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0.7.0 • Public • Published

This plug-in is still experimental. Feel free to provide feedback and even contribute.

Call for Feedback

A poll about the use of single-spa's unloadApplication() function has been opened. It will help the development of this plug-in if you could visit this poll and voted.

Please vote even if you don't know about this function (it is a possible vote option).



  • FEATURE: FOUC prevention. CSS mounting now awaits the load event of CSS LINK elements before mounting the micro-frontends.


  • Fix: Dynamic CSS links from other micro-frontends were being disabled by unloading a micro-frontend using the singleMife or multiMife CSS strategy if said links were loaded after the micro-frontend.
  • Fix: Dynamic CSS links would not be tracked under the multiMife CSS strategy after unloading all parcels of a project and then loading another.


Vite plug-in to convert Vite-based projects to single-spa root or micro-frontend applications.

This Vite plug-in is an opinionated way of making Vite-based front-end projects work with single-spa.


NOTE: This document assumes the use of TypeScript Vite projects. However, all of this applies to JavaScript projects as well. Just know that any file name with extension .ts probably exists as a .js file.

Install the NPM package as a development dependency.

npm i -D vite-plugin-single-spa

In reality, what is installed as development dependency is a matter of how you build your final product. For example, if you want to use npm with --omit=dev to build the project, then you'll need all packages used by Vite's build command to have been installed as regular dependencies. It is up to you how you end up installing the package (dev or regular).

Now, in vite.config.ts, import the vitePluginSingleSpa function from it. It is the default export but it is also a named export:

// Either works.
import vitePluginSingleSpa from 'vite-plugin-single-spa';
import { vitePluginSingleSpa } from 'vite-plugin-single-spa';

This is the plug-in factory function. Create the plug-in by invoking the function. Pass the plug-in options as argument to the function call. The following is the content of vite.config.ts of a Vite + Svelte project created with npm create vite@latest.

import { svelte } from '@sveltejs/vite-plugin-svelte';
import { defineConfig } from 'vite';
import vitePluginSingleSpa from 'vite-plugin-single-spa';

export default defineConfig({
    plugins: [svelte(), vitePluginSingleSpa({ /* options go here */})],

The options passed to the plug-in factory function determine the type of project (root or micro-frontend). For micro-frontend projects, the server port is required, while for root projects the type is required.

Additionally for micro-frontend projects, the file src/spa.ts/js/jsx/tsx must be created. This file becomes the main export of the project and should export the single-spa lifecycle functions.

single-spa Root Projects

The single-spa root project (referred to as root config within the single-spa documentation) is the project that loads all other micro-frontends and the one that has the single-spa package installed, and the one that typically calls registerApplication() and start(). The single-spa developers advertise as a best practice to make a root project that uses no framework. In other words, that your root project be devoided of all user interface elements. This is a view I don't share, and this plug-in is capable of making a suitable Vite + XXX root project. In the end, the choice is yours.

Root Project Options

The plug-in options available to root projects are dictated by the following TypeScript type:

export type SingleSpaRootPluginOptions = {
    type: 'root';
    importMaps?: {
        type?: 'importmap' | 'overridable-importmap' | 'systemjs-importmap' | 'importmap-shim';
        dev?: string | string[];
        build?: string | string[];
    imo?: boolean | string | (() => string);
    imoUi?: boolean | 'full' | 'popup' | 'list' | {
        variant?: boolean | 'full' | 'popup' | 'list';
        buttonPos?: 'bottom-right' | 'bottom-left' | 'top-right' | 'top-left';
        localStorageKey?: string;

The type property is mandatory for root projects and must be the string 'root', as seen. The other options pertain to the use of import maps and the import-map-overrides package. Long story short: Vite, while in development mode, inserts the script @vite/client as first child in the HTML page's <head> element, and this makes the import maps non-functional, at least for the native importmap type. The solution: Make this plug-in add both the import map script and the import-map-overrides package as first children of the <head> HTML element. Why did I tell the story? Just so you know that it is impossible to use import-map-overrides with Vite "by hand" in serve mode.

The imo option is used to control the inclusion of import-map-overrides. Set it to false to exclude it; set to true to include its latest version from JSDelivr. However, production deployments should never let unknown versions of packages to be loaded without prior testing, so it really isn't good practice to just say "include the latest version". Instead, specify the desired package version as a string. The current recommended version of import-map-overrides is v3.1.1 (but always check for yourself).

    type: 'root',
    imo: '3.1.1'

Just like the case of the latest version, this will also use the JSDelivr network.

IMPORTANT: Even if you request import-map-overrides to be included, it won't be included if no import maps are present.

