Business rule code strange behavior

Good morning , we're building some forms and' we'd like to personalize the behavior with coded rules.

Unfortunately i'm facing a strange kind of problem writing a simple rule .

Within a new form with only two text fields and a simple Business rule code like this one :

if (myFirstTextField.value.length > 0 ) {

mySecondTextField.value = 'TaDa !, the value has been changed';

}

the rule correctly fires when myFirstTextField is edited but we got this RULE_ERROR :

Java arrays have no public instance fields or methods named "value"

Could you please tell me what i'm doing wrong ?

Thank you in advance.