back
loading skill details...
Master TypeScript's advanced type system including generics, conditional types, mapped types, template literals, and utility types for building type-safe…
TypeScript Advanced Types
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.
When to Use This Skill
Building type-safe libraries or frameworks
Creating reusable generic components
Implementing complex type inference logic
Designing type-safe API clients
Building form validation systems
Creating strongly-typed configuration objects
Implementing type-safe state management
Migrating JavaScript codebases to TypeScript
Core Concepts
1. Generics
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 }
2. Conditional Types
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"
3. Mapped Types
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${Capitalize<string & K>}`]: () => T[K];
};
interface Person {
name: string;
age: number;
}
type PersonGetters = Getters<Person>;
// Type: { getName: () => string; getAge: () => number; }
Filtering Properties:
type PickByType<T, U> = {
[K in keyof T as T[K] extends U ? K : never]: T[K];
};
interface Mixed {
id: number;
name: string;
age: number;
active: boolean;
}
type OnlyNumbers = PickByType<Mixed, number>;
// Type: { id: number; age: number; }
4. Template Literal Types
Purpose: Create string-based types with pattern matching and transformation.
Basic Template Literal:
type EventName = "click" | "focus" | "blur";
type EventHandler = `on${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"
5. Utility Types
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 worked examples and patterns
Detailed sections (starting with ## Advanced Patterns) live in references/details.md. Read that file when the navigation summary above is insufficient.
Best Practices
Use unknown over any: Enforce type checking
Prefer interface for object shapes: Better error messages
Use type for unions and complex types: More flexible
Leverage type inference: Let TypeScript infer when possible
Create helper types: Build reusable type utilities
Use const assertions: Preserve literal types
Avoid type assertions: Use type guards instead
Document complex types: Add JSDoc comments
Use strict mode: Enable all strict compiler options
Test your types: Use type tests to verify type behavior
Type Testing
// 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>>;
Common Pitfalls
Over-using any: Defeats the purpose of TypeScript
Ignoring strict null checks: Can lead to runtime errors
Too complex types: Can slow down compilation
Not using discriminated unions: Misses type narrowing opportunities
Forgetting readonly modifiers: Allows unintended mutations
Circular type references: Can cause compiler errors
Not handling edge cases: Like empty arrays or null values
Performance Considerations
Avoid deeply nested conditional types
Use simple types when possible
Cache complex type computations
Limit recursion depth in recursive types
Use build tools to skip type checking in productiondon't have the plugin yet? install it then click "run inline in claude" again.