getClosestResolutionIdx, Find the closest zoom level based on half of the difference of 2 resolution levels

gistfile1.js

        /**
         * Given a resolutionArray
         * resolutionArr[0] = 100
         *              [1] = 50
         *              [2] = 25
         *              [3] = 12.5
         *
         * getClosestResolutionIdx(resolutionArray, 51) === 1 // round down to 50
         * getClosestResolutionIdx(resolutionArray, 76) === 0 // round up to 100
         * getClosestResolutionIdx(resolutionArray, 10) === 3 // round up to 3
         *
         * @param resolutionArr
         * @param resolution
         * @return {Number}
         */
        getClosestResolutionIdx: function (resolutionArr, resolution) {
            if(!resolution) {
                // if resolution is not defined, return zoom level 0
                return 0;
            }
            for(var idx=0; idx<resolutionArr.length; idx++) {
                if( resolutionArr[idx] <= resolution ) {

                    var diff = resolution - resolutionArr[idx];
                    if( idx > 0 ) {
                        var halfResolutionDiff = (resolutionArr[idx-1] - resolutionArr[idx]) / 2;

                        // round to nearest zoomLevel integer
                        // ie if res1 = 10, res2 = 20
                        //    and prevRes = 14
                        // then round UP to 10 (nearest integer)
                        if( diff <= halfResolutionDiff ) {
                            // closer to resolutionArr[idx]
                            return idx;
                        }
                        else {
                            return idx-1;
                        }
                    }
                    return idx;
                }
                else {
                    if( (idx+1) === resolutionArr.length ) {
                        // default to max zoom level(idx) if we can't find something more
                        // ie previous module has much smaller scale/resolution than that of current one
                        // so default to smallest scale/resolution this module supports
                        return idx;
                    }
                }
            }
        },