If you wish to change the source of the package from JSDelivr to something else, then provide a function that returns the package's URL.

    type: 'root',
    imo: () => ``

The imoUi property controls the inclusion of the import-map-overrides user interface. The property's default value is true, which is equivalent to full. The values full, popup and list refer to the type of user interface. Refer to the package's documentation in case this is confusing.

By default, the user interface will be configured to appear in the bottom right corner and become visible on the presence of the imo-ui local storage variable. If any of this is inconvenient, specify the value of imoUi as an object with the variant, buttonPos and localStorageKey properties set to your liking.

We finally reach the importMaps section of the options. Use this section to specify the import map type and file names. The default behavior is to automatically import maps from the file src/ whenever Vite runs in serve mode (when you run the project with npm run dev), or the file src/importMap.json whenever vite runs in build mode (when you run npm run build). Note, however, that if you have no need to have different import maps, then you can omit src/ and just create src/importMap.json.

Usually, the development import map would look like this:

    "imports": {
        "@learnSspa/spa01": "http://localhost:4101/src/spa.ts",
        "@learnSspa/spa02": "http://localhost:4102/src/spa.ts"

This is because, while using npm run dev, no bundling takes place, so we directly reference the module in the /src folder.

Building the micro-frontend, on the other hand, produces a spa.js bundle at the root of the dist folder, so the building import map would look like this:

    "imports": {
        "@learnSspa/spa01": "http://localhost:4101/spa.js",
        "@learnSspa/spa02": "http://localhost:4102/spa.js"

Of course, that would be if you were building to test locally the build using npm run preview. Whenever building for deployment, it will be more like this:

    "imports": {
        "@learnSspa/spa01": "/spa01-prefix/spa.js",
        "@learnSspa/spa02": "/spa02-prefix/spa.js"

The above is a popular Kubernetes deployment option: Set the K8s ingress up so that requests that start with the micro-frontend prefix are routed to the pod that serves that micro-frontend. In the end the final look of the import map is up to you and your deployment setup.

Now, what if you want or need to specify a different file name for your import maps? No problem. Use to specify the serve-time import map file; use importMaps.buid to specify the build-time import map file.

As seen in the TypeScript definition, you can specify the type of import map you want. The four choices are the four possible options for the import-map-overrides package, and if not specified, it will default to overridable-importmap. Once again, I deviate from single-spa's recommendation of using SystemJS as the module import solution. Long story short: Native import maps, except for one bug, seem to work just fine, and I am pro minimizing package dependencies in projects. Furthermore, if you read the Vite ecosystem page, you'll see that SystemJS is recommended because, and I quote:

since browser support for Import Maps is still pending

This is no longer the case as seen in the caniuse website.

If you're confused about all this import map type thing, read all about this import map topic in the import-map-overrides website.

Using More Import Map Files

Since v0.3.0

In single-spa applications, it is common to need shared modules, and it so happens to be very practical to list them as import map entries. For example, one could have something like this:

    "imports": {
        "vue": "",
        "react": "",
        "react-dom": "",
        "@learnSspa/mifeA": "http://localhost:4101/src/spa.tsx"

Because those shared entries (vue, react, react-dom) need to be specified for both Vite modes (serve and build), the most practical thing is to have a third import map file that is used in both scenarios. To support this kind of import map construction, the properties and can also accept an array of string values to specify multiple file names. If you opt for this option, there is no "default file by omission" and you must specify all your import map files explicitly.

Create 3 import map JSON files: src/importMap.json, src/ and src/importMap.shared.json. Now specify the import map files as an array:

    type: 'root',
    importMaps: {
        dev: ['src/', 'src/importMap.shared.json'],
        build: ['src/importMap.json', 'src/importMap.shared.json'],

This is of course a mere suggestion. Feel free to arrange your import maps any way you feel is best. Two, three or 50 import map files. Create import map files until your heart is content.

single-spa Micro-Frontend Projects

A micro-frontend project in single-spa is referred to as a micro-frontend or a parcel. Micro-frontends are the ultimate goal: Pieces of user interface living as entirely separate projects.

Micro-Frontend Project Options

The plug-in options available to micro-frontend projects are dictated by the following TypeScript type:

export type SingleSpaMifePluginOptions = {
    type?: 'mife';
    serverPort: number;
    spaEntryPoints?: string | string[];
    projectId?: string;
    assetFileNames?: string;
    cssStrategy?: 'singleMife' | 'multiMife' | 'none';

The type property may be omitted, but if specified, it must be the string 'mife'.

The serverPort property is relevant to the correct configuration of single-spa and therefore, it is something this package must know, or things will become harder to configure between the micro-frontends and the root project.

Specifically, micro-frontend projects are linked to the root project by means of its import map. During development, the import map will usually point to the micro-frontend's entry file with a full URL similar to http://localhost:4444/src/spa.ts, where 4444 is the server's port number. It is very difficult to imagine a single-spa project that works with dynamic ports.

