Mastering JavaScript Functional Programming

Using tee

The first solution we'll use implies adding a function to the pipeline, which will just log its input. We want to implement something similar to the tee Linux command, which can intercept the standard data flow in a pipeline and send a copy to an alternate file or device. Remembering that /dev/tty is the usual console, we may execute something as follows and get an onscreen copy of everything that passes through the tee command:

$ ls -1 | grep "odt$" | tee /dev/tty | wc -l...the list of files with names ending in odt...4

We could write a similar function with ease:

const tee = arg => {    console.log(arg);    return arg;};

If you are aware of the uses of the comma operator, you can be quite more concise, and just write const tee = (arg) ...