Using jQuery UI .position() in a backwards compatible way

Interesting question,

have you tried passing +0 as an offset, that should work and make your code a little easier to read.

/** Prefix number with '+'/'-' or return empty string for 0. */
function offsetString(n){
    return '' + ( n < 0 ? n : '+' + n );
}

As for the isVersionAtLeast($.ui.version, 1, 9) I would only check this once.

function generateCompatibleOffsetFunction(){

  if( isVersionAtLeast($.ui.version, 1, 9) ){
    return function setOffset( options , offsetX , offsetY ){
      options.my = "left" + offsetString(offsetX ) + " top" + offsetString(offsetY );
    }
  } else {
    return function setOffset( options , offsetX , offsetY ){
      options.my = "left top";
      options.offset = "" + offsetX + " " + offsetY;
    }
  }
}

var setOffset = generateCompatibleOffsetFunction();

/** Prefix number with '+'/'-' or return empty string for 0. */
function offsetString(n){
    return '' + ( n < 0 ? n : '+' + n );
}

var posOpts = {
    at: "left top",
    of: $target
};

setOffset( posOpts, markerOffsetX, markerOffsetY );

$dropMarker.position(posOpts);

I came up with this approach (barely tested): the idea is to use the new syntax, but convert to the old syntax when a legacy jQuery UI is included (i.e. version <= 1.8).

This could be used like so:

$("#source").position(fixPositionOptions({
    my: "left+30 center",
    at: "center bottom+30",
    of: $("#target")
}));

(See also this fiddle: http://jsfiddle.net/mar10/6xtu9a4e/)

var uiVersion = $.ui.version.match(/^(\d)\.(\d+)/);

/** Make jQuery.position() arguments backwards compatible,i.e. if 
 * jQuery UI version <= 1.8, convert 
 *   { my: "left+3 center", at: "left bottom", of: $target }
 * to
 *   { my: "left center", at: "left bottom", of: $target, offset: "3  0" }
 *
 * See http://jqueryui.com/upgrade-guide/1.9/#deprecated-offset-option-merged-into-my-and-at
 * and http://jsfiddle.net/mar10/6xtu9a4e/
 */
function fixPositionOptions(opts) {
    if( opts.offset || ("" + opts.my + opts.at ).indexOf("%") >= 0 ) {
        $.error("expected new position syntax (but '%' is not supported)");
    }
    if( uiVersion[1] < 2 && uiVersion[2] <= 8 ) {
        var // parse 'left+3 center' into ['left+3 center', 'left', '+3', 'center', undefined]
            myParts = /(\w+)([+-]?\d+)?\s+(\w+)([+-]?\d+)?/.exec(opts.my),
            atParts = /(\w+)([+-]?\d+)?\s+(\w+)([+-]?\d+)?/.exec(opts.at),
            // convert to numbers
            dx = (myParts[2] ? (+myParts[2]) : 0) + (atParts[2] ? (+atParts[2]) : 0),
            dy = (myParts[4] ? (+myParts[4]) : 0) + (atParts[4] ? (+atParts[4]) : 0);

        opts = $.extend({}, opts, { // make a copy and overwrite
            my: myParts[1] + " " + myParts[3],
            at: atParts[1] + " " + atParts[3]
        });
        if( dx || dy ) {
            opts.offset = "" + dx + " " + dy;
        }
    }
    return opts;
}