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<p>Refactoring is a crucial process in software development, aimed at improving the non-functional attributes of your codebase without altering its external behavior. It’s essential for maintaining code quality, readability, and scalability. In this article, we’ll explore how to refactor TypeScript code in a NestJS application that uses MongoDB, focusing on a VisitorsController as our example.</p>
<figure><img alt="line of codes" src="https://cdn-images-1.medium.com/max/1024/0*wzMOPlivH5iyawS9"><figcaption>Photo by <a href="https://unsplash.com/@thomastastet?utm_source=medium&amp;utm_medium=referral">Thomas Tastet</a> on <a href="https://unsplash.com/?utm_source=medium&amp;utm_medium=referral">Unsplash</a></figcaption></figure><h3>Initial Code Overview</h3>
<p>Our starting point involves a VisitorsController in a NestJS application, responsible for handling CRUD operations for visitors. It uses decorators like @Get, @Post, @Patch, and @Delete to define routes and @Body, @Param, and @Query to handle incoming data. The controller interacts with a VisitorsService to perform business logic operations.</p>
<h3>Areas for Refactoring</h3>
<ol>
<li>
<strong>Separation of Concerns</strong>: Ensure that the controller is only responsible for handling HTTP requests and delegating business logic to the service layer.</li>
<li>
<strong>Validation</strong>: Implement DTO (Data Transfer Object) validation to ensure the integrity of incoming data.</li>
<li>
<strong>Error Handling</strong>: Centralize error handling to keep the code clean and maintainable.</li>
<li>
<strong>Query Optimization</strong>: Improve the handling of query parameters to make the code more readable and efficient.</li>
</ol>
<h3>Step-by-Step Refactoring</h3>
<h3>Step 1: Enhancing DTO Validation</h3>
<p>NestJS supports class-validator and class-transformer packages to automate validation. We’ll enhance our CreateVisitorDto and UpdateVisitorDto with validation decorators.</p>
<pre>import { IsString, IsNotEmpty, IsOptional } from 'class-validator';<br><br>export class CreateVisitorDto {<br>  @IsString()<br>  @IsNotEmpty()<br>  readonly name: string;<br><br>  // Add other properties with appropriate validation decorators<br>}<br><br>export class UpdateVisitorDto {<br>  @IsString()<br>  @IsOptional()<br>  readonly name?: string;<br><br>  // Add other properties with appropriate validation decorators<br>}</pre>
<h3>Step 2: Centralizing Error Handling</h3>
<p>NestJS provides filter mechanisms to handle exceptions in a centralized way. We’ll create an HttpExceptionFilter to catch and format errors.</p>
<pre>import { ExceptionFilter, Catch, ArgumentsHost, HttpException } from '@nestjs/common';<br>import { Response } from 'express';<br><br>@Catch(HttpException)<br>export class HttpExceptionFilter implements ExceptionFilter {<br>  catch(exception: HttpException, host: ArgumentsHost) {<br>    const ctx = host.switchToHttp();<br>    const response = ctx.getResponse&lt;Response&gt;();<br>    const status = exception.getStatus();<br>    const exceptionResponse = exception.getResponse();<br><br>    response<br>      .status(status)<br>      .json({<br>        statusCode: status,<br>        message: exceptionResponse['message'],<br>        timestamp: new Date().toISOString(),<br>        path: ctx.getRequest&lt;Request&gt;().url,<br>      });<br>  }<br>}</pre>
<h3>Step 3: Optimizing Query Parameters Handling</h3>
<p>Instead of manually checking and logging query parameters in the findAll method, we can streamline this process using a custom decorator or by leveraging NestJS pipes for validation and transformation.</p>
<h3>Step 4: Implementing Dependency Injection for Better Testing</h3>
<p>Ensure all dependencies are injected via the constructor, allowing for easier testing and better modularity. This is already correctly implemented in our initial code.</p>
<h3>Step 5: Using Async/Await Consistently</h3>
<p>For consistency and to handle asynchronous operations effectively, ensure all methods in the controller that deal with promises use async/await.</p>
<pre>@Get(':id')<br>async findOne(@Param('id') id: string) {<br>  const visitor = await this.visitorsService.findOne(id);<br>  if (!visitor) throw new NotFoundException('Visitor not found');<br>  return visitor;<br>}</pre>
<h3>Conclusion</h3>
<p>Refactoring is not about rewriting code; it’s about making it more efficient, readable, and maintainable. By following these steps, we’ve improved our VisitorsController in several key areas, making it more robust and future-proof. Remember, the goal of refactoring is to make your code better, not different. Happy coding!</p>
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