter + 1;

}

console.log(result);

// → 1024

----

The counter could also start at `1` and check for `<= 10`, but, for

reasons that will become apparent in

link:04_data.html#array_indexing[Chapter 4], it is a good idea to get

used to counting from 0.

(((loop body)))(((do loop)))(((control flow)))The `do` loop is a

control structure similar to the `while` loop. It differs only on one

point: a `do` loop always executes its body at least once, and it

starts testing whether it should stop only after that first execution.

To reflect this, the test appears after the body of the loop:

[source,javascript]

----

do {

var yourName = prompt("Who are you?");

} while (!yourName);

console.log(yourName);

----

(((Boolean,conversion to)))(((! operator)))This program will

force you to enter a name. It will ask again and again until it gets

something that is not an empty string. Applying the `!` operator will

convert a value to Boolean type before negating it, and all strings

except `""` convert to `true`. This means the loop continues going round

until you provide a name that is not the empty string.

== Indenting Code ==

(((block)))(((code structure)))(((whitespace)))(((programming

style)))You've probably noticed the spaces I put in front of some

statements. In JavaScript, these are not required—the computer will

accept the program just fine without them. In fact, even the ((line))

breaks in programs are optional. You could write a program as a single

long line if you felt like it. The role of the ((indentation)) inside

blocks is to make the structure of the code stand out. In complex

code, where new blocks are opened inside other blocks, it can become

hard to see where one block ends and another begins. With proper

indentation, the visual shape of a program corresponds to the shape of

the blocks inside it. I like to use two spaces for every open block,

but tastes differ—some people use four spaces, and some people use

((tab character))s.

== for loops ==

(((syntax)))(((while loop)))(((counter variable)))Many loops follow

the pattern seen in the previous `while` examples. First, a “counter”

variable is created to track the progress of the loop. Then comes a

`while` loop, whose test expression usually checks whether the counter

has reached some boundary yet. At the end of the loop body, the

counter is updated to track progress.

(((for loop)))(((loop)))Because this pattern is so common, JavaScript and

similar languages provide a slightly shorter and more comprehensive

form, the `for` loop.

[source,javascript]

----

for (var number = 0; number <= 12; number = number + 2)

console.log(number);

// → 0

// → 2

// … etcetera

----

(((control flow)))(((state)))This program is exactly equivalent to the

link:02_program_structure.html#loops[earlier] even-number-printing

example. The only change is that all the ((statement))s that are

related to the “state” of the loop are now grouped together.

The ((parentheses)) after a `for` keyword must contain two

((semicolon))s. The part before the first semicolon _initializes_ the

loop, usually by defining a ((variable)). The second part is the

((expression)) that _checks_ whether the loop must continue. The final

part _updates_ the state of the loop after every iteration. In most

cases, this is shorter and clearer than a `while` construct.

(((exponentiation)))Here is the code that computes 2^10^, using `for`

instead of `while`:

[source,javascript]

----

var result = 1;

for (var counter = 0; counter < 10; counter = counter + 1)

result = result * 2;

console.log(result);

// → 1024

----

(((programming style)))(((indentation)))Note that even though no block

is opened with a `{`, the statement in the loop is still indented two

spaces to make it clear that it “belongs” to the line before it.

== Breaking Out of a Loop ==

(((loop,termination of)))(((break keyword)))Having the loop's

condition produce `false` is not the only way a loop can finish. There

is a special statement called `break` that has the effect of

immediately jumping out of the enclosing loop.

This program illustrates the `break` statement. It finds the first number

that is both greater than or equal to 20 and divisible by 7.

[source,javascript]

----

for (var current = 20; ; current++) {

if (current % 7 == 0)

break;

}

console.log(current);

// → 21

----

(((remainder operator)))(((% operator)))Using the remainder

(`%`) operator is an easy way to test whether a number is divisible by

another number. If it is, the remainder of their division is zero.

(((for loop)))The `for` construct in the example does not have a part

that checks for the end of the loop. This means that the loop will

never stop unless the `break` statement inside is executed.

If you were to leave out that `break` statement or accidentally write

a condition that always produces `true`, your program would get stuck

in an _((infinite loop))_. A program stuck in an infinite loop will

never finish running, which is usually a bad thing.

ifdef::interactive_target[]

If you create an infinite loop in one of the examples on these pages,

you'll usually be asked whether you want to stop the script after a

few seconds. If that fails, you will have to close the tab that you're

working in, or on some browsers close your whole browser, in order to

recover.

endif::interactive_target[]

(((continue keyword)))The `continue` keyword is similar to `break`, in

that it influences the progress of a loop. When `continue` is

encountered in a loop body, control jumps out of the body and

continues with the loop's next iteration.

