Conditional Statements 1

if ( ... ) { ... }

An if statement is a program control construct. It controls the program flow based on a logical expression that evaluates either to true or false.

The structure of an if statement is as follows:

if (expression)
{
   statement;
}

The statement is executed only if the expression is true.

For example:

var name;
if (name == "John")
{
    say "Hello!"
}

This condition would say "Hello" if the variable name is set to "John", but not if it is set to "Mary".

The if statement can evaluate any expression that has a logical result, that is, true or false. The operators used in expressions are

Comparison Operators

<    --> less than

<=   --> less than or equal to

>    --> greater than

>=   --> greater than or equal to

Identity Operators

==   --> is equal to

!=   --> is NOT equal to

Examples:

(5 > 4)

true, 5 is greater than 4

(5 <= 4)

false, 5 is not less than or equal to 4

("John" == "Mary")

false, "John" is not equal to "Mary"

("John" != "Mary")

true, "John" is not equal to "Mary"

You can, of course, put also mathematical expressions into the if statement:

(1 + 1 = 2)

would evaluate to true; more useful perhaps would be an expression like this:

var x = 5;
var y = 10;

if (x - y <= 0)
{
  document.write("Error: Result exceeds lower limit");
  
  /* ... now do something else here ... */
}

Evaluating more than one condition

An if statement can evaluate more than one expression. All conditions must evaluate to true for the statements following the if test to be executed.

Each expression is evaluated individually first; then the results are compared based on a logical operator that stipulates the condition that the expressions must satisfy together. The operators are:

&& = logical AND

|| = logical OR

Logical AND:

if (expression1 && expression2)
{
  do something;
}

If expression1 and expression2 are true, do something.

Logical OR:

if (expression1 || expression2)
{
  do something;
}

If either expression1 or expression2 is true, do something.