Most programs do different things based on input provided by the user of the program. In order to instruct the program to vary performance for varying input, programmers use conditional statements called if statements. The format for an if statement is given below.

if (BOOLEAN EXPRESSION) statement;

where BOOLEAN EXPRESSION is replaced by any expression that evaluates to TRUE or FALSE. For example,

if (a > b) document.write(a, " is greater than ", b);

will display the value of a followed by the text " is greater than " followed by the value of b, but only if a is larger than b.

Boolean expressions can be formed by using any of the following comparison operators. 
<	less than
>	greater than
<=	less than or equal to
>=	greater than or equal to
== 	equal to
Boolean expressions can be combined into larger boolean expressions by using the symbols && (AND) or || (OR). For example,

if (a > b && a < c) document.write(a, " is the middle value<BR>"); if (a > b && a > c || a < b && a < c) document.write(a, " is not the middle value<BR>"); Finally, the ! symbol can be placed immediately before any boolean expression in order to negate its value. !TRUE is FALSE and !FALSE is TRUE. The following two boolean expressions are equivalent.

(a < b)	  and	!(a >= b)