if

Any _local variables you declare within the body of an if/then statement (ie between the curly braces) are local to that 'if' statement, and are destroyed at the end of the statement. If you know you want to use the variable outside the 'if' statement, make sure your declare it before the 'if' statement.

If the condition is nil then neither the "then" nor the "else" section get executed, but the script will proceed with no error messages.
Example code:

systemChat "script started";    // will get executed
if (nil) then
{
    systemChat "true";            // will never get executed
}
else
{
    systemChat "false";            // will never get executed
};
systemChat "script ended";        // will get executed

If you only need to choose between 2 raw values, it is possible to use the following trick to avoid using code blocks, as required by the if command, which results in greater atomicity and faster execution:

_result = [falseValue, trueValue] select condition;

The select command treats "false" as 0 and "true" as 1, therefore you can feed it a condition determining the array index of the value to be returned. Here is another example:

_result = [1,-1] select (_this < 0); // if _this is less than 0, _result will be equal to -1, otherwise it will be 1

When using if to assign values to a variable, if you not have an else block it is treated as nil and returned, setting the variable value to nil. A3 2.08.149102

private _myVar = "value";
private _condition = false;
_myVar = if (_condition) then { "another value" }; //perfectly valid sqf
systemChat str (isNil "_myVar"); //returns true
//_myVar is now nil
_myVar = if (_condition) then { "another value" } else { "value" };
systemChat _myVar; //returns "value"

This behavior can be both desirable and undesirable depending on your needs