typescript-advanced-types — detailed worked examples
Advanced Patterns
Pattern 1: Type-Safe Event Emitter
type EventMap = {
"user:created": { id: string; name: string };
"user:updated": { id: string };
"user:deleted": { id: string };
};
class TypedEventEmitter<T extends Record<string, any>> {
private listeners: {
[K in keyof T]?: Array<(data: T[K]) => void>;
} = {};
on<K extends keyof T>(event: K, callback: (data: T[K]) => void): void {
if (!this.listeners[event]) {
this.listeners[event] = [];
}
this.listeners[event]!.push(callback);
}
emit<K extends keyof T>(event: K, data: T[K]): void {
const callbacks = this.listeners[event];
if (callbacks) {
callbacks.forEach((callback) => callback(data));
}
}
}
const emitter = new TypedEventEmitter<EventMap>();
emitter.on("user:created", (data) => {
console.log(data.id, data.name); // Type-safe!
});
emitter.emit("user:created", { id: "1", name: "John" });
// emitter.emit("user:created", { id: "1" }); // Error: missing 'name'
Pattern 2: Type-Safe API Client
type HTTPMethod = "GET" | "POST" | "PUT" | "DELETE";
type EndpointConfig = {
"/users": {
GET: { response: User[] };
POST: { body: { name: string; email: string }; response: User };
};
"/users/:id": {
GET: { params: { id: string }; response: User };
PUT: { params: { id: string }; body: Partial<User>; response: User };
DELETE: { params: { id: string }; response: void };
};
};
type ExtractParams<T> = T extends { params: infer P } ? P : never;
type ExtractBody<T> = T extends { body: infer B } ? B : never;
type ExtractResponse<T> = T extends { response: infer R } ? R : never;
class APIClient<Config extends Record<string, Record<HTTPMethod, any>>> {
async request<Path extends keyof Config, Method extends keyof Config[Path]>(
path: Path,
method: Method,
...[options]: ExtractParams<Config[Path][Method]> extends never
? ExtractBody<Config[Path][Method]> extends never
? []
: [{ body: ExtractBody<Config[Path][Method]> }]
: [
{
params: ExtractParams<Config[Path][Method]>;
body?: ExtractBody<Config[Path][Method]>;
},
]
): Promise<ExtractResponse<Config[Path][Method]>> {
// Implementation here
return {} as any;
}
}
const api = new APIClient<EndpointConfig>();
// Type-safe API calls
const users = await api.request("/users", "GET");
// Type: User[]
const newUser = await api.request("/users", "POST", {
body: { name: "John", email: "john@example.com" },
});
// Type: User
const user = await api.request("/users/:id", "GET", {
params: { id: "123" },
});
// Type: User
Pattern 3: Builder Pattern with Type Safety
type BuilderState<T> = {
[K in keyof T]: T[K] | undefined;
};
type RequiredKeys<T> = {
[K in keyof T]-?: {} extends Pick<T, K> ? never : K;
}[keyof T];
type OptionalKeys<T> = {
[K in keyof T]-?: {} extends Pick<T, K> ? K : never;
}[keyof T];
type IsComplete<T, S> =
RequiredKeys<T> extends keyof S
? S[RequiredKeys<T>] extends undefined
? false
: true
: false;
class Builder<T, S extends BuilderState<T> = {}> {
private state: S = {} as S;
set<K extends keyof T>(key: K, value: T[K]): Builder<T, S & Record<K, T[K]>> {
this.state[key] = value;
return this as any;
}
build(this: IsComplete<T, S> extends true ? this : never): T {
return this.state as T;
}
}
interface User {
id: string;
name: string;
email: string;
age?: number;
}
const builder = new Builder<User>();
const user = builder
.set("id", "1")
.set("name", "John")
.set("email", "john@example.com")
.build(); // OK: all required fields set
// const incomplete = builder
// .set("id", "1")
// .build(); // Error: missing required fields
Pattern 4: Deep Readonly/Partial
type DeepReadonly<T> = {
readonly [P in keyof T]: T[P] extends object
? T[P] extends Function
? T[P]
: DeepReadonly<T[P]>
: T[P];
};
type DeepPartial<T> = {
[P in keyof T]?: T[P] extends object
? T[P] extends Array<infer U>
? Array<DeepPartial<U>>
: DeepPartial<T[P]>
: T[P];
};
interface Config {
server: {
host: string;
port: number;
ssl: {
enabled: boolean;
cert: string;
};
};
database: {
url: string;
pool: {
