feat: enable the ability to overwrite the refer origin

Author: yuhao900914

src/amplitude-client.js

@@ -789,7 +789,10 @@ var _sendParamsReferrerUserProperties = function _sendParamsReferrerUserProperti
  * @private
  */
 AmplitudeClient.prototype._getReferrer = function _getReferrer() {
-  return typeof document !== 'undefined' ? document.referrer : '';
+  const urlRefer = this._getReferrerFromUrlParam(this._getUrlParams());
+
+  if (urlRefer) return urlRefer;
+  else typeof document !== 'undefined' ? document.referrer : '';
 };
 
 /**
@@ -835,6 +838,14 @@ AmplitudeClient.prototype._getDeviceIdFromUrlParam = function _getDeviceIdFromUr
   return utils.getQueryParam(Constants.AMP_DEVICE_ID_PARAM, urlParams);
 };
 
+/**
+ * Try to fetch referrer from url params.
+ * @private
+ */
+AmplitudeClient.prototype._getReferrerFromUrlParam = function _getReferrerFromUrlParam(urlParams) {
+  return utils.getQueryParam(Constants.AMP_REFERRER_PARAM, urlParams);
+};
+
 /**
  * Parse the domain from referrer info
  * @private

src/constants.js

@@ -50,6 +50,7 @@ export default {
   REVENUE_REVENUE_TYPE: '$revenueType',
 
   AMP_DEVICE_ID_PARAM: 'amp_device_id', // url param
+  AMP_REFERRER_PARAM: 'amp_referrer', // url param for overwriting the document.refer
 
   REFERRER: 'referrer',