Void

A variable of type Void is an undefined variable.

Undefining Variables

nil can be used to undefine variables.

myVar = 1;
// ...
myVar = nil;
// myVar is undefined

String Representation

The String representation of an undefined variable depends on its inferred type:

Comparisons

No comparison with any variables will work.

Example:

// error
if (undefinedVar == definedVar) then { /* ... */ };

// error
if (undefinedVar == undefinedVar) then { /* ... */ };

Detect an Undefined Variable

The isNil command, introduced in Armed Assault, allows to detect if a variable exists or not:

HelloThere = 42;
isNil "HelloThere"; // false

HelloThere = nil;
isNil "HelloThere"; // true

isNil Workaround 1 (Operation Flashpoint v1.00)

SQS Syntax:

_nil = format ["%1", _undefinedVariable]
? (format ["%1", variableToTest] == _nil) : hint "variableToTest is nil"

SQF Syntax:

_nil = format ["%1", _undefinedVariable];
if (format ["%1", variableToTest] == _nil) then { hint "variableToTest is nil" };

isNil Workaround 2 (Operation Flashpoint v1.00)

Caution

This information is obsolete. Armed Assault introduced isNil, and Arma 2 forbade undefined variables usage in script - see ArmA: Editing#Forward Compatibility.

The alternative way to check if a variable is undefined before isNil existence is that it silently fails any expression evaluations on it:

defined = false;
if (var == var) then
{
    defined = true;
};
if (var != var) then
{
    defined = true;
};
if (!defined) then
{
    // we know var is undefined here
};

Type Inference

Suppose the variable a is undefined, and we set the variable b like so:

b = a + [];

The inferred type of b is now array - since b was the product result of an array add operation with a definite array ([]) and an unknown type (a)

b's string representation will now be "array", and b wil be considered an undefined array.