OL3 add backup url - backup

Question

I'm trying to add a backup route for a tiles with ol3. I would like to test on the errorload event if the source url starting by "http".
If "yes" : replace this tile by a custom tile.
If "no" : change the source url of this tile by another one and retry
I think i need to use something like that :
layerTile.getSource().setUrl('file:///local/{z}/{x}/{y}.jpg');
var errorTilePath='https://image.noelshack.com/fichiers/2017/14/1491403614-errortile.png';
var serverBackup='http://otile1.mqcdn.com/tiles/1.0.0/map/';
layerTile.getSource().setTileLoadFunction((function() {
var tileLoadFn = layerTile.getSource().getTileLoadFunction();
return function(tile, src) {
var image = tile.getImage();
image.onload = function() {console.log('Tile ok : ' + src); };
image.onerror = function() {
console.log('Tile error : ' + src);
console.log(tile);
if (src.substr(0,4)!='http') {
var tmp=src.split('/').reverse();
var serverBackupPath=serverBackup+tmp[2]+'/'+tmp[1]+'/'+tmp[0].split('.')[0]+'.png';
console.log("Second url : " + serverBackupPath)
src=serverBackupPath;
tile.getImage().src=src;
var image = tile.getImage();
image.onload = function() {console.log('Tile backup ok : ' + src);};
image.onerror = function() {console.log('Tile backup error : ' + src); src=errorTilePath; tile.getImage().src=src; tileLoadFn(tile, src);}
} else {
console.log('Custom tile : ');
src=errorTilePath;
tile.getImage().src=src;
}
tileLoadFn(tile, src);
};
tileLoadFn(tile, src);
};
})());
With that, I can see that the backup tile is downloaded but not visible on map.
Certainely, I misunderstood something.
Thanks in advance if somebody could help me.

Answer 1

Oups, without code my answer is null.
layerTile.getSource().setUrl('file:///local/{z}/{x}/{y}.jpg');
var serverBackup='https://{a-c}.tile.openstreetmap.org/';
var errorTilePath=urlBase+'css/images/error.png';
layerTile.getSource().setTileLoadFunction((function() {
return function(tile, src) {
if (UrlExists(src)) {
tile.getImage().src=src;
} else {
if (src.substr(0,4)=='file') {
var tmp=src.split('/').reverse();
src='https://'+['a', 'b', 'c'].sort(function() {return 0.5 - Math.random()})[0]+'.tile.openstreetmap.org/'+tmp[2]+'/'+tmp[1]+'/'+tmp[0].split('.')[0]+'.png';
if (UrlExists(src)) {
tile.getImage().src=src;
} else {
tile.getImage().src=errorTilePath;
}
} else {
tile.getImage().src=errorTilePath;
}
}
};
})());
function UrlExists(url){
try {
var http = new XMLHttpRequest();
http.open('HEAD', url, false);
http.send();
return http.status==200||http.status==403;
} catch(err){return false;}
}