Doing an assignment in the condition part of a 'if', 'while' or 'for' statement is highly not recommended for the two following reasons :

• The assignment is perhaps a bug as the '=' operator might have been used in place of the '==' operator
• If this is not a bug, this is highly error prone as another developer might read '==' instead of '=' while trying to maintain the source code

if (dayNumber = getClickedDayNumber(dayInfo)) { // Non-Compliant
  alert("day number found : " + dayNumber);
}

If you really want to do the assignment in place, then better to do an explicit comparison as well :

if ((value = someFunction()) === true) { // Compliant
  // ...
}