[Bug]: encodeValue crashes with "Cannot read properties of undefined" on MapSchema with nested Schema types

[miketests]

Bug description

Description

When encoding a MapSchema containing nested Schema instances, encodeValue in EncodeOperation.ts:37 receives undefined as the type parameter, causing a crash on type[Symbol.metadata].

The @type() decorators run correctly — Symbol.metadata is populated on all classes (verified via logging). The crash happens at encode time, not at decoration time.

Environment

• @colyseus/schema: 3.0.76
• colyseus: 0.16.5
• Node.js: 24.13.0
• TypeScript: 5.9.3
• Runtime: tsx 4.21.0 with --no-experimental-strip-types (forces esbuild, which correctly handles experimentalDecorators)
• tsconfig: "experimentalDecorators": true

Error

TypeError: Cannot read properties of undefined (reading 'Symbol(Symbol.metadata)')
    at encodeValue (EncodeOperation.ts:37:20)
    at encodeKeyValueOperation (EncodeOperation.ts:150:5)
    at Encoder.encode (Encoder.ts:119:17)
    at Encoder.encodeAll (Encoder.ts:155:21)

The crash is at this line in EncodeOperation.ts:37:

} else if (type[Symbol.metadata] !== undefined) {

where type is undefined.

Notes

• Encoding works fine with no entries in the MapSchema — the crash only happens when encoding a map entry (ADD operation).
• All class metadata is correctly populated (decorators run successfully).
• @colyseus/schema@2.0.37 does not have this issue — MapSchema encoding works correctly there.
• This may be related to how Encoder resolves the child type of a MapSchema before passing it to encodeValue.

Optional: Minimal reproduction

Minimal Reproduction

import { Schema, type, MapSchema, Encoder } from '@colyseus/schema'

class Player extends Schema {
  @type('string') sessionId: string = ''
  @type('float32') x: number = 0
}

class GameState extends Schema {
  @type({ map: Player }) players = new MapSchema<Player>()
  @type('string') mapId: string = ''
}

// Verify metadata is populated correctly
console.log('Player metadata:', (Player as any)[Symbol.metadata])
// → { '0': { type: 'string', index: 0, name: 'sessionId' }, '1': { type: 'float32', index: 1, name: 'x' } }

console.log('GameState metadata:', (GameState as any)[Symbol.metadata])
// → { '0': { type: { map: [class Player] }, index: 0, name: 'players' }, '1': { type: 'string', index: 1, name: 'mapId' } }

const state = new GameState()
const encoder = new Encoder(state)

// ✅ This works — empty state
encoder.encodeAll()

// Add a player to the map
const player = new Player()
player.sessionId = 'abc'
player.x = 100
state.players.set('abc', player)

// ❌ This crashes
encoder.encode()

[endel]

Hi @miketests, I'd suggest upgrading to @colyseus/schema v4+ and Colyseus 0.17. Please let me know if the issue persists there. https://docs.colyseus.io/migrating/0.17

[endel]

BTW this tsconfig setting is required in all versions:

"useDefineForClassFields": false

[miketests]

BTW this tsconfig setting is required in all versions:

"useDefineForClassFields": false

Thanks, that was the key! Adding "useDefineForClassFields": false to tsconfig fixes the encoding issue on both schema v3 and v4.

Tested with @colyseus/schema@4.0.19 + colyseus@0.17.8, MapSchema encoding works correctly now. The root cause was that without this setting, TypeScript (targeting ES2022+) uses Object.defineProperty for class field initialization, which overwrites the property descriptors set up by the @type() decorator in the Schema constructor.

We're currently on 0.15 and will plan a full migration to 0.17 separately. Would be great if this tsconfig requirement was mentioned in the error message or documented more prominently, it's easy to miss.

Thanks again for the quick response!