The generated JS is invalid here, somewhat unexpectedly (I suppose parentheses are required):

(defun foo (obj)

  (create :abc (let ((x (gethash "blah" obj)))

                 (if x 123 456))))

function foo(obj) {

    var x;

    return { 'abc' : x = obj['blah'], x ? 123 : 456 };

};