Apprendre Practical Patterns for Async Typing | Advanced Typing and Asynchronous Patterns

When building asynchronous workflows in TypeScript, you gain powerful tools for error handling and for ensuring type safety throughout your code. TypeScript lets you define the exact shape of data you expect to receive from APIs, process in background jobs, or iterate over asynchronously. This helps you catch mistakes at compile time rather than at runtime, leading to more reliable applications and fewer production bugs.

You often interact with APIs that return data in unpredictable formats. With TypeScript, you can define interfaces or types that represent the expected response, and use them to validate the data as soon as it arrives. This not only improves error handling—by making it clear when data does not match expectations—but also enables better tooling and auto-completion in your editor.


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// Define the expected type for the API response
type User = {
  id: number;
  name: string;
  email: string;
};

// Fetch data from an API and ensure the response matches the User type
async function fetchUser(userId: number): Promise<User> {
  const response = await fetch(`https://api.example.com/users/${userId}`);
  const data: unknown = await response.json();

  // Type guard to validate the response
  if (
    typeof data === "object" &&
    data !== null &&
    typeof (data as any).id === "number" &&
    typeof (data as any).name === "string" &&
    typeof (data as any).email === "string"
  ) {
    return data as User;
  } else {
    throw new Error("API response does not match expected User type");
  }
}

By specifying the User type and validating the structure of the response, you protect your code from unexpected data and ensure downstream consumers can rely on the shape of the object. This pattern is especially important in asynchronous code, where errors can be harder to trace.

TypeScript also shines when orchestrating complex async workflows, such as processing a queue of background jobs. You can use types to describe the jobs, their payloads, and the results, making your workflow robust and easy to maintain.


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// Define a type for job payloads
type JobPayload = {
  jobId: string;
  data: { value: number };
};

// Define a type for job results
type JobResult = {
  jobId: string;
  success: boolean;
  result?: number;
  error?: string;
};

// Async function to process a single job
async function processJob(payload: JobPayload): Promise<JobResult> {
  try {
    // Simulate async work (e.g., database operation)
    const result = payload.data.value * 2;
    return { jobId: payload.jobId, success: true, result };
  } catch (e) {
    return { jobId: payload.jobId, success: false, error: String(e) };
  }
}

// Process a queue of jobs asynchronously with type safety
async function processJobQueue(queue: JobPayload[]): Promise<JobResult[]> {
  const results: JobResult[] = [];
  for (const job of queue) {
    const result = await processJob(job);
    results.push(result);
  }
  return results;
}

This approach ensures that both the input and output of each async step are well-defined, allowing you to handle errors gracefully and avoid accidental type mismatches.

Async iterators in TypeScript let you use the for await...of syntax to consume data from sources that produce values asynchronously. By typing the iterator, you ensure each value yielded matches your expectations, reducing the risk of runtime errors and making your code easier to reason about.

1. Why is it important to type API responses in asynchronous code?

2. What is an async iterator in TypeScript?

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Section 3. Chapitre 3

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