The purpose of the preflight file is to populate the server-side store with default application state. The preflight function is passed the request object enabling you to populate the server-side store with customized data per route.
You can use the preflight file as a way to incrementally build your data layer. Start with a static version of your store data to get everything working then progress to using API functions and a database as needed.
Preflight can also work as a global middleware replacement for pages that require data in common with other pages that otherwise would not require an API route. A typical pattern that can be solved with preflight is needing authenticated account data on multiple pages without writing an API endpoint for each page.
Enhance looks for the preflight file in the root of your app.
app
├── api ............... data routes
│ └── index.mjs ..... override default preflight application state with api data
├── preflight.mjs ..... pre-populate server-side store
└── head.mjs .......... custom <head> component
The preflight function is passed the request object enabling you to customize data per requested route. API responses are merged with the default state returned from preflight allowing you to override default state with specific API data per request.
app/preflight.mjs
export default async function Preflight ({ req }) {
return { /* ...Your data here */ }
}app/preflight.mjs
export default async function Preflight ({ req }) {
return {
pageTitle: getPageTitle(req.path),
account: {
username: 'bobsyouruncle',
id: '23jk24h24'
}
}
}
function getPageTitle (path) {
const titleMap = {
'/': 'Home',
'/about': 'About',
'/account': 'My Account'
}
return titleMap[path] || 'My App Name'
}The data object you return from preflight will be available to your elements and the head.mjs file via the state.store
app/head.mjs
export default function Head(state) {
const { store = {} } = state
const { pageTitle = 'Enhance Starter Project' } = store
return `
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>${pageTitle}</title>
</head>
<body class="font-sans">
`
}app/preflight.mjs
export default async function Preflight ({ req }) {
return {
pageTitle: getPageTitle(req.path),
account: {
username: 'bobsyouruncle',
id: '23jk24h24'
}
}
}app/api/index.mjs
export async function get() {
return {
json: {
account: {
username: 'thisshouldoverride',
id: '39nr34n2'
}
}
}
}The account object will be overridden by the API response.