> For the complete documentation index, see [llms.txt](https://hanfak.gitbook.io/workspace/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hanfak.gitbook.io/workspace/testing-reading-maintaining/legacy-code.md).

# Legacy Code

## What?

* Many different definitions (See links)
  * but mainly about code that is made sometime ago by developers who are no longer around,
  * using technologies/libraries which are outdated or not supported,
  * and probably written with bad structure,
  * poor or little testing,
  * poorly engineered
  * not kept upto date
  * Code that predates current team
  * that has not been touched in living memory
  * But they are still used by the business and need to be maintained, thus making it hard to do so.
* Legacy Code is valuable code you’re afraid to change.
  * Without tests, makes it hard to change without knowing if something has broken
    * code without tests is tricky to change without introducing a regression somewhere.
    * But Code with tests can also be Legacy Code
      * Poorly written tests can get in the way
        * ie Only there for test coverage, doing too much, hard to understand
    * Also some code bases, or parts/modules may not have test but easy to change
      * Much rarer
  * Better definition is valuable code you're afraid to change
    * For example, searching for a bug and making a change to fix it
    * Contributing factors:
      * Unfamiliarity with the code plays a lot
        * We overestimate the complexity of unfamiliar code.
        * Not working on the code base recently or changing context from another codebase/domain can affect remembering how it works or navigating it
      * Good tests make you comfortable changing unfamiliar code
      * It gets better after a few months.
        * Time spent working on it, improves ability to work with it
        * The way you feel about it, becomes less dread and more acceptance
      * Most of the code is terrible
        * At the time it might have been good, but it rot sets in

<https://understandlegacycode.com/all-articles/>

## Reasons for dealing with legacy code

* It's legacy cause it works and does not need to changed (Rarely does), but there are cases when it does
* It needs to change
  * internal reasons
    * New features
    * new bugs
  * external reasons
    * ports to new hardware or operating systems
    * new versions of dependencies, language
    * security, vulnerabilities
    * new laws
    * new versions of api
    * api being decomissioned
    *