In this book, you will learn about the most-used data structures and algorithms. Why should we use JavaScript to learn about data structures and algorithms? We have already answered this question. JavaScript is very popular and is appropriate for learning about data structures because it is a functional language. Also, this can be a very fun way of learning something new, as it is very different from (and easier than) learning about data structures with a standard language such as C, Java, or Python. And who said data structures and algorithms were only made for languages such as C and Java? You might need to implement some of these languages while developing for the frontend as well.

Learning about data structures and algorithms is very important. The first reason is that data structures and algorithms can solve the most common problems efficiently. This will make a difference to the quality of the source code you write in the future (including performance; if you choose the incorrect data structure or algorithm, depending on the scenario, you may have some performance issues). Secondly, algorithms are studied in college together with the introductory concepts of computer science. And finally, if you are planning on getting a job with one of the greatest Information Technology (IT) companies (such as Google, Amazon, Microsoft, eBay, and so on) data structures, and algorithms are the subjects of interview questions.

Let's get started!

One of the pros of the JavaScript language compared to other languages is that you do not need to install or configure a complicated environment to get started with it. Every computer has the required environment already, even though the user may never write a single line of source code. All we need is a browser!

To execute the examples in this book, it is recommended that you have a modern browser installed such as Google Chrome or Firefox (you can use the one you like the most), an editor of your preference (such as Visual Studio Code), and a web server (XAMPP or any other of your preference, but this step is optional). Chrome, Firefox, VS Code, and XAMPP are available for Windows, Linux, and Mac OS.

The simplest environment that you can use for JavaScript development is a browser. The modern browsers (Chrome, Firefox, Safari, and Edge) have a functionality called Developer Tools. To access the DevTools in Chrome, you can click on the menu in the upper-right corner, More Tools | Developer Tools:

When you open the DevTools, you will see the Console tab, and you will be able to write all your JavaScript code in its command-line area, as demonstrated in the following screenshot (to execute the source code, you need to press Enter):

The second environment option you might want to install on your computer is also simple, but it requires installing a web server. If an HTML file contains only simple JavaScript code that does not require any request to a server (Ajax calls), it can be executed in the browser by right-clicking on the HTML file and selecting the Open with option. The code we will develop throughout this book is simple and it can be executed using this approach. However, it is always nice to have a web server installed.

There are many open source and free options available to you. If you are familiar with PHP, XAMPP (https://www.apachefriends.org) is a good option, and it is available for Linux, Windows, and Mac OS.

Since we will be focusing on JavaScript for the server side and the browser, there is also a simpler web server you can install in Chrome. It is the Web Server for Chrome extension, and this can be downloaded at https://goo.gl/pxqLmU. After installing it, you can access it through the Chrome URL chrome://apps:

After opening the Web Server extension, you can CHOOSE FOLDER you want to serve in the browser. You can create a new folder in which you can execute the source code we will implement throughout this book, or you can download the source code from this book and extract it to a directory of your preference and access it through the informed URL (the default is http://127.0.0.1:8887):

All examples from this book can be executed by accessing http://127.0.0.1:8887/examples. You will find an index.html with a list of all examples, as demonstrated in the following screenshot:

The third option is having an environment that is 100 percent JavaScript! For this environment, we need to have Node.js installed. Go to http://nodejs.org, and download and install Node.js. After installing it, open the Terminal application (if you are using Windows, open the Command Prompt with Node.js, which was installed with Node.js) and run the following command:

    npm install http-server -g

Make sure you type the command and don't copy and paste it. Copying the command might give you some errors. You can also execute the command as an administrator. For Linux and Mac systems, use the following command:

    sudo npm install http-server -g

This command will install http-server, which is a JavaScript server. To start a server and run the examples from this book in the Terminal application, change the directory to the folder that contains the book's source code and type http-server, as displayed in the following screenshot:

To execute the examples, open the browser and access the localhost on the port specified by the http-server command.

