Data Types & Variables

JavaScript defines four basic data types as follows:

numbers

may be any number, such as: 23, 5.7, or 34e6

strings

any alpha-numeric data, such as: "Hello, this is page 1"

boolean

used for results of decisions and may only be true or false, not any other value, such as 0 or -1

null

represented by the special keyword 'null' which represents nothing

These data types apply to both literals and variables. When using variables, the data type is determined by the interpreter, based on the contents of the variable.

For example, if the variable contains the value "Hello", the interpreter treats the variable as a string data type automatically. If the variable contained the value 10, the interpreter would treat the variable as a number.

When declaring variables in JavaScript, the following rules apply:

• variables should be declared using the keyword 'var' at the variable's first occurrence.
• names are case-sensitive.
• variable names must start with a letter or an underscore, for example

  name or _ctrl_val are valid names, but
  1stname and 1234 are not.

• may contain letter, numbers and underscores only
• cannot be a reserved word, such as null or alert
• names must be meaningful