MMMRjs a product of BYTES Genesis
Comments

The comments such a This is Mean. are pointless.

And the comment 

// To call/run this function we can type mean(Your array of numbers separated with commas ',');


is a bit strange. Are they targeting people who don't know basic JavaScript syntax? Then why mention commas, but not the brackets ([...]) needed for an array.

It probably would be a good idea to instead use a standard comment format for documentation such as jsdoc.

Sorting

The comparison function used for sorting ((a, b) =&gt; a - b) is pointless, because that is the default.

Mutating the input array

In median you are creating a copy of the input array with .slice() before sorting it, so that it isn't modified, which is a good thing. You should do the same in mode and range.

Validating the input

You should consider validating the input. mean, mode and range break or give unexpected results if you give them an empty array. It's probably best to check if they are receiving an array at all.