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GitHub Actions: Custom JS Actions

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GitHub Actions: Custom JS Actions
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GitHub Actions: Custom JS Actions

"A shell script is a quick fix. A Custom JavaScript Action is a professional tool."

In Module 111, we learned how to use "GitHub Script" inside a YAML file. But what if you have a complex logic (like "Calculating Cost of Cloud Resources") and you want to Reuse it in 50 different repositories? You shouldn't copy-paste 100 lines of JavaScript into every YAML.

You should build a Custom JavaScript Action. This 1,500+ word guide explores the "Toolkit" of action development: action.yml, the @actions/core library, and the Binary Distribution strategy of 2026.

1. The Anatomy: action.yml

This is the "Contract" of your action. It tells GitHub: "My name is TopicTrick Deployer, and I need two inputs: 'token' and 'destination'."

yaml




2. Code: The @actions/core Library


Instead of console.log, you use a professional library provided by GitHub.


javascript




3. The "Dist" Problem: Handling Node_Modules


GitHub runners do NOT run npm install for your custom action. They expect a Single File with all dependencies included.


    

  • The Solution: @vercel/ncc (The Node Compiler).
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  • You write your code in 10 different files with many libraries.
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  • You run ncc build index.js.
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  • It gives you One single index.js file that you commit to your repo.
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4. Input-Output Lifecycle


A great action should be "Pure."


    

  • Input: What does the user want? (e.g., user_id, message).
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  • Logic: Perform the work.
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  • Output: Tell the user what happened so the NEXT step in their YAML can use it.
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  • Example: An "Image Opimizer" action takes a .png, outputs a .webp, and provides an output named compressed_size.
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5. Testing: The "Mock" Runner


How do you test your action without pushing code and waiting 5 minutes for a GitHub Runner?


    

  • You use a tool like act.
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  • It simulates the GitHub environment on your laptop.
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  • You can run your custom action locally, fix the bugs in 1 second, and only push when it's perfect.
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Summary: The Development Checklist


    

  1. Metadata: Define clear inputs and outputs in action.yml.
    2. 

  2. Core Lib: Use @actions/core for input-handling and failure reporting.
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  3. NCC Build: Always compile your Node.js code into a single distribution file.
    4. 

  4. Version: Use Semantic Versioning (v1, v2) for your action repo.
    5. 

  5. Documentation: Write a README that shows exactly how to use the uses syntax for your action.
    6. 



Custom Actions are the "Plugins" of the DevOps world. By mastering the development of Node-based automation, you gain the ability to build and sell (or share) professional-grade tools that solve complex problems for thousands of other developers. You graduate from "Running code" to "Architecting the Toolchain."




Part of the GitHub Mastery Course — engineering the tools.