TLDR: Choose one, stick with it, try not to have multiple node versions per machine/VM if you can help it, and update & test your packages in small frequent inc

TLDR: Choose one, stick with it, try not to have multiple node versions per machine/VM if you can help it, and update & test your packages in small frequent increments if you can. Otherwise it gets to be a nightmare real quick. Yarn 1.x came out because NPM was really slow and buggy back in the day. Yarn was fast and buggy. Then Yarn 2 came out, was even faster and less buggy, but not backward compatible. I've never actually seen it used in the wild, even now (it's just yarn and NPM on all the projects I've worked on). Bigger differences will come trying to upgrade multiple packages at once, especially across major versions (e.g. 2.x to 3.x). It's going to be hard no matter what, but `yarn upgrade-interactive` makes it a bit simpler, while npm has third party plugins that do similar things. There's also complexities involved if you need to switch the underlying `node` version across projects, since npm and yarn both need different node versions depending on their own version. Their final outputs are similar (a node_modules folder where all your JS dependencies and sub-dependencies live), but their internal workings are different, and you shouldn't use both together because they will conflict with each other (sometimes). Generally, switching back and forth isn't hard on smaller projects: just delete the lock files and delete the node_modules folder and reinstall everything from scratch. But you really don't want to get into the habit of doing that. There's no real benefit (just choose one and stick with it; either is fine these days), and you may introduce hard-to-catch bugs related to one of your thousands of dependencies. Especially on bigger projects, erasing the lockfile means that it will try to use the designated SemVer in your package.json to get the desired version, but that may be different than the actual version that was being used, which is stored in the lockfile as a manual override. I'm not sure why these package managers had two sources of truth, but it's a nightmare.