Before we start diving in to the various data structures and algorithms, let's have a quick overview of the JavaScript language. This section will present the JavaScript basics required to implement the algorithms we will create in the subsequent chapters.

To start, let's look at the two different ways we can use JavaScript code on an HTML page. The first example is demonstrated by the following code. We need to create an HTML file (01-HelloWorld.html) and write this code in it. In this example, we are declaring the script tag inside the HTML file and, inside the script tag, we have the JavaScript code:

<!DOCTYPE html> 
<html> 
  <head> 
    <meta charset="UTF-8"> 
  </head> 
  <body> 
    <script> 
      alert('Hello, World!'); 
    </script> 
  </body> 
</html> 

For the second example, we need to create a JavaScript file (we can save it as 01- HelloWorld.js) and, inside this file, we will insert the following code:

alert('Hello, World!'); 

Then, our HTML file will look similar to this:

<!DOCTYPE html> 
<html> 
  <head> 
    <meta charset="UTF-8"> 
    <title></title> 
  </head> 
  <body> 
    <script src="01-HelloWorld.js"></script> 
  </body> 
</html> 

The second example demonstrates how to include a JavaScript file inside an HTML file.

By executing any of these two examples, the output will be the same. However, the second example is the most used by JavaScript developers.

Variables store data that can be set, updated, and retrieved whenever necessary. Values that are assigned to a variable belong to a type. In JavaScript, the available types are number, string, boolean, function, and object. We also have undefined and null, along with arrays, dates, and regular expressions.

Although JavaScript has different available variable types, it is not a strongly typed language such as C/C++, C#, and Java. In strongly typed languages, we need to declare the type of the variable along with its declaration (for example, in Java, to declare an integer variable, we use int num = 1;). In JavaScript, we only need to use the keyword var, and we do not need to declare the variable type. For this reason, JavaScript is not a strongly typed language. However, there are discussions and a specification in draft mode for optional static typing (https://github.com/dslomov/typed-objects-es7) that can become part of the JavaScript specification (ECMAScript) in the future. We can also use TypeScript in case we want to type our variables when working with JavaScript. We will learn more about ECMAScript and TypeScript later in this chapter.

The following is an example of how to use variables in JavaScript:

var num = 1; // {1} 
num = 3; // {2} 
var price = 1.5; // {3} 
var myName = 'Packt'; // {4} 
var trueValue = true; // {5} 
var nullVar = null; // {6} 
var und; // {7}

• In line {1}, we have an example of how to declare a variable in JavaScript (we are declaring a number). Although it is not necessary to use the var keyword declaration, it is a good practice to always specify when we declare a new variable.
• In line {2}, we updated an existing variable. JavaScript is not a strongly typed language. This means you can declare a variable, initialize it with a number, and then update it with a string or any other datatype. Assigning a value to a variable that is different from its original type is also not a good practice.
• In line {3}, we also declared a number, but this time it is a decimal floating point. In line {4}, we declared a string; in line {5}, we declared a boolean. In line {6}, we declared a null value, and in line {7}, we declared an undefined variable. A null value means no value, and undefined means a variable that has been declared but not yet assigned a value.

If we want to see the value of each variable we declared, we can use console.log to do so, as listed in the following code snippet:

console.log('num: ' + num); 
console.log('myName: ' + myName); 
console.log('trueValue: ' + trueValue); 
console.log('price: ' + price); 
console.log('nullVar: ' + nullVar); 
console.log('und: ' + und); 

The console.log method also accepts more than just arguments. Instead of console.log('num: ' + num), we can also use console.log('num: ', num). While the first option is going to concatenate the result into a single string, the second option allows us to add a description and also visualize the variable content in case it is an object.

We will discuss functions and objects later in this chapter.

The scope refers to where in the algorithm we can access the variable (it can also be a function when we work with function scopes). There are local and global variables.

