create-next-app always uses yarn if it is installed

[tsriram]

Feature request

Is your feature request related to a problem? Please describe.

I have both npm and yarn installed in my laptop. When I create a new app using npm init next-app (or npx create-next-app), Next always uses yarn as the package manager. This is because it just checks if yarns is installed and decides to use it.

Describe the solution you'd like

Ideally, I'd like this command to check if the command is run using npm or yarn and use the same as package manager for the new app. I'd suggest to use is-npm package (or just copy those few lines 😛, if we don't want to add a dependency) and check if code is run using npm or yarn. If for any reason this module isn't able to determine the used package manager, we can use the current logic as a fallback mechanism.

I'm happy to submit a PR is this makes sense.

[timneutkens]

Hey! this is a duplicate of #10647

Feel free to add checking for it similar to https://github.com/sindresorhus/is-npm/blob/master/index.js. I'd like to avoid adding additional deps

[tsriram]

Hi @timneutkens! I've made the changes locally but couldn't figure out how to test it. Would be great if you could share a quick note on how to run/test this locally. I'll add it to the README.md or CONTRIBUTING.md for future contributors.

[timneutkens]

Tbh I'm not sure how to test this behavior correctly without fully running npm or yarn. Let's open a PR first and figure out how to test it later 👍

[balazsorban44]

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