JavaScript basics

Before we start diving into 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 take a look at the two different ways we can use JavaScript code on an HTML page:

The first way is demonstrated by the previous code. We need to create an HTML file and write this code on it. In this example, we are declaring the script tag inside the HTML file and, inside the script tag, we have the JavaScript code.

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:

Then, our HTML file will look similar to this:

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 best practice.

Note

You may find JavaScript include statements or JavaScript code inside the head tag in some examples on the Internet. As best practice, we will include any JavaScript code at the end of the body tag. This way, the HTML will be parsed by the browser and displayed before the scripts are loaded. This boosts the performance of the page.

Variables

Variables store data that can be set, updated, and retrieved whenever needed. Values that are assigned to a variable belong to a type. In JavaScript, the available types are numbers, strings, Booleans, functions, and objects. We also have undefined and null, along with arrays, dates, and regular expressions.

Note

Although JavaScript has different available variable types, it is not a strongly typed language such as C/C++, C#, Java. In strongly typed languages, we need to declare the type of the variable along with its declaration (in Java, for example, 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.

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

• On 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 good practice to always specify when we declare a new variable.
• On 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 good practice.
• On line {3}, we also declared a number, but this time it is a decimal floating point. On line {4}, we declared a string; on line {5}, we declared a Boolean. On line {6}, we declared a null value, and on 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. Take a look at the following:

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

Note

We have three ways of outputting values in JavaScript that we can use with the examples of this book. The first one is alert('My text here'), which outputs an alert window on the browser, and the second one is console.log('My text here'), which outputs text on the Console tab of the debug tool (Google Developer Tools or Firebug, depending on the browser you are using). The third way is outputting the value directly on the HTML page that is rendered by the browser using document.write('My text here'). You can use the option that you feel most comfortable with.

The console.log method also accepts more than just arguments. Instead of console.log("num: "+ num), we can also use console.log("num: ", num).

We will discuss functions and objects later in this chapter.

Variable scope

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:

• 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  referencing the myOtherVariable global variable and assigning 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.

Operators

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:

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

Arithmetic operator

Description

+

Addition

-

Subtraction

*

Multiplication

/

Division

%

Modulus (remainder of a division operation)

++

Increment

--

Decrement

On line {2}, we have the assignment...