What exactly does “/usr/bin/env node” do at the beginning of node files?

#!/usr/bin/env node is an instance of a shebang line: the very first line in an executable plain-text file on Unix-like platforms that tells the system what interpreter to pass that file to for execution, via the command line following the magic #! prefix (called shebang).

Note: Windows does not support shebang lines, so they’re effectively ignored there; on Windows it is solely a given file’s filename extension that determines what executable will interpret it. However, you still need them in the context of npm.^[1]

The following, general discussion of shebang lines is limited to Unix-like platforms:

^{In the following discussion I’ll assume that the file containing source code for execution by Node.js is simply named file.}

• You NEED this line, if you want to invoke a Node.js source file directly, as an executable in its own right – this assumes that the file has been marked as executable with a command such as chmod +x ./file, which then allows you to invoke the file with, for instance, ./file, or, if it’s located in one of the directories listed in the $PATH variable, simply as file.
  • Specifically, you need a shebang line to create CLIs based on Node.js source files as part of an npm package, with the CLI(s) to be installed by npm based on the value of the "bin" key in a package’s package.json file; also see this answer for how that works with globally installed packages. Footnote [1] shows how this is handled on Windows.
• You do NOT need this line to invoke a file explicitly via the node interpreter, e.g., node ./file

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Optional background information:

#!/usr/bin/env <executableName> is a way of portably specifying an interpreter: in a nutshell, it says: execute <executableName> wherever you (first) find it among the directories listed in the $PATH variable (and implicitly pass it the path to the file at hand).

This accounts for the fact that a given interpreter may be installed in different locations across platforms, which is definitely the case with node, the Node.js binary.

By contrast, the location of the env utility itself can be relied upon to be in the same location across platforms, namely /usr/bin/env – and specifying the full path to an executable is required in a shebang line.

Note that POSIX utility env is being repurposed here to locate by filename and execute an executable in the $PATH.
The true purpose of env is to manage the environment for a command – see env‘s POSIX spec and Keith Thompson’s helpful answer.

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It’s also worth noting that Node.js is making a syntax exception for shebang lines, given that they’re not valid JavaScript code (# is not a comment character in JavaScript, unlike in POSIX-like shells and other interpreters).

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^{[1] In the interest of cross-platform consistency, npm creates wrapper *.cmd files (batch files) on Windows when installing executables specified in a package’s package.json file (via the "bin" property). Essentially, these wrapper batch files mimic Unix shebang functionality: they invoke the target file explicitly with the executable specified in the shebang line – thus, your scripts must include a shebang line even if you only ever intend to run them on Windows – see this answer of mine for details.
Since *.cmd files can be invoked without the .cmd extension, this makes for a seamless cross-platform experience: on both Windows and Unix you can effectively invoke an npm-installed CLI by its original, extension-less name.}