if/else extracting part of a UK postcode

I'm trying to extract the first part of a UK postcode using an if/else from the full postcode in javascript

Basically, if the postcode is in the format;

AB99 9CD it needs to return AB

A9 9BC it needs to return A

A99 9BC it needs to return A

AB9C 9DE it needs to return AB.

I thought about trying

if (fieldname == LETTER1 LETTER2 NUM NUM NUM LETTER3 LETTER 4) 
    return LETTER 1 LETTER 2;

and so on, then using the else as the error message, so that it's recognising the format & returning the first one or two letters but haven't got a clue what code to use.