Advanced TypeScript: A Deep Dive into Modern TypeScript Development

Introduction

TypeScript has become the go-to language for building scalable JavaScript applications. In this comprehensive guide, we'll explore advanced TypeScript concepts that will enhance your development skills and help you write more type-safe code.

1. Advanced Type System Features

Conditional Types

Understanding complex type relationships:

type IsArray<T> = T extends any[] ? true : false;
type IsString<T> = T extends string ? true : false;

// Usage
type CheckArray = IsArray<string[]>; // true
type CheckString = IsString<"hello">; // true

// More complex conditional types
type UnwrapPromise<T> = T extends Promise<infer U> ? U : T;
type ArrayElement<T> = T extends (infer U)[] ? U : never;

// Example usage
type PromiseString = UnwrapPromise<Promise<string>>; // string
type NumberArray = ArrayElement<number[]>; // number

Template Literal Types

Leveraging string literal types for better type safety:

type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'DELETE';
type APIEndpoint = '/users' | '/posts' | '/comments';

type APIRoute = `${HTTPMethod} ${APIEndpoint}`;

// Valid routes
const validRoute: APIRoute = 'GET /users';
const validRoute2: APIRoute = 'POST /posts';

// Error: Type '"PATCH /users"' is not assignable to type 'APIRoute'
// const invalidRoute: APIRoute = 'PATCH /users';

// Dynamic template literal types
type PropEventType<T extends string> = `on${Capitalize<T>}`;
type ButtonEvents = PropEventType<'click' | 'hover' | 'focus'>;
// Results in: 'onClick' | 'onHover' | 'onFocus'

2. Advanced Generics

Generic Constraints and Defaults

Creating flexible yet type-safe generic interfaces:

interface Database<T extends { id: string }> {
    find(id: string): Promise<T | null>;
    create(data: Omit<T, 'id'>): Promise<T>;
    update(id: string, data: Partial<T>): Promise<T>;
    delete(id: string): Promise<boolean>;
}

// Implementation example
class MongoDatabase<T extends { id: string }> implements Database<T> {
    constructor(private collection: string) {}

    async find(id: string): Promise<T | null> {
        // Implementation
        return null;
    }

    async create(data: Omit<T, 'id'>): Promise<T> {
        // Implementation
        return { id: 'generated', ...data } as T;
    }

    async update(id: string, data: Partial<T>): Promise<T> {
        // Implementation
        return { id, ...data } as T;
    }

    async delete(id: string): Promise<boolean> {
        // Implementation
        return true;
    }
}

Mapped Types and Key Remapping

Advanced type transformations:

type Getters<T> = {
    [K in keyof T as `get${Capitalize<string & K>}`]: () => T[K]
};

interface Person {
    name: string;
    age: number;
}

type PersonGetters = Getters<Person>;
// Results in:
// {
//     getName: () => string;
//     getAge: () => number;
// }

// Advanced key remapping with filtering
type FilteredKeys<T, U> = {
    [K in keyof T as T[K] extends U ? K : never]: T[K]
};

interface Mixed {
    name: string;
    count: number;
    isActive: boolean;
    data: object;
}

type StringKeys = FilteredKeys<Mixed, string>;
// Results in: { name: string }

3. Advanced Decorators

Custom Property Decorators

Creating powerful metadata-driven decorators:

function ValidateProperty(validationFn: (value: any) => boolean) {
    return function (target: any, propertyKey: string) {
        let value: any;

        const getter = function() {
            return value;
        };

        const setter = function(newVal: any) {
            if (!validationFn(newVal)) {
                throw new Error(`Invalid value for ${propertyKey}`);
            }
            value = newVal;
        };

        Object.defineProperty(target, propertyKey, {
            get: getter,
            set: setter,
            enumerable: true,
            configurable: true,
        });
    };
}

class User {
    @ValidateProperty((value) => typeof value === 'string' && value.length > 0)
    name: string;

    @ValidateProperty((value) => typeof value === 'number' && value >= 0)
    age: number;

    constructor(name: string, age: number) {
        this.name = name;
        this.age = age;
    }
}

4. Advanced Utility Types

Custom Utility Types

Building powerful type transformations:

// Deep Partial type
type DeepPartial<T> = {
    [P in keyof T]?: T[P] extends object
        ? DeepPartial<T[P]>
        : T[P];
};

// Deep Required type
type DeepRequired<T> = {
    [P in keyof T]-?: T[P] extends object
        ? DeepRequired<T[P]>
        : T[P];
};

// Deep Readonly type
type DeepReadonly<T> = {
    readonly [P in keyof T]: T[P] extends object
        ? DeepReadonly<T[P]>
        : T[P];
};

// Example usage
interface Config {
    server: {
        port: number;
        host: string;
        options: {
            timeout: number;
            retries: number;
        };
    };
    database: {
        url: string;
        name: string;
    };
}

type PartialConfig = DeepPartial<Config>;
// Now we can have partial nested objects
const config: PartialConfig = {
    server: {
        port: 3000
        // host and options can be omitted
    }
};

5. Type-Safe API Patterns

Builder Pattern with Type Safety

Implementing the builder pattern with full type safety:

class RequestBuilder<T = {}> {
    private data: T;

    constructor(data: T = {} as T) {
        this.data = data;
    }

    with<K extends string, V>(
        key: K,
        value: V
    ): RequestBuilder<T & { [key in K]: V }> {
        return new RequestBuilder({
            ...this.data,
            [key]: value,
        });
    }

    build(): T {
        return this.data;
    }
}

// Usage
const request = new RequestBuilder()
    .with('url', 'https://api.example.com')
    .with('method', 'GET')
    .with('headers', { 'Content-Type': 'application/json' })
    .build();

// Type is inferred correctly
type Request = typeof request;
// {
//     url: string;
//     method: string;
//     headers: { 'Content-Type': string };
// }

6. Advanced Error Handling

Type-Safe Error Handling

Creating a robust error handling system:

class Result<T, E extends Error> {
    private constructor(
        private value: T | null,
        private error: E | null
    ) {}

    static ok<T>(value: T): Result<T, never> {
        return new Result(value, null);
    }

    static err<E extends Error>(error: E): Result<never, E> {
        return new Result(null, error);
    }

    map<U>(fn: (value: T) => U): Result<U, E> {
        if (this.value === null) {
            return new Result(null, this.error);
        }
        return new Result(fn(this.value), null);
    }

    mapError<F extends Error>(fn: (error: E) => F): Result<T, F> {
        if (this.error === null) {
            return new Result(this.value, null);
        }
        return new Result(null, fn(this.error));
    }

    unwrap(): T {
        if (this.value === null) {
            throw this.error;
        }
        return this.value;
    }
}

// Usage example
function divide(a: number, b: number): Result<number, Error> {
    if (b === 0) {
        return Result.err(new Error('Division by zero'));
    }
    return Result.ok(a / b);
}

const result = divide(10, 2)
    .map(n => n * 2)
    .unwrap(); // 10

Conclusion

These advanced TypeScript patterns demonstrate the language's power in creating type-safe and maintainable applications. By mastering these concepts, you'll be better equipped to build robust applications that leverage TypeScript's type system to its fullest potential.

Additional Resources

• TypeScript Documentation

• TypeScript Deep Dive

• TypeScript GitHub Repository

Share your experiences with these patterns in the comments below! What advanced TypeScript features have you found most useful in your projects?

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Tags: #typescript #javascript #webdevelopment #programming #typing

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