CSCI 121 - Lecture 10 - JavaScript Variables, Data Types and Objects*

Topics Covered

JavaScript Variables

Think of variables as "containers" or "buckets" for holding data values.

In the example below, x, y and z are variables.

                var x = 5;
                var y = 6;
                var z = x + y;

Above, z will hold the value 11.

JavaScript Identifiers

All JavaScript variables must be identified with unique names.
These unique names are called identifiers.
Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).
The general rules for constructing names for variables (unique identifiers) are:

Names can contain letters, digits, underscores, and dollar signs.
Names must begin with a letter.
Names can also begin with $ and _ (but we will not use it in this tutorial).
Names are case sensitive (y and Y are different variables).
Reserved words (like JavaScript keywords) cannot be used as names

Assignment Operator

In JavaScript, the equal sign (=) is an "assignment" operator, not an "equal to" operator.
This is different from algebra. The following does not make sense in algebra:

                x = x + 5;

In JavaScript, however, it makes perfect sense: it assigns the value of x + 5 to x. (It calculates the value of x + 5 and puts the result into x. The value of x is incremented by 5.)
The "equal to" operator is written like == in JavaScript.

JavaScript Data Types

JavaScript variables can hold many data types: numbers, strings, arrays, objects and more:

                    var length = 16;                               // Number
                    var lastName = "Johnson";                      // String
                    var cars = ["Saab", "Volvo", "BMW"];           // Array
                    var x = {firstName:"John", lastName:"Doe"};    // Object

The Concept of Data Types

In programming, data types is an important concept.
To be able to operate on variables, it is important to know something about the type.
Without data types, a computer cannot safely solve this:

                var x = 16 + "Volvo";

Does it make any sense to add "Volvo" to sixteen? Will it produce an error or will it produce a result?
JavaScript will treat the example above as:

                var x = "16" + "Volvo";

JavaScript evaluates expressions from left to right. Different sequences can produce different results:

                var x = 16 + 4 + "Volvo";

Result: 20Volvo

                var x = "Volvo" + 16 + 4;

Result: Volvo164

To be continued...

* Examples and content from W3Schools.com.