== Updating variables succinctly ==

(((assignment)))(((+= operator)))(((-= operator)))(((/=

operator)))(((*= operator)))(((state)))(((side effect)))Especially

when looping, a program often needs to “update” a variable to hold a

value based on that variable's previous value.

// test: no

[source,javascript]

----

counter = counter + 1;

----

JavaScript provides a shortcut for this:

// test: no

[source,javascript]

----

counter += 1;

----

Similar shortcuts work for many other operators, such as `result *= 2` to

double `result` or `counter -= 1` to count downward.

This allows us to shorten our counting example a little more.

[source,javascript]

----

for (var number = 0; number <= 12; number += 2)

console.log(number);

----

(((++ operator)))(((-- operator)))For `counter += 1` and `counter -=

1`, there are even shorter equivalents: `counter++` and `counter--`.

== Dispatching on a value with switch ==

(((syntax)))(((conditional execution)))(((dispatching)))(((if

keyword,chaining)))It is common for code to look like this:

// test: no

[source,javascript]

----

if (variable == "value1") action1();

else if (variable == "value2") action2();

else if (variable == "value3") action3();

else defaultAction();

----

(((colon character)))(((switch keyword)))There is a construct called

`switch` that is intended to solve such a “dispatch” in a more direct

way. Unfortunately, the syntax JavaScript uses for this (which it

inherited from the C/Java line of programming languages) is somewhat

awkward—a chain of `if` statements often looks better. Here is an

example:

[source,javascript]

----

switch (prompt("What is the weather like?")) {

case "rainy":

console.log("Remember to bring an umbrella.");

break;

case "sunny":

console.log("Dress lightly.");

case "cloudy":

console.log("Go outside.");

break;

default:

console.log("Unknown weather type!");

break;

}

----

(((fallthrough)))(((comparison)))(((break keyword)))(((case

keyword)))(((default keyword)))You may put any number of `case` labels

inside the block opened by `switch`. The program will jump to the

label that corresponds to the value that `switch` was given or to

`default` if no matching value is found. It starts executing

statements there, even if they're under another label, until it

reaches a `break` statement. In some cases, such as the `"sunny"` case

in the example, this can be used to share some code between cases (it

recommends going outside for both sunny and cloudy weather). But

beware: it is easy to forget such a `break`, which will cause the

program to execute code you do not want executed.

== Capitalization ==

(((capitalization)))(((variable,naming)))(((whitespace)))Variable

names may not contain spaces, yet it is often helpful to use multiple

words to clearly describe what the variable represents. These are

pretty much your choices for writing a variable name with several

words in it:

----

fuzzylittleturtle

fuzzy_little_turtle

FuzzyLittleTurtle

fuzzyLittleTurtle

----

(((camel case)))(((programming style)))(((underscore character)))The

first style can be hard to read. Personally, I like the look of the

underscores, though that style is a little painful to type. The

((standard)) JavaScript functions, and most JavaScript programmers,

follow the bottom style—they capitalize every word except the first.

It is not hard to get used to little things like that, and code with

mixed naming styles can be jarring to read, so we will just follow

this ((convention)).

(((Number function)))(((constructor)))In a few cases, such as the

`Number` function, the first letter of a variable is also capitalized.

This was done to mark this function as a constructor. What a

constructor is will become clear in

link:06_object.html#constructors[Chapter 6]. For now, the important

thing is not to be bothered by this apparent lack of ((consistency)).

== Comments ==

(((readability)))Often, raw code does not convey all the information

you want a program to convey to human readers, or it conveys it in

such a cryptic way that people might not understand it. At other

times, you might just feel poetic or want to include some thoughts as

part of your program. This is what _((comment))s_ are for.

(((slash character)))(((line comment)))A comment is a piece of text

that is part of a program but is completely ignored by the computer.

JavaScript has two ways of writing comments. To write a single-line

comment, you can use two slash characters (`//`) and then the comment

text after it.

// test: no

[source,javascript]

----

var accountBalance = calculateBalance(account);

// It's a green hollow where a river sings

accountBalance.adjust();

// Madly catching white tatters in the grass.

var report = new Report();

// Where the sun on the proud mountain rings:

addToReport(accountBalance, report);

// It's a little valley, foaming like light in a glass.

----

(((block comment)))A `//` comment goes only to the end of the line. A

section of text between `/*` and `*/` will be ignored, regardless of

whether it contains line breaks. This is often useful for adding

blocks of information about a file or a chunk of program.

[source,javascript]

----

/*

I first found this number scrawled on the back of one of

my notebooks a few years ago. Since then, it has often

dropped by, showing up in phone numbers and the serial

numbers of products that I've bought. It obviously likes

me, so I've decided to keep it.

