var resolutions = [];
for(var i=0; i<= 18; i++) {
    resolutions[i] = Math.pow(2, 18 - i);
}

var tileGrid = new ol.tilegrid.TileGrid({
    origin:[0, 0],
    resolutions: resolutions
});

var baiduSource = new ol.source.TileImage({
    projection: "EPSG:3857",
    tileGrid: tileGrid,
    tileUrlFunction
 
: function(tileCoord, pixelRatio, proj) {
        if(!tileCoord){
            return "";
        }
        var [z, x, y] = tileCoord;

        if(x < 0) {
            x = `M${-x}`;
        }
        if(y < 0) {
            y = `M${-y}`;
        }
        var num = Math.floor(Math.random() * 5);
        return 
 
`http://online${num}.map.bdimg.com/tile/?qt=tile&x=${x}&y=${y}&z=${z}&styles=pl&scaler=2&udt=20180303`;
    }
});

var baiduLayer = new ol.layer.Tile({
    source: baiduSource
});

var map = new ol.Map({
    target:"map",
    layers:[
        baiduLayer
],
    view: new ol.View({
        center
 
: ol.proj.fromLonLat([121.48, 31.22]),
        zoom: 8
})
});