Since v0.4.0

The spaEntryPoints property has a default value of src/spa.ts and is used to specify all the files that export single-spa modules (modules that export bootstrap, mount and unmount). Most of the time, there is only one such file (the one that exports the micro-frontend), but if the micro-frontend exposes parcels, those are specified here as well. If you need to specify more than one file, use an array of strings.

The default file name is certainly handy, but it is opinionated: It is a TypeScript file (the plug-in's author's preference). If your file name differs, even if only by file extension, use spaEntryPoints to specify it.

Since v0.6.0

The assetFileNames option works as documented in Rollup's documentation, with one exception: It can only be a string. Functions may not be passed at this time. If this is something that people need, open an issue in the project's repository and request it.

projectId: Since v0.2.0; cssStrategy: Since v0.4.0

At the bottom we see projectId and cssStrategy. These are necessary for CSS mounting and unmounting. In a single-spa-enabled application, there will be (potentially) many micro-frontends coming and going in and out of the application's page. The project ID is used to name the CSS bundles during the micro-frontend building process. Then, at runtime, this identifier is used to discriminate among all the CSS link elements in the page to properly mount and unmount them. If not provided, the plug-in will use the project's name from package.json. Make sure the project has a name, or provide a value for this property.

NOTE: The projectId value will be trimmed to 20 characters if a longer value is found/specified.

The cssStrategy has to do with what you, the developer, want to do with the exported micro-frontend/parcel. Read the following section to understand this fully.

Mounting and Unmounting CSS

Since v0.4.0

IMPORTANT: CSS lifecycle logic has been completely replaced by a new algorithm as of v0.4.0. The previous cssLifecycle object is no longer available, and in its stead you can import the cssLifecycleFactory function.

This plug-in provides an extension module that provides ready-to-use single-spa lifecycle functions for the bundled CSS. It is very simple to use. The following is an example of the spa entry file src/spa.tsx of a Vite + React project created with npm create vite@latest:

import React from 'react';
import ReactDOMClient from 'react-dom/client';
// @ts-expect-error
import singleSpaReact from 'single-spa-react';
import App from './App';
import { cssLifecycleFactory } from 'vite-plugin-single-spa/ex';

const lc = singleSpaReact({
    rootComponent: App,
    errorBoundary(err: any, _info: any, _props: any) {
        return <div>Error: {err}</div>
// IMPORTANT:  Because the file is named spa.tsx, the string 'spa'
// must be passed to the call to cssLifecycleFactory.
const cssLc = cssLifecycleFactory('spa', /* optional factory options */);
export const bootstrap = [cssLc.bootstrap, lc.bootstrap];
export const mount = [cssLc.mount, lc.mount];
export const unmount = [cssLc.unmount, lc.unmount];

The lifecycle factory algorithm needs to know which entry point it should be creating the lifecycle object for, so it is very important that the name passed to the factory coincides exactly with the file name (minus the extension).

The object created by the factory (in the example, stored in the cssLc variable), must be used for every exported/created single-spa lifecycle object that comes out of the same file (module).

NOTE: The optional factory options control the behavior of the FOUC-prevention feature and control over what is logged to the console.

Console Logging

Since v0.7.0

The option named logger in the options parameter of the cssLifecycleFactory() function can be used to control what gets logged to the browser's console from the CSS mounting algorithms. Its default value is true and means "log to the console". If set to false, then nothing is logged to the console.

There is a third possiblity: To provide a custom logger object. The logger only needs to implement the 4 console methods debug(), info(), warn() and error(). Use this approach if you need fine-grained control over what gets logged to the console, or to even do fancier logging, such as posting the console entries to a remote structured log storage, like a Seq server.

CSS Strategy

The cssLifecycleFactory function covers both CSS strategies (singleMife and multiMife).

This new CSS mounting algorithm supports multiple SPA entry points, with single or multiple exports per entry point, and mixing micro-frontends with parcels in the same project should also be possible. If you intend to either mount multiple instances of the same parcel or micro-frontend, or export more than one single-spa lifecycle object, you must set the options' cssStrategy property to multiMife.

If, however, your project simply exports a single micro-frontend or a single parcel that doesn't expect to be instantiated more than once simultaneously, then cssStrategy can be set to singleMife, which is the default value for this property.

Deactivating CSS Mounting

Since v0.6.0

The CSS mounting algorithm in this package relies on some naming convention around bundled CSS files. If you plan to roll out your own CSS mounting algorithm, you may set cssStrategy to none. This will effectively deactivate the CSS bundle renaming that takes place during building. This also deactivates cssLifecycleFactory.

Currently investigating if usage of cssLifecycleFactory can be detected in order to emit a warning.

