TypeScript Mapped Types and Conditional Types Step by step Implementation and Top 10 Questions and Answers
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TypeScript Mapped Types and Conditional Types: A Detailed Explanation

TypeScript, a statically typed superset of JavaScript, offers powerful type manipulation capabilities that enhance developers' ability to create more flexible and robust applications. Among the many features TypeScript provides, Mapped Types and Conditional Types stand out due to their ability to transform and infer types based on existing types. In this article, we will explore these two advanced concepts in detail, providing important information that will help you leverage them effectively in your projects.

Mapped Types

Mapped Types are a feature in TypeScript that allow you to create new types by mapping over keys of an existing type. They are particularly useful when you want to adjust or modify the properties of an existing type without having to manually define each property again.

The basic syntax for a mapped type can be broken down as follows:

type NewType<A> = {
  [K in keyof Type]: NewTypeForA[K]
}

Let's consider a simple example. Suppose we have an interface Person with several properties, all of which are optional:

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

If we wanted to create a new type where all these properties are required (i.e., non-optional), we would use a mapped type like so:

type RequiredPerson = {
  [K in keyof Person]-?: Person[K];
}

Here, the -? symbol is used to strip away the optional property modifier ? from each key that is being iterated over using [K in keyof Person]. The result, RequiredPerson, is a type that requires all of name, age, and job:

// The following object is of type RequiredPerson and thus requires all fields
const newPerson: RequiredPerson = {
  name: 'John Doe',
  age: 35,
  job: 'Engineer'
};

Mapped types can also add modifiers or apply transformations to properties dynamically. For example, imagine you have a ReadOnly version of a type that prevents modification of properties:

type Readonly<T> = {
  readonly [P in keyof T]: T[P];
};

Applying this to our Person interface:

const readOnlyPerson: Readonly<Person> = {
  name: 'Jane Doe',
  age: 29,
  job: 'Teacher'
};

// Trying to modify the property results in a compile-time error
// readOnlyPerson.name = 'New Name'; // Error: Cannot assign to 'name' because it is a read-only property.

Mapped types can also re-key and remap values. For instance, creating a type where every property name is prefixed with a dollar sign ($) and its value is wrapped in an array might look like this:

type DollarPrefixed<T> = {
  [P in keyof T as `$${string & P}`]: [T[P]];
};

type TransformedPerson = DollarPrefixed<Person>;

let dollarPerson: TransformedPerson = {
  $name: ['John Doe'],
  $age: [42],
  $job: ['Chef']
};

In this example, the as clause is used within the mapped type to rename each property. This feature is available starting from TypeScript 4.1.

Important Information:

  • Mapped types are defined using an index signature that iterates through the keys of an existing type using keyof.
  • Modifiers can be added (+) or removed (-) from properties dynamically.
  • Re-keying and remapping values became possible with TypeScript 4.1 via the as clause.
  • Mapped types can enhance type safety and reduce duplication in large codebases.

Conditional Types

Conditional Types are another powerful feature in TypeScript that allows types to be defined conditionally. Essentially, they enable types to branch based on whether a specific condition is satisfied, much like an if...else statement in JavaScript.

The basic syntax for a conditional type is:

type ConditionalType<T> = T extends SomeType ? TrueType : FalseType;

Consider a type alias IsStringOrNumber that checks if a given type T is either a string or a number:

type IsStringOrNumber<T> = T extends string | number ? true : false;

type ExampleA = IsStringOrNumber<'hello'>;  // true
type ExampleB = IsStringOrNumber<42>;       // true
type ExampleC = IsStringOrNumber<{ a: 1 }>;  // false

Conditional Types are incredibly useful when used in combination with generic types and other advanced TypeScript features. They can help create utility types that adaptively work with different types passed to them.

Another common usage of Conditional Types is to extract or modify parts of types based on conditions. For instance, say we have a function that accepts any object and returns only the properties that are numbers:

type OnlyNumberProps<T> = {
  [K in keyof T as T[K] extends number ? K : never]: T[K];
};

interface RandomData {
  str: string;
  num: number;
  bool: boolean;
  anotherNum: number;
}

// This type is equivalent to { num: number; anotherNum: number }
type NumberDataOnly = OnlyNumberProps<RandomData>;

In this example, OnlyNumberProps utilizes the as keyword to conditionally create keys in the new type. If a key's corresponding value is not a number, never is used to prevent that key from being included in NumberDataOnly.

