Syntax errors and warnings are shown in the code editor by a wavy underline marking the location at which the error was detected. Note that the line appears only at the point at which the syntax becomes unambiguously incorrect. The actual position of the error may sometimes lie at a point in the code some way preceding the indicator. You may hover the mouse over a wavy line error indicator in order to view an explanatory message in a tooltip.
By default, a blue wavy line indicates a warning:
A red wavy line indicates a syntax error: