oliverdavies.uk/content/node.c72430aa-0c6d-4cf8-8a43-812d245ea73d.yml

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title:
  - value: 'How I work around legacy code'
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  - value: '2025-02-06T00:00:00+00:00'
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  - value: |
      <p>A few days ago, I mentioned <a href="/daily/2025/02/03/testable">a method that was over 150 lines long</a>.</p>

      <p>It had too many responsibilities, nested conditions, no dependency injection and no tests.</p>

      <p>Changing it would be risky, so how would I go about it?</p>

      <p>Let's assume I have this PHP method that contains the existing logic:</p>

      <pre><code class="php">function doSomething() {
        // 150 lines of legacy code...
      }
      </code></pre>

      <p>I can create a new class that's clean and simple, with whatever automated tests and checks I want.</p>

      <p>Let's say it has an <code>execute()</code> method that returns a boolean value:</p>

      <pre><code class="php">class NewService {

        public function execute() {
          return TRUE;
        }

      }
      </code></pre>

      <p>Because it's been tested in isolation, we can be confident it works as needed, so I can use it in the legacy function.</p>

      <p>I want to try and find a seam where I can use the new service and check for a result.</p>

      <p>Ideally, this would be as soon as possible within the function:</p>

      <pre><code class="php">function doSomething() {
        $newResult = $this-&gt;newService-&gt;execute();

        if ($newResult) {
          return $newResult;
        }

        // 150 lines of legacy code...
      }
      </code></pre>

      <p>If the new service returns a result, use it and return early, and don't execute the legacy code.</p>

      <p>Otherwise, run the original code as it would have been before, falling back to the original logic and result.</p>

      <p>Over time, more logic can be migrated into the new service until the legacy code is no longer used and can be removed.</p>

      <h2 id="here%27s-the-thing">Here's the thing</h2>

      <p>Writing tests for legacy code can be difficult or sometimes impossible.</p>

      <p>This approach means new code can be written using tests and test-driven development, dependency injection and whatever else you want without being limited by the existing code.</p>

      <p>You can incrementally migrate to the new approach and refactor out the old code, making it less risky than an all or nothing approach.</p>

      <p>Do you do the same thing or do you handle legacy code in a different way?</p>

              
    format: full_html
    processed: |
      <p>A few days ago, I mentioned <a href="/daily/2025/02/03/testable">a method that was over 150 lines long</a>.</p>

      <p>It had too many responsibilities, nested conditions, no dependency injection and no tests.</p>

      <p>Changing it would be risky, so how would I go about it?</p>

      <p>Let's assume I have this PHP method that contains the existing logic:</p>

      <pre><code class="php">function doSomething() {
        // 150 lines of legacy code...
      }
      </code></pre>

      <p>I can create a new class that's clean and simple, with whatever automated tests and checks I want.</p>

      <p>Let's say it has an <code>execute()</code> method that returns a boolean value:</p>

      <pre><code class="php">class NewService {

        public function execute() {
          return TRUE;
        }

      }
      </code></pre>

      <p>Because it's been tested in isolation, we can be confident it works as needed, so I can use it in the legacy function.</p>

      <p>I want to try and find a seam where I can use the new service and check for a result.</p>

      <p>Ideally, this would be as soon as possible within the function:</p>

      <pre><code class="php">function doSomething() {
        $newResult = $this-&gt;newService-&gt;execute();

        if ($newResult) {
          return $newResult;
        }

        // 150 lines of legacy code...
      }
      </code></pre>

      <p>If the new service returns a result, use it and return early, and don't execute the legacy code.</p>

      <p>Otherwise, run the original code as it would have been before, falling back to the original logic and result.</p>

      <p>Over time, more logic can be migrated into the new service until the legacy code is no longer used and can be removed.</p>

      <h2 id="here%27s-the-thing">Here's the thing</h2>

      <p>Writing tests for legacy code can be difficult or sometimes impossible.</p>

      <p>This approach means new code can be written using tests and test-driven development, dependency injection and whatever else you want without being limited by the existing code.</p>

      <p>You can incrementally migrate to the new approach and refactor out the old code, making it less risky than an all or nothing approach.</p>

      <p>Do you do the same thing or do you handle legacy code in a different way?</p>

              
    summary: null
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