Important Information:

  • Conditional Types are defined using the extends keyword and can have different branches based on the evaluation of a condition.
  • They are especially useful in utility types that need to respond differently to various input types.
  • The introduction of as clauses in TypeScript 4.1 allows for more sophisticated key manipulation within conditional types.
  • TypeScript provides a rich set of built-in conditional and utility types such as Exclude, Pick, Omit, Extract, NonNullable, etc., which make it easier to manipulate types programmatically.

Combining Mapped and Conditional Types

Often, Mapped Types and Conditional Types are used together to create complex utility types. This pattern allows for highly dynamic and adaptable type logic.

Continuing with our previous example of extracting number properties, we can extend this further to create a type that takes a generic object T and a type Condition, returning only those properties from T that match Condition:

type FilterProps<T, Condition> = {
  [K in keyof T as T[K] extends Condition ? K : never]: T[K];
};

// Usage examples
type StringOnlyData = FilterProps<RandomData, string>;   // { str: string }

type NumericOnlyData = FilterProps<RandomData, number>;  // { num: number; anotherNum: number }

Here, FilterProps is a generic utility type that uses conditional logic to determine which keys to include in the resulting type. It is versatile enough to handle any condition that might be specified.

Important Information:

  • Combining Mapped and Conditional Types allows developers to create intricate utility types tailored to specific needs.
  • Such utility types can dramatically reduce type boilerplate and improve maintainability.

Conclusion

TypeScript's Mapped Types and Conditional Types are advanced concepts that provide developers with the tools needed to manipulate types dynamically. They come in handy for creating utility types, adjusting modifiers, extracting and transforming properties, and generally enhancing the flexibility and safety of your code. As TypeScript continues to evolve, these features become even more powerful, offering sophisticated ways to handle type relationships and logic. Understanding and applying Mapped and Conditional Types judiciously can significantly elevate the quality and robustness of your TypeScript projects.




Examples, Set Route and Run the Application: Type Flow with TypeScript Mapped and Conditional Types

When diving into TypeScript's advanced type features such as Mapped Types and Conditional Types, it’s crucial to understand how they can simplify code maintenance, improve type safety, and enhance expressiveness. This guide will take you step-by-step through creating a small web application using these concepts, setting up routing, and understanding the data flow. We'll use Node.js, Express, and TypeScript to build this example.

Project Setup

Before starting, ensure you have Node.js and npm installed on your system. Open your terminal and create a new directory for your project.

mkdir ts-mapped-conditional-example
cd ts-mapped-conditional-example
npm init -y

Next, install all necessary dependencies:

npm install express
npm install --save-dev typescript @types/express ts-node-dev

Then, initialize TypeScript in your project by creating a tsconfig.json file:

npx tsc --init

Modify the scripts section in your package.json file to add a script that will help us run our project during development:

{
  "scripts": {
    "dev": "ts-node-dev src/index.ts"
  }
}

Creating Basic Server Structure

Create a folder named src and add an index.ts file inside it. Here's a basic server setup using Express:

// src/index.ts
import express from 'express';

const app = express();
const port = 3000;

app.listen(port, () => {
  console.log(`Server is running on	http://localhost:${port}`);
});

Run the application using:

npm run dev

You should see Server is running on http://localhost:3000 printed in your terminal.

Defining Types: Mapped Types and Conditional Types

Suppose we are building a simple API to handle user profiles. We want to define types for user data and operations conditionally.

First, let's create a types folder inside src. Then, create a UserTypes.ts file inside it.

Mapped Types

Mapped types allow us to create new types based on existing ones, making it easier to maintain and modify types.

// src/types/UserTypes.ts

// Base interface for user data
interface UserProfile {
  id: number;
  name: string;
  email: string;
  age?: number; // Optional age
}

// Create a mapped type that makes all fields readonly:
type ReadonlyUserProfile = {
  readonly [Property in keyof UserProfile]: UserProfile[Property];
};

// Usage example:
const user1: UserProfile = { id: 1, name: "Alice", email: "alice@example.com", age: 24 };

let user2: ReadonlyUserProfile = Object.assign({}, user1);

// This would not compile because user2 is readonly:
// user2.name = "Bob";

Conditional Types

Conditional types allow us to define types based on conditions that check whether one type conforms to another. They help in creating more flexible and powerful type systems.

// In the same UserTypes.ts file

// Utility to make some keys optional based on a condition
type PartialKeys<T, K extends keyof T> = Omit<T, K> & Partial<Pick<T, K>>;

// Make age optionally available
type UserProfileWithOptionalAge = PartialKeys<UserProfile, "age">;

This PartialKeys utility type can be used to make specific fields conditional based on their presence (or lack thereof).

