A dive into npx: What happens when you run npx?

#npm #npx #beginners

npx is something that is very handy nowadays, combining both installation as well as utilization of the tool in one command.

It makes things very straightforward, but have you wondered what actually happens behind the scenes? Let's take a look.

What happens when you use npx?

Suppose you ran npx package-name.
Let's divide it into steps:

Creation of temporary directory
Running npm
Creation of CLI "shim"
npx executes the shim

This is the basic overview; now let's dig into each one and get an idea.

Step 1: Creation of temporary directory

First step of the process: creating a temporary directory.
It can be /tmp/npx-<RANDOM_ID>.

Step 2: Running npm

The following npm command is run:
npm install package-name --no-save
This will install into the temp directory and there won’t be any global installation.

Step 3: npm generates a CLI "shim"

Okay, now the npm installation is done; now we need to craft a bridge from npm to npx, right?

Like me, you would also have thought, "What is a shim?" Let's understand that first.

What is a shim?

A shim is a tiny autogenerated script that acts as the bridge between npx and your package's real executable file.
So you can think of it as a launcher or a shortcut.
npx automatically creates this file for every package that has a bin entry.

Example of shim creation

Suppose you have this entry in the package.json:

"bin": {
  "<cli-name>": "<path-to-your-real-cli-or-wrapper-file>"
}

npm creates a shim at:

/tmp/npx-<RANDOM_ID>/node_modules/.bin/<cli-name>

The shim contains something like:

node ../<PACKAGE_NAME>/<path-to-your-real-cli-file>

Step 4: npx executes the shim

npx will run the shim that was created in the earlier step.

/tmp/npx-<RANDOM_ID>/node_modules/.bin/<cli-name>

The shim then launches the actual CLI file inside:

/tmp/npx-<RANDOM_ID>/node_modules/<PACKAGE_NAME>/<path-to-your-real-cli-file>

Wrapping up

This article stemmed from my curiosity about how this whole npx thing works. Since I was creating packages with npx compatibility, it was essential for me to understand the behind-the-scenes to properly add in the needful.

So this principle applies everywhere: don’t consider anything as a black box. When you get time, understand the behind-the-scenes and you will have a better experience working on it.

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