Let's look at an example:

var myVariable = 'global'; 
myOtherVariable = 'global'; 
 
function myFunction() { 
  var myVariable = 'local'; 
  return myVariable; 
} 
 
function myOtherFunction() { 
  myOtherVariable = 'local'; 
  return myOtherVariable; 
} 
 
console.log(myVariable); //{1} 
console.log(myFunction()); //{2} 
 
console.log(myOtherVariable); //{3} 
console.log(myOtherFunction()); //{4} 
console.log(myOtherVariable); //{5} 

The above code can be explained as follows:

• Line {1} will output global because we are referring to a global variable.
• Line {2} will output local because we declared the myVariable variable inside the myFunction function as a local variable, so the scope will only be inside myFunction.
• Line {3} will output global because we are referencing the global variable named myOtherVariable that was initialized on the second line of the example.
• Line {4} will output local. Inside the myOtherFunction function, we referenced the myOtherVariable global variable and assigned the value local to it because we are not declaring the variable using the var keyword.
• For this reason, line {5} will output local (because we changed the value of the variable inside myOtherFunction).

You may hear that global variables in JavaScript are evil, and this is true. Usually, the quality of JavaScript source code is measured by the number of global variables and functions (a large number is bad). So, whenever possible, try avoiding global variables.

We need operators when performing any operation in a programming language. JavaScript also has arithmetic, assignment, comparison, logical, bitwise, and unary operators, among others. Let's take a look at these:

var num = 0; // {1}  
num = num + 2; 
num = num * 3;  
num = num / 2;  
num++; 
num--; 
 
num += 1; // {2} 
num -= 2; 
num *= 3; 
num /= 2; 
num %= 3; 
 
console.log('num == 1 : ' + (num == 1)); // {3}  
console.log('num === 1 : ' + (num === 1));  
console.log('num != 1 : ' + (num != 1));  
console.log('num > 1 : ' + (num > 1)); 
console.log('num < 1 : ' + (num < 1));  
console.log('num >= 1 : ' + (num >= 1));  
console.log('num <= 1 : ' + (num <= 1)); 
 
console.log('true && false : ' + (true && false)); // {4}  
console.log('true || false : ' + (true || false));  
console.log('!true : ' + (!true)); 

In line {1}, we have the arithmetic operators. In the following table, we have the operators and their descriptions:

In line {2}, we have the assignment operators. In the following table, we have the operators and their descriptions:

In line {3}, we have the comparison operators. In the following table, we have the operators and their descriptions:

Finally, in line {4}, we have the logical operators. In the following table, we have the operators and their descriptions:

JavaScript also supports bitwise operators, which are shown as follows:

console.log('5 & 1:', (5 & 1)); 
console.log('5 | 1:', (5 | 1)); 
console.log('~ 5:', (~5)); 
console.log('5 ^ 1:', (5 ^ 1)); 
console.log('5 << 1:', (5 << 1)); 
console.log('5 >> 1:', (5 >> 1)); 

The following table contains a more detailed description of the bitwise operators:

The typeof operator returns the type of the variable or expression. For example, have a look at the following code:

console.log('typeof num:', typeof num); 
console.log('typeof Packt:', typeof 'Packt'); 
console.log('typeof true:', typeof true); 
console.log('typeof [1,2,3]:', typeof [1,2,3]); 
console.log('typeof {name:John}:', typeof {name:'John'}); 

The output will be as follows:

    typeof num: number
    typeof Packt: string
    typeof true: boolean
    typeof [1,2,3]: object
    typeof {name:John}: object
 

According to the specification, there are two data types in JavaScript:

• Primitive data types: Null, undefined, string, number, boolean, and symbol
• Derived data types/objects: JavaScript objects, including functions, arrays, and regular expressions 

JavaScript also supports the delete operator, which deletes a property from an object:

var myObj = {name: 'John', age: 21}; 
delete myObj.age; 
console.log(myObj); //outputs Object {name: "John"} 

In this book's algorithms, we will be using some of these operators.

In JavaScript, true and false are a little bit tricky. In most languages, the boolean values true and false represent the true/false results. In JavaScript, a string such as Packt evaluates to true.

