Scripter Javascript Tutorial

37 Variable Scope

MDN: Grammar and Types: Variable Scope

An important concept to understand about functions is scope. Whenever a variable a declared within a function, then that variable is local to the function. This means the variable cannot be called from outside the function.

function add( operand1, operand2 ) {
	var sum = operand1 + operand2;
	return sum;
}
Trace(operand1);

In the above example, the Trace() function tries to use the operand1 variable, which is declared as one of the function’s arguments. Because the variable is within the function but is called from outside the function, Scripter will display the following error in the console:

[JS Exception] ReferenceError: Can't find variable: operand1 line:5

The variable operand1 simply does not exist outside of the add function. However, variables declared outside of the function can be used from within the function. The following is a contrived, almost useless example, but the concept will be important to grasp when discussing Scripter’s own variables which can be used from anywhere in a script.

function add() {
	var result = opr1 + opr2;
	return result;
}

var opr1 = 2;
var opr2 = 3;
var sum = add(); // sum = 5

In the above example:

• Line 1: The add function is declared but without any arguments.

• Line 2: The result is determined by the two variables which are declared on lines 6–-7;

• Line 3: The result is returned.

• Line 8: The add function is called without any arguments. Note the parentheses are still needed to actually invoke the function.