Elysia.js: Guard Schema Type Inference Issue In Plugins

by SLV Team 56 views

Hey guys! Let's dive into a tricky issue in Elysia.js that can stump you when working with plugins, specifically how guard schemas interact with the onAfterHandle lifecycle hook. If you've ever scratched your head wondering why your types aren't being inferred correctly in your Elysia.js plugins, you're in the right place. We're going to break down the problem, explore the root cause, and, most importantly, look at some solutions and workarounds. So, buckle up, and let's get started!

🐛 The Problem: Type Inference with Guards and onAfterHandle

So, here's the deal. Imagine you're building an Elysia.js plugin and want to use the .guard() method to define a schema, say for the request body. Now, you also want to tap into the .onAfterHandle() lifecycle hook to do something after your handler has done its thing. Seems straightforward, right? But here's where it gets interesting: the types you defined in your guard schema might not be correctly inferred within your onAfterHandle function. Specifically, the body parameter, which you'd expect to have the type you defined in the guard, often shows up as unknown. What's up with that?

To put it simply, the key issue lies in how Elysia.js, in conjunction with TypeScript, handles generic type inference in fluent APIs. When you chain methods like .guard() and .onAfterHandle(), TypeScript sometimes struggles to "look back" and remember the schema you defined earlier in the chain. This can lead to unexpected unknown types, which isn't ideal for a type-safe environment like TypeScript.

📋 Expected Behavior

Ideally, if you define a body schema in your guard, like t.Any(), you'd expect the body parameter in onAfterHandle to be correctly inferred as any. This would allow you to work with the request body without needing to resort to type assertions or other workarounds. Imagine the clarity and confidence you'd have knowing your types are flowing correctly!

📋 Actual Behavior

Unfortunately, the reality often differs. Instead of the expected type, you often find the body parameter typed as unknown. This means you lose the type safety you were aiming for, and you might need to add extra code to handle the potentially unknown type. It's not the end of the world, but it's definitely a bump in the road to smooth plugin development.

🔬 A Minimal Reproduction

Let's look at some code to make this crystal clear. Here’s a snippet that highlights the problem:

import { Elysia, t } from 'elysia'

// ❌ Problem: body type is `unknown` in onAfterHandle
const problemPlugin = new Elysia({ name: 'problem' })
  .guard({
    as: 'scoped',
    body: t.Any()  // Should make body type `any`
  })
  .onAfterHandle({ as: 'scoped' }, ({ body }) => {
    // Hover over `body` -> Type is `unknown` ❌
    // Expected: Type should be `any` ✅
    console.log(body)
  })

// ✅ Comparison: Same schema works in route afterHandle
const app = new Elysia()
  .post('/', ({ body }) => body, {
    body: t.Any(),
    afterHandle({ body }) {
      // Hover over `body` -> Type is `any` ✅
      console.log(body)
    }
  })

In this example, you can see that within the problemPlugin, the body type in onAfterHandle is unknown, even though we defined it as t.Any() in the guard. However, in the direct route example, the type inference works as expected. This contrast highlights the specific issue with plugins and onAfterHandle.

TypeScript Root Cause

To really understand what's going on, we need to peek under the hood at TypeScript's type system. The issue stems from a limitation in how TypeScript infers generic types in fluent APIs. Let's simplify the situation with a bit of TypeScript code:

// Simplified reproduction of the TypeScript issue
interface Schema { body?: any }

class Builder<CurrentSchema extends Schema = {}> {
  setSchema<NewSchema extends Schema>(schema: NewSchema): Builder<CurrentSchema & NewSchema> {
    return new Builder()
  }

  // The problem: HandlerSchema is inferred only from the first parameter
  useHandler<
    HandlerSchema extends Schema,  // ← Inferred from `handlerSchema` parameter only
    Handler extends (ctx: {
      body: HandlerSchema['body'] extends undefined
        ? CurrentSchema['body'] extends undefined
          ? unknown
          : CurrentSchema['body']
        : HandlerSchema['body']
    }) => void
  >(
    handlerSchema: HandlerSchema,  // ← TypeScript only looks at this
    handler: Handler
  ): void {
    handler({ body: 'runtime-value' } as any)
  }
}

