Master TypeScript's advanced type system including generics, conditional types, mapped types, template literals, and utility types for building type-safe applications. Use when implementing complex type logic, creating reusable type utilities, or ensuring compile-time type safety in TypeScript projects.
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Comprehensive guidance for mastering TypeScript's advanced type system including generics, conditional types, mapped types, template literal types, and utility types for building robust, type-safe applications.
Purpose: Create reusable, type-flexible components while maintaining type safety.
Basic Generic Function:
function identity<T>(value: T): T {
return value;
}
const num = identity<number>(42); // Type: number
const str = identity<string>("hello"); // Type: string
const auto = identity(true); // Type inferred: boolean
Generic Constraints:
interface HasLength {
length: number;
}
function logLength<T extends HasLength>(item: T): T {
console.log(item.length);
return item;
}
logLength("hello"); // OK: string has length
logLength([1, 2, 3]); // OK: array has length
logLength({ length: 10 }); // OK: object has length
// logLength(42); // Error: number has no length
Multiple Type Parameters:
function merge<T, U>(obj1: T, obj2: U): T & U {
return { ...obj1, ...obj2 };
}
const merged = merge({ name: "John" }, { age: 30 });
// Type: { name: string } & { age: number }
Purpose: Create types that depend on conditions, enabling sophisticated type logic.
Basic Conditional Type:
type IsString<T> = T extends string ? true : false;
type A = IsString<string>; // true
type B = IsString<number>; // false
Extracting Return Types:
type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;
function getUser() {
return { id: 1, name: "John" };
}
type User = ReturnType<typeof getUser>;
// Type: { id: number; name: string; }
Distributive Conditional Types:
type ToArray<T> = T extends any ? T[] : never;
type StrOrNumArray = ToArray<string | number>;
// Type: string[] | number[]
Nested Conditions:
type TypeName<T> = T extends string
? "string"
: T extends number
? "number"
: T extends boolean
? "boolean"
: T extends undefined
? "undefined"
: T extends Function
? "function"
: "object";
type T1 = TypeName<string>; // "string"
type T2 = TypeName<() => void>; // "function"
Purpose: Transform existing types by iterating over their properties.
Basic Mapped Type:
type Readonly<T> = {
readonly [P in keyof T]: T[P];
};
interface User {
id: number;
name: string;
}
type ReadonlyUser = Readonly<User>;
// Type: { readonly id: number; readonly name: string; }
Optional Properties:
type Partial<T> = {
[P in keyof T]?: T[P];
};
type PartialUser = Partial<User>;
// Type: { id?: number; name?: string; }
Key Remapping:
type Getters<T> = {
[K in keyof T as `get${Capitalize<string & K>}`]: () => T[K];
};
interface Person {
name: string;
age: number;
}
type PersonGetters = Getters<Person>;
// Type: { getName: () => string; getAge: () => number; }
Filtering Properties:
type PickByType<T, U> = {
[K in keyof T as T[K] extends U ? K : never]: T[K];
};
interface Mixed {
id: number;
name: string;
age: number;
active: boolean;
}
type OnlyNumbers = PickByType<Mixed, number>;
// Type: { id: number; age: number; }
Purpose: Create string-based types with pattern matching and transformation.
Basic Template Literal:
type EventName = "click" | "focus" | "blur";
type EventHandler = `on${Capitalize<EventName>}`;
// Type: "onClick" | "onFocus" | "onBlur"
String Manipulation:
type UppercaseGreeting = Uppercase<"hello">; // "HELLO"
type LowercaseGreeting = Lowercase<"HELLO">; // "hello"
type CapitalizedName = Capitalize<"john">; // "John"
type UncapitalizedName = Uncapitalize<"John">; // "john"
Path Building:
type Path<T> = T extends object
? {
[K in keyof T]: K extends string ? `${K}` | `${K}.${Path<T[K]>}` : never;
}[keyof T]
: never;
interface Config {
server: {
host: string;
port: number;
};
database: {
url: string;
};
}
type ConfigPath = Path<Config>;
// Type: "server" | "database" | "server.host" | "server.port" | "database.url"
Built-in Utility Types:
// Partial<T> - Make all properties optional
type PartialUser = Partial<User>;
// Required<T> - Make all properties required
type RequiredUser = Required<PartialUser>;
// Readonly<T> - Make all properties readonly
type ReadonlyUser = Readonly<User>;
// Pick<T, K> - Select specific properties
type UserName = Pick<User, "name" | "email">;
// Omit<T, K> - Remove specific properties
type UserWithoutPassword = Omit<User, "password">;
// Exclude<T, U> - Exclude types from union
type T1 = Exclude<"a" | "b" | "c", "a">; // "b" | "c"
// Extract<T, U> - Extract types from union
type T2 = Extract<"a" | "b" | "c", "a" | "b">; // "a" | "b"
// NonNullable<T> - Exclude null and undefined
type T3 = NonNullable<string | null | undefined>; // string
// Record<K, T> - Create object type with keys K and values T
type PageInfo = Record<"home" | "about", { title: string }>;
Detailed sections (starting with ## Advanced Patterns) live in references/details.md. Read that file when the navigation summary above is insufficient.
unknown over any: Enforce type checkinginterface for object shapes: Better error messagestype for unions and complex types: More flexible// Type assertion tests
type AssertEqual<T, U> = [T] extends [U]
? [U] extends [T]
? true
: false
: false;
type Test1 = AssertEqual<string, string>; // true
type Test2 = AssertEqual<string, number>; // false
type Test3 = AssertEqual<string | number, string>; // false
// Expect error helper
type ExpectError<T extends never> = T;
// Example usage
type ShouldError = ExpectError<AssertEqual<string, number>>;
any: Defeats the purpose of TypeScript
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