Guide · Open source
How to use co: generator-based flow control for Node.js
Updated · written with AI from the project's own documentation and checked against our catalogue data
co is an MIT-licensed JavaScript library for Node.js that lets you write asynchronous code in a sequential style by running generator functions and resolving the values they yield, such as thunks and promises. This guide is for developers who maintain older code built around generators or want a small helper instead of a full framework. The README in the source we reviewed is empty, so this page sticks to what the project description and our analysis support.
cotj/co · ★ 11.8k · MIT Install options & downloads Quick start
- 1Check your Node.js setup. Confirm Node.js is installed, since it is the stated prerequisite for co.
- 2Clone the repository. Run git clone https://github.com/tj/co to get the source code locally.
- 3Read the source. The README is empty in the source we reviewed, so open the library files directly to learn its behaviour.
- 4Write a generator-based flow. Wrap your async steps in generator functions and yield each thunk or promise you need to wait for.
- 5Test your flow. Run your code and check that results and errors arrive in the order you expect.
What co does
co is described as generator-based flow control for Node.js. A generator function pauses at each yield, and co resumes it once the yielded value is resolved. The project description says it supports thunks, promises and similar patterns.
This lets callback-heavy or promise-chained code read top to bottom, in the order things happen.
Get the source code
The only install instruction in the source is a clone of the repository. Clone it with the command below, then look through the files to see how the library is structured.
Our analysis lists the install method as a package manager with Node.js as the prerequisite, but the source does not give a package-manager command, so check the repository for the exact steps before you rely on one.
- Install Node.js first, since it is the stated prerequisite.
- Clone the repository from GitHub.
git clone https://github.com/tj/coUnderstand the generator pattern
Write your async steps as generator functions and yield each asynchronous operation. co takes each yielded thunk or promise, waits for its result, and sends the value back into the generator.
Because the control flow reads sequentially, error handling and branching sit alongside the steps they affect rather than in nested callbacks.
Decide whether co fits your project
co is a good match when you already use generator functions, when you are reading or migrating code written in co style, or when you want a minimal helper rather than a framework.
For new code, native async/await covers the same need with built-in language syntax. Our analysis gives this as general reasoning, not something the README states.
- Keep co if your codebase already uses generators around it.
- Consider native async/await for new projects.
- Compare with Bluebird or async if you need a wider set of utilities.
Troubleshooting and limits
The source we reviewed does not include a usage guide, examples or an API reference. If something behaves unexpectedly, read the library source and tests in the repository rather than assuming behaviour.
Verify the Node.js version your project uses against the library's code before you deploy.
What people use it for
- Flattening a callback-heavy Node.js script into sequential steps.
- Maintaining an older codebase that already uses generator functions for async work.
- Migrating code written in co style toward native async/await over time.
- Adding a small flow-control helper to a project without pulling in a full framework.
Frequently asked questions
How do I install co?
The source lists a package-manager install method with Node.js as the prerequisite. The only command in the source is git clone https://github.com/tj/co, so clone the repository to get the code and check it for installation details.
What are the requirements for co?
The stated prerequisite is Node.js. The source does not list other requirements, so confirm the Node.js version against the repository code.
Is co free to use?
Yes. co is released under the MIT licence, which permits use, modification and redistribution, subject to the licence terms.
Is there a good alternative to co?
Native async/await covers the same need in modern JavaScript without a library. Bluebird and async are other options for promise and flow-control utilities.
Why does co not work for me?
The README in the source is empty, so there is no troubleshooting guide to point to. Check the repository's code and tests, and confirm your Node.js version.
How does co compare with async/await?
co uses generator functions and yields thunks or promises, while async/await is built into the language. For new projects, async/await is usually the simpler choice, as our general analysis notes.
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Our verdict
Use co if you already maintain generator-based Node.js code or want a minimal helper with an MIT licence. Pick native async/await for new projects, or Bluebird or async if you need a broader toolkit, since the source offers no usage guide.
Install options & downloads