⚠️ WARNING: Firebase have introduced first-party support for Web Frameworks. SvelteKit support is experimental. This adapter may be deprecated in future as it cannot support the same level of integration as the official team and tooling. Use at your own risk.

svelte-adapter-firebase

Firebase adapter for SvelteKit.

Utilise the Firebase Hosting CDN with dynamic content served by SvelteKit on Cloud Functions!

✔️ SSR on Cloud Functions
✔️ Integrates with existing Cloud Functions!
✔️ Local production testing with Firebase Emulator
✔️ Mitigate cold-starts with minInstances
✔️ Multiple Hosting Sites

Contents
Setup
- Beta Adapter Version Compatibility
Configuration Overview
Details
- firebase.json Configurations
- Adapter Configurations
How it works
Firebase Emulator local Testing
Deployment
Caveats
Non-goals
FAQ
Contributing
- external contributions

Setup

The adapter reads firebase.json to determine output dirs for Server scripts & Static assets, without this file the adapter cannot know how your Firebase app is configured. Hosting & Cloud Functions are required.

In your standard SvelteKit project:

npm install --save-dev svelte-adapter-firebase
add adapter to svelte.config.js:

+import firebase from "svelte-adapter-firebase";

/** @type {import('@sveltejs/kit').Config} */
export default {
  kit: {
+   adapter: firebase(),
  },
};

Setup firebase.json with firebase init.
npm run build. Read and repeat, the output is meant as a guide. IE: after your first build, you need to manually add the produced Cloud Function in index.js

Beta Adapter Version Compatibility

This adapter has not been tested beyond the versions listed below, use at your own risk:

Adapter Version	SvelteKit Version
`0.15.0`	`1.1.1`
`0.14.5`	`1.0.0-next.587`
`0.14.4`	`1.0.0-next.443`
`0.14.3`	`1.0.0-next.443`
`0.14.2`	`1.0.0-next.405`
`0.14.0`	`1.0.0-next.330`
`0.13.1`	`1.0.0-next.180`
`0.13.0`	`1.0.0-next.168`
`0.12.x`	`1.0.0-next.165`
`0.11.x`	`1.0.0-next.155`
`NA`	`1.0.0-next.152-154`
`0.10.x`	`1.0.0-next.132`
`0.9.1`	`1.0.0-next.122`
`0.9.0`	`1.0.0-next.120`
`0.8.x`	`1.0.0-next.111`
`NA`	`1.0.0-next.109-110`
`0.7.x`	`1.0.0-next.107`
`0.6.x`	`1.0.0-next.103`
`0.5.x`	`1.0.0-next.54`
`0.4.x`	`1.0.0-next.46`
`0.3.x`	`1.0.0-next.27`

Note: only the versions listed have been tested together, if others happen to work, it is just coincidence. This is beta software after all.

Configuration Overview

Adapter options:

esbuildBuildOptions
- function to return an esbuild.BuildOptions object
- default: see defaultOptions object in src/index.js
firebaseJsonPath
- path to your firebase.json file, relative from where svelte build is called
- default: ./firebase.json
target
- required when firebase.json:hosting is an array (contains many site configurations)
- default: undefined
sourceRewriteMatch
- used to lookup the rewrite config to determine whether to output SSR code for Cloud Functions. See Firebase Rewrite configuration docs.
- default: **

Adapter output:

static assets (images, CSS, Client-side JavaScript) of your SvelteKit app output to the directory defined by firebase.json:hosting.public
server assets (SSR JavaScript) output alongside your Cloud Functions defined by firebase.json:functions.source

Details

Setup outlines the steps most commonly used with a single SvelteKit app. Here we go into the details of each configuration and how it interacts with the firebase.json config.

The 3 step process is:

select Hosting config from firebase.json. If more than one site present in config, match svelte.config.js:target field with either firebase.json:hosting[].site or .target fields.
output static assets to the directory in the public field
identify the rewrite rule for SSR to determine Cloud Function output. The rewrite rule is determined by a lookup of the rewrites.source against sourceRewriteMatch

`firebase.json` Configurations

Due to the relaxed rules of firebase.json we can have many valid configs. At a minimum, one or more Hosting sites is required with an associated Functions config if a Cloud Function rewrite is used. These are the combintations:

single Hosting site with Cloud Function rewrite

{
  "hosting": {
    "public": "<someDir>",
    "rewrites": [
      {
        "source": "**",
        "function": "<functionName>"
      }
    ]
  },
  "functions": {
    "source": "<anotherDir>"
  }
}

multiple Hosting site with Cloud Function rewrite

{
  "hosting": [
    {
      "site": "blog",
      "public": "<someDir>",
      "rewrites": [
        {
          "source": "**",
          "function": "<functionName>"
        }
      ]
    },
    {
      // another site config
    }
  ],
  "functions": {
    "source": "<anotherDir>"
  }
}

To correctly lookup the blog site, target will need to be set in svelte.config.js:

import firebase from "svelte-adapter-firebase";

/** @type {import('@sveltejs/kit').Config} */
export default {
  kit: {
    adapter: firebase({ target: "blog" }),
  },
};

Adapter Configurations

Detailed examples of the adapter configuration options.