The following table can help us better understand how true and false work in JavaScript:

Let's consider some examples and verify their output:

function testTruthy(val) { 
  return val ? console.log('truthy') : console.log('falsy'); 
} 
 
testTruthy(true); // true 
testTruthy(false); // false 
testTruthy(new Boolean(false)); // true (object is always true) 
 
testTruthy(''); // false 
testTruthy('Packt'); // true 
testTruthy(new String('')); // true (object is always true) 
 
testTruthy(1); // true 
testTruthy(-1); // true 
testTruthy(NaN); // false 
testTruthy(new Number(NaN)); // true (object is always true) 
 
testTruthy({}); // true (object is always true) 
 
var obj = { name: 'John' }; 
testTruthy(obj); // true 
testTruthy(obj.name); // true 
testTruthy(obj.age); // age (property does not exist) 

The two equals operators supported by JavaScript can cause a little bit of confusion when working with them.

When using ==, values can be considered equal even when they are of different types. This can be confusing even for a senior JavaScript developer. Let's analyze how == works using the following table:

If x and y are of the same type, then JavaScript will use the equals method to compare the two values or objects. Any other combination that is not listed in the table gives a false result.

The toNumber and toPrimitive methods are internal and evaluate the values according to the tables that follow.

The toNumber method is as follows:

Finally, toPrimitive is as follows:

Let's verify the results of some examples. First, we know that the output of the following code is true (string length > 1):

console.log('packt' ? true : false); 

Now, what about the following code? Let's take a look:

console.log('packt' == true); 

The output is false, so let's understand why:

• First, it converts the boolean value using toNumber, so we have packt == 1.
• Then, it converts the string value using toNumber. Since the string consists of alphabetical characters, it returns NaN, so we have NaN == 1, which is false.

What about the following code? Let's take a look:

console.log('packt' == false); 

The output is also false, and the following is why:

• First, it converts the boolean value using toNumber, so we have packt == 0.
• Then, it converts the string value using toNumber. Since the string consists of alphabetical characters, it returns NaN, so we have NaN == 0, which is false.

What about the === operator? This is much easier. If we are comparing two values of different types, the result is always false. If they have the same type, they are compared according to the following table:

If x and y are different types, then the result is false. Let's consider some examples:

console.log('packt' === true); //false  
console.log('packt' === 'packt'); //true 
var person1 = {name:'John'};  
var person2 = {name:'John'}; 
console.log(person1 === person2); //false, different objects 

JavaScript has a similar set of control structures as the C and Java languages. Conditional statements are supported by if...else and switch. Loops are supported by the while, do...while, and for constructs.

The first conditional statement we will take a look at is the if...else construct. There are a few ways we can use the if...else construct.

We can use the if statement if we want to execute a block of code only if the condition (expression) is true, as follows:

var num = 1; 
if (num === 1) { 
  console.log('num is equal to 1'); 
} 

We can use the if...else statement if we want to execute a block of code and the condition is true or another block of code just in case the condition is false (else), as follows:

var num = 0; 
if (num === 1) { 
  console.log('num is equal to 1'); 
} else { 
  console.log('num is not equal to 1, the value of num is ' + num); 
} 

The if...else statement can also be represented by a ternary operator. For example, take a look at the following if...else statement:

if (num === 1) { 
  num--; 
} else { 
  num++; 
} 

It can also be represented as follows:

(num === 1) ? num-- : num++; 

Also, if we have several expressions, we can use if...else several times to execute different blocks of code based on different conditions, as follows:

var month = 5; 
if (month === 1) { 
  console.log('January'); 
} else if (month === 2) { 
  console.log('February'); 
} else if (month === 3) { 
  console.log('March'); 
} else { 
  console.log('Month is not January, February or March'); 
} 

Finally, we have the switch statement. If the condition we are evaluating is the same as the previous one (however, it is being compared to different values), we can use the switch statement:

var month = 5; 
switch (month) { 
  case 1: 
    console.log('January'); 
    break; 
  case 2: 
    console.log('February'); 
    break; 
  case 3: 
    console.log('March'); 
    break; 
  default: 
    console.log('Month is not January, February or March'); 
} 

One thing that is very important in a switch statement is the use of the case and break keywords. The case clause determines whether the value of switch is equal to the value of the case clause. The break statement stops the switch statement from executing the rest of the statement (otherwise, it will execute all the scripts from all case clauses below the matched case until a break statement is found in one of the case clauses). Finally, we have the default statement, which is executed by default if none of the case statements are true (or if the executed case statement does not have the break statement).