min: number;
max: number;
};
};
}
type ReadonlyConfig = DeepReadonly<Config>;
// All nested properties are readonly
type PartialConfig = DeepPartial<Config>;
// All nested properties are optional
Pattern 5: Type-Safe Form Validation
type ValidationRule<T> = {
validate: (value: T) => boolean;
message: string;
};
type FieldValidation<T> = {
[K in keyof T]?: ValidationRule<T[K]>[];
};
type ValidationErrors<T> = {
[K in keyof T]?: string[];
};
class FormValidator<T extends Record<string, any>> {
constructor(private rules: FieldValidation<T>) {}
validate(data: T): ValidationErrors<T> | null {
const errors: ValidationErrors<T> = {};
let hasErrors = false;
for (const key in this.rules) {
const fieldRules = this.rules[key];
const value = data[key];
if (fieldRules) {
const fieldErrors: string[] = [];
for (const rule of fieldRules) {
if (!rule.validate(value)) {
fieldErrors.push(rule.message);
}
}
if (fieldErrors.length > 0) {
errors[key] = fieldErrors;
hasErrors = true;
}
}
}
return hasErrors ? errors : null;
}
}
interface LoginForm {
email: string;
password: string;
}
const validator = new FormValidator<LoginForm>({
email: [
{
validate: (v) => v.includes("@"),
message: "Email must contain @",
},
{
validate: (v) => v.length > 0,
message: "Email is required",
},
],
password: [
{
validate: (v) => v.length >= 8,
message: "Password must be at least 8 characters",
},
],
});
const errors = validator.validate({
email: "invalid",
password: "short",
});
// Type: { email?: string[]; password?: string[]; } | null
Pattern 6: Discriminated Unions
type Success<T> = {
status: "success";
data: T;
};
type Error = {
status: "error";
error: string;
};
type Loading = {
status: "loading";
};
type AsyncState<T> = Success<T> | Error | Loading;
function handleState<T>(state: AsyncState<T>): void {
switch (state.status) {
case "success":
console.log(state.data); // Type: T
break;
case "error":
console.log(state.error); // Type: string
break;
case "loading":
console.log("Loading...");
break;
}
}
// Type-safe state machine
type State =
| { type: "idle" }
| { type: "fetching"; requestId: string }
| { type: "success"; data: any }
| { type: "error"; error: Error };
type Event =
| { type: "FETCH"; requestId: string }
| { type: "SUCCESS"; data: any }
| { type: "ERROR"; error: Error }
| { type: "RESET" };
function reducer(state: State, event: Event): State {
switch (state.type) {
case "idle":
return event.type === "FETCH"
? { type: "fetching", requestId: event.requestId }
: state;
case "fetching":
if (event.type === "SUCCESS") {
return { type: "success", data: event.data };
}
if (event.type === "ERROR") {
return { type: "error", error: event.error };
}
return state;
case "success":
case "error":
return event.type === "RESET" ? { type: "idle" } : state;
}
}
Type Inference Techniques
1. Infer Keyword
// Extract array element type
type ElementType<T> = T extends (infer U)[] ? U : never;
type NumArray = number[];
type Num = ElementType<NumArray>; // number
// Extract promise type
type PromiseType<T> = T extends Promise<infer U> ? U : never;
type AsyncNum = PromiseType<Promise<number>>; // number
// Extract function parameters
type Parameters<T> = T extends (...args: infer P) => any ? P : never;
function foo(a: string, b: number) {}
type FooParams = Parameters<typeof foo>; // [string, number]
2. Type Guards
function isString(value: unknown): value is string {
return typeof value === "string";
}
function isArrayOf<T>(
value: unknown,
guard: (item: unknown) => item is T,
): value is T[] {
return Array.isArray(value) && value.every(guard);
}
const data: unknown = ["a", "b", "c"];
if (isArrayOf(data, isString)) {
data.forEach((s) => s.toUpperCase()); // Type: string[]
}
3. Assertion Functions
function assertIsString(value: unknown): asserts value is string {
if (typeof value !== "string") {
throw new Error("Not a string");
}
}
function processValue(value: unknown) {
assertIsString(value);
// value is now typed as string
console.log(value.toUpperCase());
}
