How to avoid “Synchronous XMLHttpRequest on the main thread” warning in UI5?

I’m staring my UI5 with the following bootstrapping in index.js:

], (Core, Shell, ComponentContainer) => {
    "use strict";

    new Shell("", {
        app: new ComponentContainer("", {
            height: "100%",
            name: "webapp"
        appWidthLimited: false

According to the UI5 documentation:

Static dependencies are loaded in the dependency declaration array of the sap.ui.define call. These dependencies are always loaded in advance before executing the defined module.

Do I understand it correctly that in such case the modules will be always loaded in a synchronous way and there is no real way to avoid the «Synchronous XMLHttpRequest on the main thread» warning?

Can I, perhaps, just wrap new sap.m.Shell(sId?, mSettings?) with async/await?

Update #1:

I’ve checked the loading with ?sap-ui-xx-nosync=warn and got the following results:

enter image description here

For some reason i18n/ is loaded synchronously. The only place where I’m accessing the i18n is:

const oBundle = myI18nModel.getResourceBundle();

But following the documentation, I could not grasp why myI18nModel.getResourceBundle() leads to the synchronous loading.

Update #2:

After a deep exploration of no sync XHR sample, I found out the reason for the sync XHR warning. That was "description": "{{appDescription}}" and "title": "{{appTitle}}" in manifest.json which is explicitly noted:

"title": "Here the i18n bundles can't be loaded with 'async: true' yet!",
"description": "Same here. Hence, no 'i18n' in this '' section!"

After replacing it with a static values the warning is gone.


Early development of UI5 was heavily based on synchronous XHRs. Legacy applications or some outdated documentation topics might still refer to APIs that have either the asynchronous option disabled by default or no async option at all such as: sap.ui.controller, sap.ui.component, sap.ui.*fragment, sap.ui.*view,, sap.ui.requireSync,, and the constructor function of the ComponentContainer as mentioned in the question.

In order to reduce the number of sync XHR calls:

  1. Follow the documented guidelines:

    Particularly when bootstrapping, use the data-sap-ui-async="true" option, and, if the app has a Component.js, use the declarative sap/ui/core/ComponentSupport module in data-sap-ui-oninit instead of instantiating sap.ui.core.ComponentContainer manually. For example:

      &lt!-- ... -->
      <script id="sap-ui-bootstrap" ...
    <body id="content" class="sapUiBody">
      <div data-sap-ui-component
        data-settings='{ "id": "rootComponent" }'

    This automatically creates a ComponentContainer, pushes it to the DOM, and loads the Component.js and manifest.json asynchronously altogether while avoiding inline scripts or a separate bootstrap script at the same time.

  1. Run the app with the xx-nosync bootstrap option. For example, in the URL:


    UI5 will then log “loading … with sync XHR” in the browser console for each of the files fetched synchronously (Ignore other [nosync] messages). For example:

    • If the app uses a v2.ODataModel, it might complain that the module sap/ui/thirdparty/datajs was loaded synchronously. In that case, add the following to the bootstrap config:
      <script id="sap-ui-bootstrap" ...
    • If a ResourceModel is created manually in JS, make sure to enable the async flag there:
      const i18nModel = new ResourceModel({ // required by "sap/ui/model/resource/ResourceModel"
        bundleName: "demo.i18n.i18n",
        supportedLocales: [""],
        fallbackLocale: "",
        async: true, // <--
      i18nModel.getResourceBundle().then(resourceBundle => {
        this.getOwnerComponent().setModel(i18nModel, "i18n");
        resourceBundle.getText(/*...*/); // See sap/base/i18n/ResourceBundle

    Check the API reference of the reported APIs. Most of the times, they’re either deprecated or they come with an async flag set to false by default.

There are still some APIs left which have no asynchronous replacements yet such as:

  • The CLDR file being fetched with sync XHR when instantiating sap/ui/core/LocaleData (Used in e.g. Calendar or DatePicker. See Issue #2345).
  • When resolving i18n Handlebar-like "{{...}}"-syntax in manifest.json

With the latest stable version of UI5, however, most of the synchronous requests can be eliminated.
Here is a sample that has no sync XHR:

To answer the question: sap.ui.define and sap.ui.require do load dependencies asynchronously if the data-sap-ui-async config is set to true (Replaces data-sap-ui-preload="async"). Also make sure to add dependent libs to the manifest.json in order to prevent a major performance bottleneck.

Wrapping a synchronous content request with async-await/Promise won’t unfortunately help. Better look for asynchronous alternatives. For example, the ComponentContainer has an async flag in the constructor settings which, if enabled, fetches the Component.js and manifest.json files asynchronously:

new ComponentContainer({
  height: "100%",
  name: "webapp",
  async: true, // or
  manifest: true, // sets the `async` option automatically to true

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