TypeScript Academy · Lesson

Strongly-typed builders

Design chainable builders that accumulate typed options; ensure required steps and valid combinations using generics and conditional types.

Lesson 3 of 38 steps

Strongly-typed builders is a free TypeScript Academy lesson on CoddyKit. This is lesson 3 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the TypeScript Academy learning path, and your progress syncs across the web and the CoddyKit app. The TypeScript Academy course includes 3 lessons in total.

Intro

Goal: Build chainable APIs that encode progress in the type. Each step returns a new typed state so invalid orders fail at compile time.

Accumulate options in a generic
Gate steps with flags
Keep DX simple

State flags

Each step returns a new type of state; build is accessible only when the required flags are true.

type Flags = { method: boolean; url: boolean }

type With<F extends Partial<Flags>> = { flags: F }

type True = true

type Builder<F extends Partial<Flags>, O extends Record<string, unknown>> = With<F> & {
  method<M extends "GET" | "POST">(m: M): Builder<F & { method: True }, O & { method: M }>
  url<U extends `/${string}`>(u: U): Builder<F & { url: True }, O & { url: U }>
  build(this: Builder<{ method: True; url: True }, any>): O
}

function createBuilder(): Builder<{}, {}> {
  return {
    flags: {},
    method(m) { return { ...this, flags: { ...this.flags, method: true }, method: this.method, url: this.url, build: this.build } as any },
    url(u) { return { ...this, flags: { ...this.flags, url: true }, method: this.method, url: this.url, build: this.build } as any },
    build() { return this as any }
  } as any
}

const b = createBuilder()
const good = b.method("GET").url("/users").build()   // ok
// const bad = b.build() // error: build requires method & url flags

Accumulate options

Accumulate options in the O generic; each call returns a type that adds new properties.

type B2<F extends Partial<Flags>, O extends Record<string, unknown>> = With<F> & {
  header<K extends string, V extends string>(k: K, v: V): B2<F, O & { headers: Record<K, V> }>
  query<K extends string, V extends string>(k: K, v: V): B2<F, O & { query: Record<K, V> }>
  method<M extends "GET" | "POST">(m: M): B2<F & { method: True }, O & { method: M }>
  url<U extends `/${string}`>(u: U): B2<F & { url: True }, O & { url: U }>
  build(this: B2<{ method: True; url: True }, any>): O
}

function builder2(): B2<{}, {}> { return {} as any }

const conf = builder2()
  .method("POST")
  .url("/login")
  .header("x-id", "42")
  .query("next", "home")
  .build()

Valid combinations

Restrict combinations with conditional types: make the body free only for POST.

type Method = "GET" | "POST"

type BodyIfPost<M extends Method, B> = M extends "POST" ? { body: B } : {}

type B3<F extends Partial<Flags>, O extends Record<string, unknown>> = With<F> & {
  method<M extends Method>(m: M): B3<F & { method: True }, O & { method: M }>
  url<U extends `/${string}`>(u: U): B3<F & { url: True }, O & { url: U }>
  body<B>(b: B & (O extends { method: "POST" } ? unknown : never)): B3<F, O & BodyIfPost<O extends { method: infer M } ? Extract<M, Method> : Method, B>>
  build(this: B3<{ method: True; url: True }, any>): O
}

function builder3(): B3<{}, {}> { return {} as any }

const ok = builder3().method("POST").url("/u").body({ a: 1 }).build()
// const badBody = builder3().method("GET").url("/u").body({ a: 1 }).build() // compile-time error

Variadic pipeline

Gather steps as a collection with a Variadic tuple array; manage input/output with Parameters/ReturnType.

type Step<A extends any[], R> = (...args: A) => R

type Pipeline<P extends Step<any, any>[]> = {
  use<S extends Step<any, any>>(...s: [S]): Pipeline<[...P, S]>
  run<A extends Parameters<P[0]>>(this: Pipeline<P>, ...args: A): ReturnType<P[number]>
}

function pipeMake(): Pipeline<[]> { return {} as any }

const pipe = pipeMake()
  .use((a: string) => a.length)
  .use((n: number) => n % 2 === 0)
// pipe.run("abc") // demo; implementation omitted

Tips

Tips:

Keep state as a phantom generic; don't burden the runtime.

Keep step names small and fixed (method, url).

Separate difficult combinations with conditional types; keep the chain clean.

Builder gating check

Quick check: How do we ensure .method() is called before .build()?

Answer it in the interactive lesson

Recap

Recap: Carry typed state along the chain, accumulate options in generics, catch invalid sequences at compile time.

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Frequently Asked Questions

Is the “Strongly-typed builders” lesson free?

Yes — the full text of “Strongly-typed builders” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the TypeScript Academy course, upgrade to CoddyKit PRO. The TypeScript Academy course includes 3 lessons in total.

What will I learn in “Strongly-typed builders”?

Design chainable builders that accumulate typed options; ensure required steps and valid combinations using generics and conditional types. You practise TypeScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start TypeScript Academy?

No prior experience is required. TypeScript Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 3.

How long does the “Strongly-typed builders” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

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