// Test case
const builder = new Builder()
  .setSchema({ body: 'string' })  // ✅ Sets CurrentSchema = { body: string }
  .useHandler(
    {},  // ❌ HandlerSchema = {} (empty object)
    ({ body }) => {
      // ❌ body type is `unknown`
      // Because HandlerSchema['body'] = undefined
      // TypeScript can't "look back" at previous .setSchema() call
    }
  )

This code illustrates how TypeScript infers generic types only from the current function call in a chain, not from previous calls. In the context of Elysia.js, this means that when .onAfterHandle() is called, TypeScript only sees the parameters passed directly to it and doesn't "remember" the schema defined in the preceding .guard() call. It's like TypeScript has a short memory in this specific scenario!

🧪 Full Test Cases

To drive the point home, let's look at some comprehensive test cases that demonstrate where the type inference works and where it falls short:

import { Elysia, t } from 'elysia'

// Test 1: Plugin with guard + onAfterHandle (BROKEN)
const pluginWithGuard = new Elysia({ name: 'test1' })
  .guard({
    as: 'scoped',
    body: t.String()  // Define body as string
  })
  .onAfterHandle({ as: 'scoped' }, ({ body }) => {
    // ❌ body type: unknown (should be string)
    console.log('Plugin body type:', typeof body)
  })

// Test 2: Direct route afterHandle (WORKS)
const directRoute = new Elysia()
  .post('/', ({ body }) => body, {
    body: t.String(),
    afterHandle({ body }) {
      // ✅ body type: string (correct)
      console.log('Route body type:', typeof body)
    }
  })

// Test 3: Guard with callback (WORKS)
const guardWithCallback = new Elysia()
  .guard({
    body: t.String()
  }, (app) => app
    .onAfterHandle(({ body }) => {
      // ✅ body type: string (correct)
      console.log('Guard callback body type:', typeof body)
    })
  )

// Test 4: Different types all broken in plugin
const multiTypePlugin = new Elysia({ name: 'test4' })
  .guard({
    as: 'scoped',
    body: t.String(),
    query: t.Object({ name: t.String() }),
    headers: t.Object({ 'x-token': t.String() })
  })
  .onAfterHandle({ as: 'scoped' }, ({ body, query, headers }) => {
    // ❌ ALL are unknown: body, query, headers
    // Should be: string, { name: string }, { 'x-token': string }
  })

These tests clearly show that the issue is specific to plugins using .guard() and .onAfterHandle() in combination. Direct routes and guard callbacks, on the other hand, handle type inference correctly.

🔍 Technical Analysis

Root Cause: TypeScript Generic Inference Limitation

As we've touched on, the heart of the matter is a known limitation in TypeScript's generic type inference. This isn't just an Elysia.js quirk; it's a broader TypeScript challenge. You can dive deeper into this rabbit hole by checking out these GitHub issues:

The Problem in Detail:

  1. .guard() sets the schema in Ephemeral['schema']. Think of this as Elysia.js storing the schema information for later use.
  2. .onAfterHandle() needs to infer the handler types from its parameters. This is where TypeScript's inference engine kicks in.
  3. TypeScript can only infer generics from the current function call, not from previous calls in the chain. It's like trying to assemble a puzzle without looking at the whole picture.
  4. Therefore, onAfterHandle({ as: 'scoped' }, handler) only sees the { as: 'scoped' } parameter and doesn't have access to the guard schema. Ouch!

Why route afterHandle works:

In contrast, when you define afterHandle directly within a route, TypeScript has all the information in one place:

// All info in one object literal - TypeScript can see everything
{
  body: t.String(),
  afterHandle({ body }) { ... }  // ← TypeScript knows body is string
}

TypeScript can see the body schema and the afterHandle function in the same scope, so it can correctly infer the type of body. It's like having all the puzzle pieces laid out in front of you.

