Openlayer 3: geoJSON clustering failed

Ive been trying to use the basic example from openlayer website to create a simple clustering map for my geoJSON data. However, Im not getting any luck to generate any clusters. Below is my code, many thanks for help:

var baseLayer = new ol.layer.Tile({
    name: 'MapQuest',
    source: new ol.source.BingMaps({
        key: 'An2UbYREMJPy-Cbi2z8Z52eZWO7LryKQjdG9oRKiZGU984drPE5rPYgCeR_mwLGo',
        imagerySet: 'AerialWithLabels'
    })
});

var view = new ol.View ({
    center: [19425838, -4409677], // projection in EPSG: 3857
    zoom: 6,
    maxZoom: 28,
    minZoom: 1
    });
var source = new ol.source.GeoJSON({object: geojson_tdgjobs}); // local .js file passing geojson feature into a variable
var clustersSource = new ol.source.Cluster({
        distance: 10,
        source: source
    });
var styleCache = {};
var clusters = new ol.layer.Vector({
    source: clustersSource,
    style: function(feature, resolution) {
        var size = feature.get('features').length;
        var style = styleCache[size];
        if (true) {
            style = [
                new ol.style.Style({
                    image: new ol.style.Circle({
                        radius: resolution,
                        stroke: new ol.style.Stroke({
                            color: "#fff"
                        }),
                        fill: new ol.style.Fill({
                            color: '#3399CC'
                        })
                    }),
                    text: new ol.style.Text({
                        text: size.toString(),
                        fill: new ol.style.Fill({
                            color: '#fff'
                        })
                    })
                })];
            styleCache[size] = style;
        }
        return style;
    }
});
var layers = [baseLayer, clusters];
var map = new ol.Map({
        controls: ol.control.defaults().extend([new ol.control.ScaleLine({})]),
        target: 'map',
        renderer: 'canvas',
        view: view,
        layers: layers
    });