Loops are often used when we work with arrays (which are the subject of the next chapter). Specifically, we use the for loop in our algorithms.

The for loop is the same as in C and Java. It consists of a loop counter that is usually assigned a numeric value, then the variable is compared against another value (the script inside the for loop is executed while this condition is true), and finally, the numeric value is increased or decreased.

In the following example, we have a for loop. It outputs the value of i on the console, where i is less than 10; i is initiated with 0, so the following code will output the values 0 to 9:

for (var i = 0; i < 10; i++) { 
  console.log(i); 
} 

The next loop construct we will look at is the while loop. The block of code inside the while loop is executed while the condition is true. In the following code, we have a variable, i, initiated with the value 0, and we want the value of i to be output while i is less than 10 (or less than or equal to 9). The output will be the values from 0 to 9:

var i = 0; 
while (i < 10) { 
  console.log(i); 
  i++; 
} 

The do...while loop is similar to the while loop. The only difference is that in the while loop, the condition is evaluated before executing the block of code, and in the do...while loop, the condition is evaluated after the block of code is executed. The do...while loop ensures that the block of code is executed at least once. The following code also outputs the values from 0 to 9:

var i = 0; 
do { 
  console.log(i); 
  i++; 
} while (i < 10); 

Functions are very important when working with JavaScript. We will also use functions in our examples.

The following code demonstrates the basic syntax of a function. It does not have arguments or the return statement:

function sayHello() { 
  console.log('Hello!'); 
} 

To execute this code, we simply use the following statement:

sayHello(); 

We can also pass arguments to a function. Arguments are variables with which a function is supposed to do something. The following code demonstrates how to use arguments with functions:

function output(text) { 
  console.log(text); 
} 

To use this function, we can use the following code:

output('Hello!'); 

You can use as many arguments as you like, as follows:

output('Hello!', 'Other text'); 

In this case, only the first argument is used by the function, and the second one is ignored. A function can also return a value, as follows:

function sum(num1, num2) { 
  return num1 + num2; 
} 

This function calculates the sum of two given numbers and returns its result. We can use it as follows:

var result = sum(1, 2); 
output(result); // outputs 3 

JavaScript objects are very simple collections of name-value pairs. There are two ways of creating a simple object in JavaScript. The first way is as follows:

var obj = new Object(); 

And the second way is as follows:

var obj = {}; 

We can also create an entire object, as follows:

obj = { 
  name: { 
    first: 'Gandalf', 
    last: 'the Grey' 
  }, 
  address: 'Middle Earth' 
}; 

As we can see, to declare a JavaScript object, [key, value] pairs are used, where the key can be considered an attribute of the object and the value is the property value. All classes that we will create in this book are JavaScript objects, such as Stack, Set, LinkedList, Dictionary, Tree, Graph, and so on.

