function defaults(target, source, overlay) {
	        for (var key in source) {
	            if (source.hasOwnProperty(key)
	                && (overlay ? source[key] != null : target[key] == null)
	            ) {
	                target[key] = source[key];
	            }
	        }
	        return target;
	    }
@param {*} target
@param {*} source
@param {boolen} [overlay=false]
@memberOf module:zrender/core/util