New Line at the End of file

In most cases, it is recommended to include a new line at the end of text / source files. POSIX compliance is one historical reason, but it also ensures that tools, for example cat, return cleanly at the end of their output:

Not so nice

$ cat file-without-newline-at-eol.js
reticulateSplines()
    .then( stopSlackingOff );
commenceLoafing();yourcoolhostname:yourcoolproject yourcoolusername$

Your prompt ends up combined with the last line as the end of the source file.

Better!

$ cat file-with-newline-at-eol.js
compileKernel()
    .then( continueBuilding );
waitForIt();
yourcoolhostname:yourcoolproject yourcoolusername$

Your prompt stands alone on a new line.

Tools

The wpcalypso eslint config deems it an error if files do not "end with a newline (LF)"
Prettier reformats files to add a newline at the end "by design"
Most (if not all) major editors have settings to add newlines at the end of source files. For example:
- nano: enabled by default. To disable, you supply the -L flag
- Atom: enabled by default. To disable, you override settings for the core package, whitespace