FOUC Prevention

Since v0.7.0

The object created with the cssLifecycleFactory() function prevents FOUC (flash of unstyled content) by delaying the mounting operation until all the CSS files that the plug-in handles (the CSS bundles created by Vite) are loaded and ready to be used by the browser. This is implemented by subscribing to the LINK element's load and error events, and only returning if the CSS loads, an error occurs, or the specified time elapses (a timeout event).

The following options, which are set when calling cssLifecycleFactory(), control the behavior of this feature:

export type CssLifecycleFactoryOptions = {
    logger?: boolean | ILogger;
    loadTimeout?: number;
    failOnTimeout?: boolean;
    failOnError?: boolean;

Refer to Console Logging previously in this document to see about the logger option.

Set the loadTimeout property to the amount of time to wait for the load or error events to fire before taking action. Which action? That depends on the other two properties.

When set to true, failOnTimeout throws an error that aborts the micro-frontend mounting process, but when set to false, only a warning is logged to the console.

The last property, failOnError, works identically to failOnTimeout, except that it applies to the times where the CSS fetching process fails.

The default values are, respectively, 1500ms, false and false.

FOUC Prevention Known Facts & Issues

  1. single-spa, by default, emits minified error message #31 if the mounting process takes 3000ms or more. Avoid setting loadTimeout to 3000ms or higher.
  2. The CSS mounting algorithm dismounts CSS by disabling CSS LINK elements, and then, if necessary, these are re-enabled. Even though a network call is generated in the Network tab of the developer tools, a load event is never fired, meaning that FOUC prevention only works on the very first time the CSS is mounted.

Important Notes About Generating Multiple Instances of a Parcel or Micro-Frontend

While an incredible library, single-spa was not designed to support the mounting of more than one instance of a micro-frontend simultaneously, but some of us like this idea. This CSS mounting algorithm pretend to support as much as possible this scenario, as well as the mounting of multiple instances of the same parcel. However, single-spa is not really prepared for this, so do this at your own risk.

Speaking of which, there is a bug in the singleSpaSvelte() function exported by the single-spa-svelte NPM package that prevents exporting the way single-spa recommends. Detailed information can be found in this blog post or in the logged issue at GitHub. The blog post and the issue were written prior to the existence of vite-plugin-single-spa v0.4.0, so make the necessary changes in the proposed workaround.

While I haven't tested every helper for every framework/library, there is the possibility that the bug found in the single-spa-svelte package may exist in others, in which case the factory workaround may work for those too.

Why You Must Choose the CSS Strategy

The new algorithm is robust and seems to work just fine under the conditions set by the singleMife strategy, but there is a price to pay: This new algorithm is incompatible with the idea of using single-spa's unloadApplication for the purposes of HMR. In order to allow people to keep the possibility of using unloadApplication, the user can choose the singleMife strategy to keep this HMR ability.

Vite Environment Information

Since v0.2.0

The same extension module that provides CSS lifecycle functions also provides basic information about the Vite environment. Specifically, it exports the viteEnv object which is described as:

export const viteEnv: {
    serving: boolean,
    built: boolean,
    mode: string

The serving property will be true if the project is being served by Vite in serve mode. The built property will be true if the project is being run after being built. The mode property is just the mode used when Vite was run, and by default is the string development for serve mode, and production for builds.

Unlike cssLifecycleFactory which is only useful to micro-frontend projects, viteEnv is available to root projects as well.

To make use of it for whatever purpose, import it:

import { viteEnv } from 'vite-plugin-single-spa/ex';

In-Depth Coverage On the Topic

This plug-in was born as the result of investigating single-spa, Svelte and the general use of Vite-based projects. This investigation was documented in the form of a blog in hashnode. Feel free to read it in order to fully understand how this plug-in works and the reasons behind its behavior.


  • [x] Multiple import map files per Vite command (to support shared dependencies marked external in Vite)
  • [x] Single-SPA parcels
  • [x] Multiple single-spa entry points
  • [x] Logging options
  • [x] Asset file name pattern
  • [x] CSS load event to prevent FOUC
  • [x] Logger object for cssLifecycleFactory to allow full control of what renders in the console
  • [ ] Input file name pattern?
  • [ ] Specify import maps as objects instead of file names
  • [ ] Possibly remove the need for CSS strategies (modify multiMife so it can re-bootstrap safely)
  • [ ] CSS blocking="render" attribute on injected LINK elements (experimental feature)? Instead of load event to prevent FOUC.
  • [ ] Allow media query specification for injected CSS LINK elements? (not sure if this is relevant to this plug-in)
  • [ ] Option to set development entry point? (there might be a simpler solution)
  • [ ] SvelteKit support for root projects?

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