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.


  • Cause

    Historically, UI5 1.x had been offering APIs based on synchronous XHRs. Legacy applications or outdated documentation topics might still refer to:

    • APIs that have the asynchronous option disabled by default such as the sap.ui.core.ComponentContainer, sap.ui.core.Core#loadLibrary, etc..
    • APIs that have no async option at all such as sap.ui.controller, sap.ui.component, sap.ui.*fragment, sap.ui.*view,*, sap.ui.requireSync, oCore.createComponent, sap.ushell.Container.getService, etc.. Replacement for them are documented in the API reference.


    In order to get rid of sync XHRs in UI5:

    1. Do not use deprecated APIs such as those mentioned above.

    2. 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:

        <!-- ... -->
        <script id="sap-ui-bootstrap" src="..."
          data-sap-ui-resourceroots='{ "my.demo": "./" }'
      <body id="content" class="sapUiBody">
        <div data-sap-ui-component
          data-settings='{ "id": "myRootComponent" }'

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

      In the application's Component.js definition, additionally, implement the "sap.ui.core.IAsyncContentCreation" marker interface to allow creating the component's content fully asynchronously (in addition to stricter error handling).

      // In Component.js
      return UIComponent.extend("my.demo.Component", {
        metadata: {
          interfaces: [ "sap.ui.core.IAsyncContentCreation" ],
          manifest: "json",
        // ...
    1. For UI5 1.x: run the app with the xx-nosync bootstrap option. For example, in the URL:


      UI5 will then log "loading … with sync XHR" or "[nosync] loading module '…'" in the browser console for each file fetched synchronously (Ignore other [nosync] messages). For example:

      • Standalone apps using sap.ui.model.odata.v2.ODataModel with older UI5 versions might require sap/ui/thirdparty/datajs synchronously (C.f. issue #3134). In that case, add the module to the sap-ui-modules bootstrap config:

        <script id="sap-ui-bootstrap" src="..."

        In an FLP scenario, the datajs module should be already preloaded by the launchpad.

      • If a ResourceModel is created manually in JS, make sure to enable the async flag there:

        const i18nModel = new ResourceModel({ // sap/ui/model/resource/ResourceModel
          bundleName: "demo.i18n.i18n",
          supportedLocales: [""],
          fallbackLocale: "",
          async: true, // <--
        const resourceBundle = await i18nModel.getResourceBundle();
        this.getOwnerComponent().setModel(i18nModel, "i18n");
        resourceBundle.getText("...", [/*...*/]); // 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.

    2. When requesting UI5 theming parameters via sap.ui.core.theming.Parameters.get, use the asynchronous variant as mentioned in the API description. If there are standard UI5 controls requesting theming parameters too early - causing a sync XHR - create an issue in OpenUI5 GitHub.

    Known issues

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

    • OpenUI5 issue #2345: CLDR being fetched with sync XHR when instantiating sap/ui/core/LocaleData (Used in e.g. Calendar or DatePicker).
    • OpenUI5 issue #3331: i18n Handlebar-like "{{...}}"-syntax in manifest.json resulting in sync XHRs. (Should not be an issue if the *.properties files are included in the generated application bundle Component-preload.js). → ✅ Fixed with ad5cf65.
    • OpenUI5 issue #3134: v2.ODataModel requiring datajs.js synchronously if created with preload: true in manifest.json. → ✅ Fixed with 337b176.

    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 which can replace 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 or Promise won't 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({ // required from "sap/ui/core/ComponentContainer"
      height: "100%",
      name: "my.demo",
      async: true, // or
      manifest: true, // sets the `async` option automatically to true
      // ...