The basic if statement takes the form:
if ( expression )
{ statement }
The expression should be in parentheses. A single statement does not need curly brackets but multiple statements should be grouped within curly brackets.
For example:
var a=2;
var b=5;
if (a == b)
{
document.writeln("Behold! a and b are the same!");
}
The code fragment above would cause no alert, whereas the code fragment below would
var c=5;
var d=5;
if (c == d)
{
document.writeln("Behold! c and d are the same!");
}
For more details of the expressions that can be used to compare two operands, see comparison operators.
You can test more than one thing at a time. The first code fragment below tests if both a and be are the same, and c and d. It will not generate a message in this example. The second code fragment tests if a = b OR c = d, and it will produce an alert in this case.
if ((a == b) && (c == d))
{
document.writeln("Wow! a = b and c = d!");
}
if ((a == b) || (c == d))
{
document.writeln("One out of 2 isn't bad! a = b or c = d!");
}
(The javascript for the 4 tests above follow in the code, but with each document.writeln commented out. Just view source and un-comment them to try the code out).
To carry out one action if the condition is true, and another if it is false, use an else. For example:
if (a == b)
{
document.writeln("Behold! a and b are the same!");
}
else
{
document.writeln("Beware! a and b are different!");
}
You can combine an else clause with a nested if/else statement to produce an else/if statement. For example:
if (a == b)
{
document.writeln("Behold! a and b are the same!");
}
else if (a < b )
{
document.writeln("a is less than b!");
}
else
{
document.writeln("a is greater than b!");
}
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© 2002. Copyright Sue Nethercott.