JavaScript on input field

export function IO_field_4_OnKeyDown(item, keyCode, modifiers) {

var value = parseInt(item.ProcessValue);

var HL = 5.0;

var LL = 0.0;

var operator = Tags("@UserName").Read();

if (keyCode == 13) {

if (operator == 'user') {

if (value > HL) {

value = HL;

} else if (value < LL) {

value = LL;

} else {

value = value;

}

} else {

value = value;

}

}

Tags("DB_HMI_Parameters_Temperature_RoC").Write(value);

}

 I want to first save operator name from system Tag "@UserName" to my variable operator, then when he presses Enter button after change the value i input field (keyCode == 13) I run the rest of the script.
Then I check is my variable operator is equal to string 'user', and if it is then I limit his input between 0 and 5, but if it is not than there is not limit.
I tested this script but it is not working, even when I am logged in as 'user' it does not limit anything.

Any idea what I am doing wrong here?