Business Rules have several quirky behaviors that will be addressed in a future release. Please read and understand these points before writing your first business rule. 

Why do I need a local variable for certain cases ?

There are rules scenarios where you can't use a  control directly in a rule expression and you are forced to define a local variable, perform the computation using the local variable and assign the value to the control at the conclusion of the computation.

For instance, assume a form that calculates a total from a list of subtotals. The total is a control T and subtotals are represented as a repeat of controls S. In other words, T holds the total of adding all values in S. You could write the rules as:

T.value = 0; 
for (var i = 0; i < S.value.length; i++) {   
    T.value = T.value + S[i].value; 
} 

The code above makes logical sense but it just won't work. In a nutshell this particularity of the  implementation can be resumed this way:

The right hand side of an assignment expression resolves to the value of a  control passed to a rule when it is invoked. The left hand side uses a reference to the control'.

Lets work with a concrete example to understand the concept. Assume that there are two subtotals in the form, 12 and 14 and the total is correctly set to 26. Now, you add 50 as a third subtotal and the rule fires as a consequence. The values of the controls involved in the rule are passed as parameters to the rule: the subtotals 12,14 and 50 and the current total (value of T), 26. The core of the matter is in the expression:

T.value = T.value + S[i].value; 

The expression will be evaluated three times (since there are three subtotals) but for every iteration, the T.value operand on the right hand side will always evaluate to 26 which is not the desired behavior and will lead to an incorrect result. Again, this happens because the right hand side of that expression is resolved based on the values initially passed to the rule as parameters.

 explicitly imposes this restriction for the sake of efficiency and there would be a significant impact in performance if the restriction was not in place. Lets rewrite the rule using a local variable:

var tot = 0; 
for (var i = 0; i < S.value.length; i++) {   
    tot = tot + S[i].value; 
} 
T.value = tot; 

This rule will produce the expected result.

Why does my rule not execute?

A rule will not execute when circular logic is detected. Currently circular dependencies are not permitted. This example tries to set the value of the same control used to trigger the rules execution. This is not permitted and this rule will not execute. This restriction may be lifted in a future release.

if (color.value === 'red') {   
    color.value = 'blue'; 
} 

The rule will simply not be executed and you will find these errors in the frevvo.log file.

21:41:42.812 |-INFO [RuleProcessor] - [set color blue] rule dependency 'color.value' because the same control property is also being assigned: 'color.value = 'blue';'
21:41:42.812 |-WARN [RuleProcessor] - [set color blue] This rule will never be triggered in this form because no valid dependencies were found!

Why does a commented out code line still execute?

A rule with a code line inside a comment block may still execute. For example this rule uses two different Javascript comment styles. The commented out lines in both cases will execute even though you would expect them to be ignored. Do note use comments for this purpose.

if (form.load)
{
  PO.value = 'A-234';
  // FN.value = 'Nancy';
  /* LN.value = 'Doe'; */
}