Sams Teach Yourself JavaScript in 24 Hours

Data Types in JavaScript

In some computer languages, you have to specify the type of data a variable will store: for example, a number or a string. In JavaScript, you don't need to specify a data type in most cases. However, you should know the types of data JavaScript can deal with.

These are the basic JavaScript data types:

• Numbers, such as 3, 25, or 1.4142138. JavaScript supports both integers and floating-point numbers.

• Boolean, or logical values. These can have one of two values: true or false. These are useful for indicating whether a certain condition is true.

  By the Way

  You'll learn more about Boolean values, and about using conditions in JavaScript, in Hour 7.

  
  

• Strings, such as "I am a jelly doughnut". These consist of one or more characters of text. (Strictly speaking, these are String objects, which you'll learn about later in this hour.)

• The null value, represented by the keyword null. This is the value of an undefined variable. For example, the statement document.write(fig) will result in this value (and an error message) if the variable fig has not been previously used or defined.

Although JavaScript keeps track of the data type currently stored in each variable, it doesn't restrict you from changing types midstream. For example, suppose you declared a variable by assigning it a value:

total = 31;

This statement declares a variable called total and assigns it the value of 31. This is a numeric variable. Now suppose you changed the value of total:

total = "albatross";

This assigns a string value to total, replacing the numeric value. JavaScript will not display an error when this statement executes; it's perfectly valid, although it's probably not a very useful total.