--use-npm should be automatically enabled with npx and npm init/create

Is your proposal related to a problem?

By default create-react-app will create new projects with yarn if it is installed, even if one of the following npm commands is used to create it:

npx create-react-app
npm init react-app
npm create react-app

In this case, I think it’s likely that the user intends on using npm for their new app, even if they use yarn for other projects. I’ve had to keep both package managers installed regardless of my preference to contribute to different projects on the same machine. This lead to some situations where I wanted to use npm by default, but I would keep forgetting that create-react-app would use yarn because I still had it installed.

Describe the solution you’d like

Ideally, it would help if create-react-app could detect when it’s being called via npx (which is also used by npm create/init) and default to turning on --use-npm in this case. create-react-app should still default to yarn when it’s installed and the user directly calls create-react-app to prevent confusion and breaking changes.

Describe alternatives you’ve considered

Of course you could try to remember to use --use-npm, but it’s easy to forget if you have yarn installed and that create-react-app still requires then flag with npm/npx commands. I think this is worth figuring out for the improved developer experience, especially since npm/yarn users frequently run into bugs when switching package managers accidentally (for example, when trying to use npm and not remembering that create-react-app was using yarn).

Additional context

I’ve attempted this myself, though unfortunately npx don’t seem to provide easy ways to detect its usage (unlike yarn which has a parent process and tons of environment variables). Instead, npx will replace the current process with the new command and provide exactly the same environment variables, making it look very similar to the Node environment that commands like create-react-app use. I’d be happy to help with implementing this if someone can help me find other ways to detect npx’s usage.

[nickmccurdy]

yarn 2 doesn’t support create, so maybe we should just stick to npx for now

[nickmccurdy]

Update: process.env.npm_config_user_agent can be used to detect yarn create, but npm create and npx do not support it. This means we can determine when yarn is used, but we can’t distinguish between using npm/npx or using create-react-app directly.

Instead, we could change create-react-app and npx create-react-app to default to npm, but replace npx create-react-app with yarn create-react-app and npm create react-app in the docs so we always have a way of telling them apart. Meanwhile, process.env.npm_config_user_agent could be used to enable yarn automatically with yarn create react-app.