*/

var myNumber = 11213;

----

== Summary ==

You now know that a program is built out of statements, which

themselves sometimes contain more statements. Statements tend to

contain expressions, which themselves can be built out of smaller

expressions.

Putting statements after one another gives you a program that is

executed from top to bottom. You can introduce disturbances in the

flow of control by using conditional (`if`, `else`, and `switch`) and

looping (`while`, `do`, and `for`) statements.

Variables can be used to file pieces of data under a name, and they

are useful for tracking state in your program. The environment is the

set of variables that are defined. JavaScript systems

always put a number of useful standard variables into your

environment.

Functions are special values that encapsulate a piece of program. You

can invoke them by writing `functionName(argument1, argument2)`. Such

a function call is an expression, and may produce a value.

== Exercises ==

(((exercises)))If you are unsure how to try your solutions to

exercises, refer to the link:00_intro.html#intro[introduction].

Each exercise starts with a problem description. Read that and try to

solve the exercise. If you run into problems, consider reading the

hints (!interactive after the exercise!)(!book at the link:hints.html#hints[end of the book]!).

Full solutions to the exercises are not included in this

book, but you can find them online at

http://eloquentjavascript.net/2nd_edition/code[_eloquentjavascript.net/code_].

If you want to learn something from the exercises, I recommend looking

at the solutions only after you've solved the exercise, or at least

after you've attacked it long and hard enough to have a slight

headache.

=== Looping a triangle ===

(((triangle (exercise))))Write a ((loop)) that makes seven calls to

`console.log` to output the following triangle:

----

#

##

###

####

#####

######

#######

----

It may be useful to know that you can find the length of a string by

writing `.length` after it.

[source,javascript]

----

var abc = "abc";

console.log(abc.length);

// → 3

----

ifdef::interactive_target[]

Most exercises contain a piece of code that you can modify to solve

the exercise. Remember that you can click code blocks to edit them.

[source,javascript]

----

// Your code here.

----

endif::interactive_target[]

!!hint!!

(((triangle (exercise))))You can start with a program that simply

prints out the numbers 1 to 7, which you can derive by making a few

modifications to the

link:02_program_structure.html#loops[even number printing example]

given earlier in the chapter, where the `for` loop was introduced.

Now consider the equivalence between numbers and strings of hash

characters. You can go from 1 to 2 by adding 1 (`+= 1`). You can go

from `"#"` to `"##"` by adding a character (`+= "#"`). Thus, your

solution can closely follow the number-printing program.

!!hint!!

=== FizzBuzz ===

(((FizzBuzz (exercise))))(((loop)))(((conditional execution)))Write a

program that uses `console.log` to print all the numbers from 1 to

100, with two exceptions. For numbers divisible by 3, print `"Fizz"`

instead of the number, and for numbers divisible by 5 (and not 3),

print `"Buzz"` instead.

When you have that working, modify your program to print `"FizzBuzz"`,

for numbers that are divisible by both 3 and 5 (and still print

`"Fizz"` or `"Buzz"` for numbers divisible by only one of those).

(This is actually an ((interview question)) that has been claimed to

weed out a significant percentage of programmer candidates. So if you

solved it, you're now allowed to feel good about yourself.)

ifdef::interactive_target[]

[source,javascript]

----

// Your code here.

----

endif::interactive_target[]

!!hint!!

(((FizzBuzz (exercise))))(((remainder operator)))(((% operator)))Going

over the numbers is clearly a looping job, and selecting what to print

is a matter of conditional execution. Remember the trick of using the

remainder (`%`) operator for checking whether a number is divisible by

another number (has a remainder of zero).

In the first version, there are three possible outcomes for every

number, so you'll have to create an `if`/`else if`/`else` chain.

(((|| operator)))(((if keyword,chaining)))The second version of the

program has a straightforward solution and a clever one. The simple

way is to add another “branch” to precisely test the given condition.

For the clever method, build up a string containing the word or words

to output, and print either this word or the number if there is no

word, potentially by making elegant use of the `||` operator.

!!hint!!

=== Chess board ===

(((chess board (exercise))))(((loop)))(((nesting,of loops)))(((newline

character)))Write a program that creates a string that represents an

8×8 grid, using newline characters to separate lines. At each position

of the grid there is either a space or a “#” character. The characters

should form a chess board.

Passing this string to `console.log` should show something like this:

----

# # # #

# # # #

# # # #

# # # #

# # # #

# # # #

# # # #

# # # #

----

When you have a program that generates this pattern, define a

((variable)) `size = 8` and change the program so that it works for

any `size`, outputting a grid of the given width and height.

ifdef::interactive_target[]