All options:

import firebase from "svelte-adapter-firebase";

/** @type {import('@sveltejs/kit').Config} */
export default {
  kit: {
    adapter: firebase({
      esbuildBuildOptions: (defaultOptions: BuildOptions) => Promise<BuildOptions> | BuildOptions,
      firebaseJsonPath: "",
      target: "svelte-func-single-site",
      sourceRewriteMatch: "",
    }),
  },
};

esbuildBuildOptions

As an escape hatch, you may optionally specify a function which will receive the final esbuild options generated by this adapter and returns a modified esbuild configuration. The result of this function will be passed as-is to esbuild. The function can be async.

For example, you may wish to add plugins, or configure the format to bundle to ESM (defaults to CJS):

import firebase from "svelte-adapter-firebase";

/** @type {import('@sveltejs/kit').Config} */
export default {
  kit: {
    adapter: firebase({
      target: "svelte-func-single-site",
      esbuildBuildOptions(defaultOptions) {
        return {
          ...defaultOptions,
          target: "esm"
          plugins: [],
        };
      },
    })
  },
};

The default options for this version are as follows:

{
	entryPoints: ['.svelte-kit/firebase/handler.js'],
	outfile: `pathToOutputDir/index.js`,
	bundle: true,
	inject: ['pathTo/shims.js'],
	platform: 'node',
  target: `node${functionRuntimeVersion}`
}

where esbuild target is computed from the Node.js runtime version defined for your Cloud Functions.

firebaseJsonPath

If the firebase.json file is not in the directory you run svelte build, then you can set a relative path in svelte.config.js:

.gitignore
firebase.json
app/                    <-- svelte build run in this dir
	package.json
	svelte.config.js
	src/
anotherApp/
	index.html
	index.css
functions/
	package.json
	index.js

import firebase from "svelte-adapter-firebase";

/** @type {import('@sveltejs/kit').Config} */
export default {
  kit: {
    adapter: firebase({
      target: "svelte-func-single-site",
      firebaseJsonPath: "../firebase.json",
    }),
  },
};

target

If firebase.json:hosting is an array of sites, then each hosting config must list a site or target field that matches the adatper's target option. For example:

// firebase.json
{
  "hosting": [
    {
      "site": "blog",
      // or
      // "target": "blog",
      "public": "<someDir>",
      "rewrites": [
        {
          "source": "**",
          "run": {
            "serviceId": "<cloudRunServiceId>"
          }
        }
      ]
    },
    {
      "site": "adminPanel",
      // or
      // "target": "adminPanel",
      "public": "<anotherDir>"
    }
  ]
}

import firebase from "svelte-adapter-firebase";

/** @type {import('@sveltejs/kit').Config} */
export default {
  kit: {
    adapter: firebase({ target: "blog" }),
    target: "#svelte",
  },
};

The Firebase config & adapter config match (firebase.json:hosting[0].site === adapter target), so therefore we know which Firebase Hosting site you want to build the SvelteKit site for.

sourceRewriteMatch

If the rewrite source pattern is not **, then svelte.config.js sourceRewriteMatch will need to be set to match your desired rewrite rule. For example:

// firebase.json
{
  "hosting": {
    "public": "<someDir>",
    "rewrites": [
      {
        "source": "/blog/**",
        "run": {
          "serviceId": "<cloudRunServiceId>"
        }
      }
    ]
  }
}

import firebase from "svelte-adapter-firebase";

/** @type {import('@sveltejs/kit').Config} */
export default {
  kit: {
    adapter: firebase({ sourceRewriteMatch: "/blog/**" }),
    target: "#svelte",
  },
};

How it works

Given

the following firebase.json configuration
a standard SvelteKit app structure
the default svelte-adapter-firebase config

// firebase.json
{
  "hosting": {
    "public": "myApp",
    "rewrites": [
      {
        "source": "**",
        "function": "ssrServer"
      }
    ],
    "predeploy": ["npm run build"]
  },
  "functions": {
    "source": "functions"
  }
}

the following Server & Static assets dirs are created:

firebase.json ("public": "myApp")
package.json
svelte.config.js
src/
	app.html
	routes/
		index.svelte
functions/
	package.json ("main": "index.js")
	index.js
	sveltekit/		<-- Server Assets dir (code to be imported to you Cloud Function)
myApp/				<-- Static Assets to go to Firebase Hosting CDN

firebase.json:functions.source dir is used to find functions/package.json whose main field is used to find the Cloud Function build dir. This is used as the server asset output dir.

