build-transformer

name: build-transformer description: Build t7m transformers for API output transformation. Use when creating new transformers, adding includes, configuring cache, or wiring transformers into Hono or Elysia routes.

Building a t7m Transformer

t7m is a TypeScript transformer layer for APIs. One class per model controls what gets exposed, loads includes in parallel, and caches repeated calls.

Quick Start

import { AbstractTransformer } from 't7m'

// 1. Define your input and output types
interface User {
    id: number
    name: string
    email: string
    password: string
}

interface PublicUser {
    name: string
    email: string
}

// 2. Extend AbstractTransformer<Input, Output>
class UserTransformer extends AbstractTransformer<User, PublicUser> {
    // 3. Implement data() — the core transformation
    data(input: User): PublicUser {
        return {
            name: input.name,
            email: input.email,
        }
    }
}

// 4. Use it
const transformer = new UserTransformer()
const result = await transformer.transform({ input: user })

Critical Rules

These are the most common mistakes. Follow them exactly.

All transform methods take a params OBJECT

// CORRECT
await transformer.transform({ input: user })
await transformer.transform({ input: user, includes: ['posts'], props: { db } })

// WRONG — never pass positional arguments
await transformer.transform(user)

Includes only work with optional output properties

type Output = {
    name: string    // Required — CANNOT be in includesMap
    avatar?: string // Optional — CAN be in includesMap
}
// Enforced by OnlyPossiblyUndefined<TOutput>

Cache uses .call(), not .get()

// CORRECT
const user = await this.cache.userProfile.call(input.userId)

// WRONG
this.cache.userProfile.get(input.userId)

transformers is a Record, not an array

// CORRECT
transformers = { author: this.authorTransformer }

// WRONG
transformers = [this.authorTransformer]

clearCacheOnTransform is on AbstractTransformer, not Cache

// CORRECT — pass to super()
constructor() {
    super({ clearCacheOnTransform: false })
}

// WRONG
new Cache(fn, { clearCacheOnTransform: false })

Detailed Patterns

• For includes, props, cache, nested transformers, and access modifiers, see reference.md
• For concurrency control (throttling external API calls), see reference.md — Concurrency Control
• For Hono middleware integration and route patterns, see hono.md
• For Elysia plugin integration and route patterns, see elysia.md