In Object-oriented programming (OOP), an object is an instance of a class. A class defines the characteristics of the object. For our algorithms and data structures, we will create some classes that will represent them. This is how we can declare a class (constructor) that represents a book:

function Book(title, pages, isbn) { 
  this.title = title; 
  this.pages = pages; 
  this.isbn = isbn; 
} 

To instantiate this class, we can use the following code:

var book = new Book('title', 'pag', 'isbn'); 

Then, we can access its properties and update them as follows:

console.log(book.title); // outputs the book title 
book.title = 'new title'; // update the value of the book title 
console.log(book.title); // outputs the updated value 

A class can also contain functions (generally also referred to as methods). We can declare and use a function/method as the following code demonstrates:

Book.prototype.printTitle = function() { 
  console.log(this.title); 
}; 
 
book.printTitle(); 

We can declare functions directly inside the class definition as well:

function Book(title, pages, isbn) { 
  this.title = title; 
  this.pages = pages; 
  this.isbn = isbn; 
  this.printIsbn = function() { 
    console.log(this.isbn); 
  }; 
} 
book.printIsbn(); 

Knowing how to program with JavaScript is important, but so is knowing how to debug your code. Debugging is very useful in helping you find bugs in your code, but it can also help you execute your code at a lower speed so that you can see everything that is happening (the stack of methods called, variable assignment, and so on). It is highly recommended that you spend some time debugging the source code of this book to see every step of the algorithm (it might help you understand it better as well).

Firefox, Safari, Edge, and Chrome support debugging. A great tutorial from Google that shows you how to use Google Developer Tools to debug JavaScript can be found at https://developer.chrome.com/devtools/docs/javascript-debugging.

You can use any text editor of your preference. However, there are other great tools that can help you be more productive when working with JavaScript as well, which are listed as follows:

• WebStorm: This is a very powerful JavaScript IDE with support for the latest web technologies and frameworks. It is a paid IDE, but you can download a 30- day trial version (http://www.jetbrains.com/webstorm).
• Sublime Text: This is a lightweight text editor and you can customize it by installing plugins. You can buy the license to support the development team, but you can also use it for free (the trial version does not expire) at http://www.sublimetext.com.
• Atom: This is also a free text editor created by GitHub. It has great support for JavaScript and it can also be customized by installing plugins (https://atom.io).
• Visual Studio Code: This is a free and open source code editor created by Microsoft, written with TypeScript. It has JavaScript autocomplete functionality with IntelliSense and provides built-in debug capability directly from the editor. It can also be customized by installing plugins (https://code.visualstudio.com).

All of the aforementioned editors are available for Windows, Linux, and Mac OS.

To debug JavaScript or ECMAScript code directly from VSCode, first, we need to install the Debugger for Chrome extension (https://goo.gl/QpXWGM).

Next, open the Web Server for Chrome extension and open the link to see the book examples in the browser (the default URL is http://127.0.0.1:8887/examples).

The following screenshot demonstrates how to debug directly from the editor:

1. In the editor, open the JavaScript file you want to debug, pass the mouse pointer near the line numbers, and click on the line to add a breakpoint (as demonstrated by 1 in the preceding screenshot). This is where the debugger will stop so we can analyze the code.
2. Once the Web Server is up and running, click on the Debug view (2), select Chrome (3), and click on the Play icon to initiate the debugging process.
3. Chrome will be opened automatically. Navigate to the desired example to evoke the code we want to debug. Once the line we added the breakpoint to is reached by the debugger, the process will stop and the editor will receive the focus.
4. We can control how the code is debugged using the top toolbar (4). We can resume the process, go to a method call, go to the next line, and restart and stop the process. It is the same behavior we have in the debugger in Chrome and other browsers.
5. The advantage of using this built-in debug functionality is that we can do everything from the editor (coding, debugging, and testing). And we also have the variables declared and call stack, we can watch variables and expressions (5), hover the mouse over a variable to see its current value (6), and see the console output as well (7).

The source code of this book was developed using Visual Studio Code and the code bundle also contains configured launch tasks so you can debug the code and the tests directly from the VSCode (all details are in the .vscode/launch.json file). All extensions recommended to run the source code from this book are also listed in the .vscode/extensions.json file.

In this chapter, we learned how to set up the development environment to be able to create or execute the examples in this book.

We also covered the basics of the JavaScript language that are needed prior to getting started with developing the algorithms and data structures covered in this book.

In the next chapter, we will learn about new functionalities introduced to JavaScript since 2015 and also how to leverage static typing and error checking using TypeScript.