if variable contains [duplicate]

You might want indexOf

if (code.indexOf("ST1") >= 0) { ... }
else if (code.indexOf("ST2") >= 0) { ... }

It checks if contains is anywhere in the string variable code. This requires code to be a string. If you want this solution to be case-insensitive you have to change the case to all the same with either String.toLowerCase() or String.toUpperCase().

You could also work with a switch statement like

switch (true) {
    case (code.indexOf('ST1') >= 0):
        document.write('code contains "ST1"');
        break;
    case (code.indexOf('ST2') >= 0):
        document.write('code contains "ST2"');        
        break;        
    case (code.indexOf('ST3') >= 0):
        document.write('code contains "ST3"');
        break;        
    }​