Sunday, 22 January 2023

Typehinting server side props

My current prefered stack is using Laravel for the backend, Vue 3 for the frontend and Inertia.js for the communication between backend & frontend and routing.

We have Models on the backend, which are the entities that make up the application. These models are sent to the frontend as json, which get passed to the Vue page component as props.

Let's take this example: we have a Post model, which has the following shape:

"id": 1,
"cover_image": "",
"title": "Laravel is great",
"excerpt": "...",
"body": "<p>...</p>",
"published_at": "2022-11-20T00:00:00.000000Z",
"author": {
"id": 1,
"name": "John Doe"

In the Vue page component we have the following:

const props = defineProps({
posts: {
type: Array,
required: true,

Our IDE knows nothing about the type of posts, other than it's an Array. We don't get any autocompletion for the posts, we need to check the backend code to see the shape of the post object, and it's very error prone, as the IDE won't warn if we make a typo.

Automatically Generating Typescript Definitions

The first step we need to take is to write Typescript definitions for our backend models. But this can be tedious to do by hand, especially when our models tend to change during development.

Thankfully, there's a useful package we can use to auto generate TS definitions for our Laravel models, based/laravel-typescript.

Once installed with composer we can generate TS definitions with this command:

php artisan typescript:generate

This generates a models.d.ts file containing the following (for our example above):

declare namespace App.Models {
export interface User {
id: number
name: string
export interface Post {
id: number
cover_image: string
title: string
body: string
published_at: string | null
author?: App.Models.User | null

Typehinting Vue Props

Now we can typehint the props like so:

import { PropType } from 'vue'
const props = defineProps({
posts: {
type: Array as PropType<App.Models.Post[]>,
required: true,

Once the post prop has been typehinted, we get intellisense autocompletion in the <script> and also in the <template> part of Vue SFCs (make sure to use the Volar VScode extension instead of the old Vetur).

This is already a massive DX improvement. We get intellisense for model properties, compile-time errors if we make a mistake or when our model definitions change.

Dynamic Types

We still have a problem. When paginating query results, Laravel wraps the array of models in an object containing more details about the pagination.

For instance, when paginating Posts, instead of a Post[] (an array of Posts), we get an object in the following shape:

"data": [
/* Posts */
"meta": {
"from": 1,
"to": 20,
"per_page": 20,
"total": 199,
"current_page": 1,
"last_page": 20,
"path": ""

We can't typehints posts as App.Models.Post[], because now it's an object containing extra meta about the pagination, in additional to a data property which contains the actual App.Models.Post[] array.

We don't want to manually create types for the paginated version of each of our models either.

Luckily, Typescript has a nifty solution for this. It's called Generics. It's basically a way to add parameters to type definitions. So this is how we can declare one type definition for paginated models:

type Paginator<T> = {
data: T[]
meta?: {
current_page: number
from: number
last_page: number
path: string
per_page: number
to: number
total: number

We declare a type Paginator which takes a parameter T. The data property is then typed as T[], which simply means - an array of the type T, which is passed as a parameter when the type is used.

Here is an example usage:

posts: Object as PropType<Paginator<App.Models.Post>>,

Now our IDE knows the exact shape of posts, and the Paginator type can be reused for any paginated model in our codebase. Amazing!