Make a zsh function alias to enforce using yarn commands when in a yarn project
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If you're anything like me, you have multiple projects (between work and side projects) where some are managed with npm and others with yarn. As the number of projects increase, the harder it gets to remember which one uses which. In this post, we will create a custom npm zsh function alias to warn yourself to use yarn commands if the a yarn.lock file exists in the same directory.

Creating the Function Alias

In order to create the npm function alias, let's first open up your .zshrc configuration file, which can be found in your user directory. You should be able to use one of the following commands to Open up the file in your preferred editor (code ~/.zshrc, vim ~/.zshrc, etc...).

The following is the script we'd like to execute each time a user types in the npm command. We are basicaly hijacking npm, so that it'll run our function alias instead of the globally installed npm package manager. The script first detects if the directory has a yarn.lock file present and if it does, then a warning message is displayed. Otherwise, the script executes the underlying npm function passing the origional parameters.

# ... more configuration ...

npm() {
  if [ -e yarn.lock ]; then
    echo "⚠️ This is a yarn project. Why don't you use it ⁉️"
    command npm $@

# ... more configuration ...

Note: This alias should be able to work both in bash or in zsh

You may be familiar with the bash function arguments $1, $2, etc... that represent the first and second parameters. We could have used those, but since we don't know how many arguments will be passed to our script, we are using $@ which represents all the arguments.

Another important things to notice is the command command. This is very important because if we had not provided it this function alias would have recursively called itself over and over again.

Sourcing our Configuration

Once you've saved your configuration file, you'll either want close and reopen your terminal, create a new tab, or reload your configuration in your current terminal/tab in order for new function alias to be recognized.

You should be able to reload your configuration by running this command in your terminal source ~/.zshrc.

Using our Function Alias

Project using npm

When we are in a project maintained by the npm package manager (there is no yarn.lock file), our function alias should just pass our commands along to the underlying npm executable. You shouldn't really notice any difference in behavior.

~/npm-project ❯ ls yarn*
zsh: no matches found: yarn*

~/npm-project ❯ npm run
Lifecycle scripts included in devpun:
    echo "Error: no test specified" && exit 0

Project using yarn

However, if you happen to be in a yarn project, then our new function alias should stop you in your tracks if you use an npm command to manage the project. At that point, just switch gears and use a support yarn command instead. It might be handy to find a good cheat sheet to help you know which command is appropriate.

~/yarn-project ❯ ls yarn*

~/yarn-project ❯ npm run
⚠ ️This is a yarn project. Why don't you use it ⁉️

~/yarn-project ❯ yarn clean
yarn run v1.19.1
$ rm -rf packages/*/{dist,.cache}
✨  Done in 0.08s.


I hope you find this function alias helpful or some of the techniques used to create it. You could possibly extend this by also making a yarn function alias or by expanding the idea to recommend or auto-translate one command to an appropriate counterpart.

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