Javascript对象文字范围问题？

Question

I'm having a hard time understanding why this.$map and this.markers are undefined. Here's the code I'm working with and I have added comments where I expect these variables to supply a value:

(function($) {
    'use strict';

    var A = {

        /**
         * Initialize the A object
         */
        init: function() {
            this.$map = this.renderMap();
            this.markers = this.getMarkers();

            this.renderMarkers();
        },

        renderMap: function() {

            var url = 'http://a.tiles.mapbox.com/v3/delewis.map-i3eukewg.jsonp';

            // Get metadata about the map from MapBox
            wax.tilejson(url, function(tilejson) {
                var map = new L.Map('map', {zoomControl: false});

                var ma = new L.LatLng(42.2625, -71.8028);

                map.setView(ma, 8);

                // Add MapBox Streets as a base layer
                map.addLayer(new wax.leaf.connector(tilejson));

                return function() {
                    return map;
                };
            });
        },

        renderMarkers: function() {
            var geojsonLayer = new L.GeoJSON(null, {
                pointToLayer: function (latlng){
                    return new L.CircleMarker(latlng, {
                        radius: 8,
                        fillColor: "#ff7800",
                        color: "#000",
                        weight: 1,
                        opacity: 1,
                        fillOpacity: 0.8
                    });
                }
            });

            geojsonLayer.addGeoJSON(this.markers); // this.markers is undefined
            this.$map.addLayer(geojsonLayer); // this.$map is undefined

        },

        getMarkers: function() {
            $.getJSON("/geojson/", function (data) {
                return data;
            });
        }
    };

    /**
     * A interactions
     */
    $(document).ready(function() {
        A.init()
    });

})(jQuery.noConflict());

I have spent much of the day searching and I think I'm missing something fundamental here, but I don't get it.

Answer 1

Neither the renderMap , nor getMarkers methods return any value, consequently their return value is undefined .

It looks like you are trying to initialize these fields from an ajax request, which is not necessarily a good idea.

What you probably ought to do is something like:

getMarkers: function(callback){
    var result = this;
    $.getJSON("url", function(jsonData){ 
        result.markers = jsonData; 
        if(callback) callback()
     });
},

which will lazily initialize the fields of the object as they become available.

NB : AJAX is asynchronous you cannot rely on this callback setting the member quickly, or indeed ever (it could fail). This suggests you need to think a bit more about your design, and try to use callbacks more.

eg modify the getMarkers and renderMap functions as above to take a callback that is called after the data is stored then change init to:

init: function(){
    var res = this;
    var post_init_callback = function(){
        if(res.$map != undefined && res.markers!=undefined){
            //this will only run after both ajax calls are complete
            res.renderMarkers();  
        }
    };
    this.getMarkers(post_init_callback);
    this.renderMap(post_init_callback);
},

Answer 2

The problem here is that you call return inside another function. What you're essentially doing is defining getMarkers (the simplest example) as this:

getMarkers: function() {
    $.getJSON('/geojson/', some_random_func);
}

At which point it becomes obious that getMarkers doesn't actually return anything (ergo undefined). The same goes for your renderMap function. Also in this case your "some_random_func" is defined as function(data) { return data; } function(data) { return data; } but what does it return it to? Truth is that the some_random_func is called by jQuery itself, and AFAIK jQuery doesn't care at all for the return-value of it's success-function.