Fundamentals of JavaScript

Variable declaration

In JavaScript, we use let to declare a variable. Here’s an example:

let persons; // declaring a variable

Here, the variable persons is declared, but its value is undefined.

Note: It is a good practice to initialize every variable.

Variable initialization

Assigning a value to a variable at the time of its declaration is called initialization. For example, the following code demonstrates the initialization of a variable in JavaScript:

In the code above, we have initialized the variable persons:

JavaScript supports the following:

• Number: Numeric values (5, 3.0, -1, etc.)
• String: String values (Educative, JavaScript, etc.)
• Boolean: Boolean values (true, false)

The following code demonstrates variables storing values of different types in JavaScript:

In the code above, we have initialized different variables to define their types.

Operations depend on the type of data

The type of data also tells which operations are permissible on a specific piece of data and indicates the semantics (or meaning) of those operations. The following code demonstrates the effect of the + operation on different types of data in JavaScript:

Every input is a string in JavaScript

When we use a prompt() statement in JavaScript, it always returns a string of characters. Even the integer, 25, is considered a string, as shown in the following code widget:

Note: Please type an integer value in each prompt after you run the following code.

In the code above, integers are treated as strings and they are concatenated due to the + operator in the output.

We must convert the input value to the suitable data type to apply the correct operation. We should use the parseInt() to convert it into an integer, as demonstrated below:

Note: Even if the input value is a floating-point, parseInt() converts it into an integer. The result contains only the integer part in the output.

Similarly, we can use the parseFloat() to store a floating-point value in a variable, as demonstrated below:

Operators in JavaScript

Like any other programming language, JavaScript supports several types of operators. We have already seen the assignment operator, =, which assigns a value to a variable in our programs. The following are various forms of the assignment operator:

Commonly used operators

There are several other commonly used operators in JavaScript, including:

• Arithmetic operators: These are used for arithmetic operations (addition, subtraction, multiplication, etc.).

• Comparison operators: These are used in conditional expressions (less than, greater than, etc.).

• Logical operators: These are used to combine conditional expressions (and, or, not).

• Compound assignment operators: These are the combinations of arithmetic and assignment operators.