Why plugin onAfterHandle breaks:

But in the plugin scenario, the information is spread across different calls:

.guard({ body: t.String() })      // ← Step 1: info here
.onAfterHandle({}, ({ body }) =>  // ← Step 2: TypeScript can't "look back"

TypeScript can't "look back" from the .onAfterHandle() call to the .guard() call, so it can't infer the body type. It's like trying to assemble the puzzle with some pieces hidden away.

💡 Current Workarounds

Okay, enough with the problem. Let's talk solutions! While we can't change TypeScript's fundamental behavior, we can employ some clever workarounds to get our types flowing correctly.

Option 1: Type Assertion (Recommended)

The simplest and often most practical solution is to use a type assertion. This tells TypeScript, "Hey, I know what I'm doing; treat this value as this type."

.onAfterHandle({ as: 'scoped' }, ({ body }) => {
  const typedBody = body as any  // Safe because guard validates at runtime
  console.log(typedBody)
})

In this case, we're asserting that body is any. This is safe because the guard has already validated the body at runtime, so we can be confident in its type. It's like adding a little note to the puzzle piece to remind yourself where it goes.

Option 2: Use Guard with Callback

Another approach is to use the guard with a callback function. This allows you to define the schema and the onAfterHandle logic within the same scope, giving TypeScript the context it needs to infer the types correctly.

.guard({
  as: 'scoped',
  body: t.Any()
}, (app) => app
  .onAfterHandle(({ body }) => {
    // ✅ body type is correctly inferred
  })
)

This pattern keeps the type information together, making it easier for TypeScript to do its job. It's like keeping all the puzzle pieces for one section in the same box.

Option 3: Move Logic to Route Level

If the logic you're performing in onAfterHandle is specific to a particular route, you can move it directly into the route's afterHandle option. This avoids the plugin-related type inference issue altogether.

// Instead of plugin onAfterHandle, let routes define their own
app.post('/', handler, {
  body: t.Any(),
  afterHandle({ body }) {
    // ✅ Works correctly
  }
})

This approach keeps the logic and its type information close together, ensuring correct type inference. It's like building a mini-puzzle within the larger one.

🎯 Possible Solutions for Elysia

While the root cause is a TypeScript limitation, the Elysia.js team could potentially implement some changes to improve the developer experience in this area. Here are a few ideas:

Option A: Support Schema in onAfterHandle First Parameter

One approach would be to allow schema definitions directly within the onAfterHandle method:

// Allow schema definition in lifecycle methods
.onAfterHandle({
  as: 'scoped',
  body: t.Any()  // ← Currently not supported
}, ({ body }) => {
  // Would fix the type inference
})

This would give TypeScript the necessary context to infer the types correctly. It's like adding a label directly to the slot where the puzzle piece goes.

Option B: Improve Type Definitions

The Elysia.js type definitions could be enhanced to better merge Ephemeral['schema'] with handler inference. However, this might be limited by TypeScript's inherent capabilities. It's like trying to reshape the puzzle pieces to fit together better.

Option C: Documentation

At the very least, clearly documenting this limitation and recommending workarounds would be a huge help for plugin authors. A little guidance can go a long way in navigating these tricky type inference issues. It's like providing a guide to assembling the puzzle.

📊 Impact

This type inference issue can affect any plugin that:

  1. Uses .guard() to define a schema.
  2. Uses .onAfterHandle() (or other lifecycle methods) that need to access the schema types.
  3. Exports the plugin for use in other applications.

This is particularly relevant for plugins dealing with authentication, logging, and validation, where schema definitions and lifecycle hooks are commonly used.

🔖 Environment

For context, this issue has been observed in the following environment:

  • Elysia version: 1.4.13
  • TypeScript version: 5.9.2
  • Bun version: 1.2.21

Wrapping Up

So, there you have it! We've taken a deep dive into a tricky type inference issue in Elysia.js plugins, explored the root cause in TypeScript's generic inference, and looked at several workarounds and potential solutions. While this is a TypeScript limitation, understanding the problem and applying the right techniques can help you build robust and type-safe Elysia.js plugins. Keep coding, keep exploring, and don't let those pesky type inference issues get you down!

Remember, this is fundamentally a TypeScript limitation, but Elysia could potentially provide workarounds or better documentation to help plugin authors navigate this challenge. Happy coding, guys!