TypeScript Cloud Functions

Because we use the above method to determine the output dir, the server assets are output to the correct place when using TypeScript.

firebase.json ("public": "myApp")
package.json
svelte.config.js
src/
  app.html
  routes/
    index.svelte
functions/
  package.json ("main": "lib/index.js")
  index.ts
  lib/
    index.js
    sveltekit/	<-- Server assets output to functions/lib
myApp/				<-- Static assets to go to Firebase Hosting CDN

Output with Multiple Sites

In a multi-site setup, the site or target field from hosting config in firebase.json is used as the server output dir:

firebase.json ("site": "myCoolSite","public": "myApp")
package.json
svelte.config.js
src/
	app.html
	routes/
		index.svelte
functions/
	package.json
	index.js
	myCoolSite/		<-- Server assets
myApp/				<-- Static assets to go to Firebase Hosting CDN

The final piece is to write the actual Cloud Function source code to reference the output server assets. The code is printed during svelte build and should be placed in your index.js or index.ts manually.

This is a flexible solution that allows integrating with other Cloud Functions in your project. You can edit the provided code as you see fit. The import/require of the generated code will not change unless you change the firebase.json:hosting.site or package.json:main fields, so you shouldn't need to update this code after adding it.

Firebase Emulator local Testing

Test your production build locally before pushing to git or deploying!

build your app: svelte-kit build
install Function dependencies: pnpm install --prefix functions
start the emulator: firebase emulators:start

Deployment

firebase deploy 🎉

Caveats

Using firebase.json:hosting[].site is preferred to firebase.json:hosting[].target as Firebase Deploy Targets only supports Hosting, Storage & Databases and not Functions. This means you can use Deploy targets target field to identify your site for the adapter to build, but you CANNOT use Deploy Targes when deploying as you need to deploy the Hosting & Functions at the same time for this solution to work as expected.
Firebase Hosting Preview Channels currently lacks first-party support for SSR applications. This adapter doesn't attempt to remedy this issue and doesn't produce a different SSR Function for preview channel deployments.
⚠️ Cloud Function rewrites only support us-central1, other regions will error. The official warning about this can be found in these docs.

Non-goals

Write Cloud Function code directly into .js file instead of printing in console.

Firebase Cloud Functions have a long history of people configuring their index files completely differently, some even generating them from directories. Accommodating these would be a headache. Instead, all we look for is a match against this string, ${name} =, where name is your Cloud Functions name. We may make this configurable to a specific file in future.

Additionally, this allows for users to customise their Firebase Cloud Function API like runWith() options for memory/CPU, min/max Instances and VPC/Ingress/Egress configuration settings, without complex support for options in the adapter. This keeps the Function config where it should, close to the userland code.

Handle the deployment of the app to Firebase.

Firebase apps consist of many different services with the CLI providing optional deployments. We do not want to dictate full deployments with your frontend nor perform partial deployments if it does not fit your app. The only option then is to leave it to you 🎉

FAQ

Why is the Cloud Function code output to the terminal for me to add manually instead of being written to functions/index.js?

See non-goals Write Cloud Function code directly into .js file instead of printing in console.

Firebase libs in SvelteKit

As recommended in the SvelteKit FAQ, please use Firebase JS SDK v9 as the older version of the SDK has issues and a larger bundle size.

Cold Starts

Since the purpose of using this adapter is to leverage the Firebase Hosting CDN, you should consider improving the user experience with targetted caching/TTLs.

If cold start are still an issue for your application, Cloud Functions has support for minInstances which will keep X number of instances warm. From the docs:

A minimum number of instances kept running incur billing costs at idle rates. Typically, to keep one idle function instance warm costs less than $6.00 a month. The Firebase CLI provides a cost estimate at deployment time for functions with reserved minimum instances. Refer to Cloud Functions Pricing to calculate costs.

-Firebase docs

To implement this, configure your runWith options like so:

const myRuntimeOptions = {
	memory: "1GB",
+	minInstances: 1,
}
exports.myFunc = functions.runWith(myRuntimeOptions).https.onRequest(async (request, response) => {
	...
});

Note: this is still single concurrency (if an instance does not exist to handle a request when it hits the backend a new Function instance is created). Watch this space!

1.0.0 will not be published until the SvelteKit Adapter API is declared stable and SvelteKit is released for general use.

Contributing

Contributions of any kind welcome, just follow the guidelines!

Short version:

git clone https://github.com/jthegedus/svelte-adapter-firebase.git
asdf install
pnpm i

See asdf to install set it up.

external contributions

While building this adapter some issues were found with upstream components, these are captured here should someone wish to contribute them:

Cloud Function validation code linked in utils.js is from two different sources which indicates that it is being validated by firebase-tools in two separate places. PR a fix there.

jthegedus/svelte-adapter-firebase