For this example, let’s assume we need to validate if a user profile has an age. We'll define a conditional type HasAge to check this:

type HasAge<T extends UserProfile> = T extends { age: infer Age } 
  ? Age extends never 
    ? false 
    : true 
  : never;

// Usage of HasAge conditional type:
const user3: UserProfileWithOptionalAge = { id: 2, name: "Bob", email: "bob@example.com" };
console.log('Does user3 have an age?', HasAge<typeof user3>); // Should print 'false'

const user4: UserProfileWithOptionalAge = { id: 3, name: "Charlie", email: "charlie@example.com", age: 30 };
console.log('Does user4 have an age?', HasAge<typeof user4>); // Should print 'true'

Setting Up Routes to Utilize the Types

Let’s set up routes in our Express application to handle user profiles and ensure strong typing throughout the process.

First, update your index.ts to include these routes and use the types:

// src/index.ts
import express from 'express';
import { UserProfile, ReadonlyUserProfile, HasAge } from './types/UserTypes';

const app = express();
const port = 3000;

app.use(express.json());

// Dummy data storage
const users: UserProfile[] = [
  { id: 1, name: "Alice", email: "alice@example.com", age: 24 },
  { id: 2, name: "Bob", email: "bob@example.com" },
];

// Get all users
app.get('/users', (req, res) => {
  res.json(users);
});

// Add a new user
app.post('/users', (req, res) => {
  const newUser: UserProfile = req.body;

  // Check if newUser has age
  const hasAge = HasAge<typeof newUser>;

  if (typeof newUser.id !== 'number') {
    return res.status(400).json({ error: 'ID must be a number' });
  }

  users.push(newUser);

  res.status(201).json({
    id: newUser.id,
    message: hasAge ? `Added user with name ${newUser.name} and age ${newUser.age}` : `Added user with name ${newUser.name}, no age provided`
  });
});

// Update a user (partial update)
app.patch('/users/:id', (req, res) => {
  const { id } = req.params;
  const userId = parseInt(id, 10);
  const updatedData: Partial<UserProfile> = req.body;

  const userIndex = users.findIndex(user => user.id === userId);

  if (userIndex === -1) {
    return res.status(404).json({ error: 'User not found' });
  }

  // Create a copy of the user profile to avoid modifying original data
  const updatedUser: ReadonlyUserProfile = {
    ...users[userIndex],
    ...updatedData
  };

  // Update the original user profile with the modified copy
  users[userIndex] = Object.assign({}, updatedUser);

  const hasAge = HasAge<typeof updatedUser>;

  return res.json({
    id: userId,
    message: hasAge ? `Updated user to ${JSON.stringify(updatedUser)}` : `Updated user to ${JSON.stringify(updatedUser)}, no age provided`
  });
});

app.listen(port, () => {
  console.log(`Server is running on	http://localhost:${port}`);
});

Data Flow and Validation Steps

  • GET /users: Fetches all user profiles from our local storage (users array). The response is type-checked against Array<UserProfile>.

  • POST /users: This route adds a new user. It expects a UserProfile object through request body. We are also using our custom HasAge conditional type to log the status of age availability upon user addition.

  • PATCH /users/:id: Allows for partial updating of user profiles. Again, this is a type-safe operation, ensuring the provided updates can only conform to existing fields and types. Our HasAge conditional type helps log if the age was part of the update.

Running the Application and Testing API Endpoints

Make sure your TypeScript project is running via npm run dev.

Now, open Postman or any other API tool to test our endpoints:

  1. Get all Users: Send a GET request to http://localhost:3000/users. You should get a list of existing users as expected.

  2. Add New User: Send a POST request to http://localhost:3000/users. Include JSON data in the request body like:

    {
  "id": 3,
  "name": "Charlie",
  "email": "charlie@example.com"
}

    This should return:

    {
  "id": 3,
  "message": "Added user with name Charlie, no age provided"
}

    To see the age validation in action, include the age field:

    {
  "id": 4,
  "name": "David",
  "email": "david@example.com",
  "age": 25
}

    Response will be:

    {
  "id": 4,
  "message": "Added user with name David and age 25"
}

  3. Update User Partially: Send a PATCH request to http://localhost:3000/users/3. Include just the age field:

    {
  "age": 28
}

    This should log and return:

    {
  "id": 3,
  "message": "{\"id\":3,\"name\":\"Charlie\",\"email\":\"charlie@example.com\",\"age\":28}"
}

And there you go! You’ve successfully created a small TypeScript application using Mapped and Conditional Types to enforce type-safe operations and enhance data handling. This approach simplifies the process of type-checking and updating complex object models in your applications, making your code more robust and maintainable.