Is there a way to join the elements in an js array, but let the last separator be different?

There's no predefined function, because it's quite simple.

var a = ['a', 'b', 'c'];
var str = a.slice(0, -1).join(',')+' or '+a.slice(-1);

There's also a specification problem for the main use case of such a function which is natural language formatting. For example if we were to use the Oxford comma logic we would have a different result than what you're looking for:

// make a list in the Oxford comma style (eg "a, b, c, and d")
// Examples with conjunction "and":
// ["a"] -> "a"
// ["a", "b"] -> "a and b"
// ["a", "b", "c"] -> "a, b, and c"
exports.oxford = function(arr, conjunction, ifempty){
    let l = arr.length;
    if (!l) return ifempty;
    if (l<2) return arr[0];
    if (l<3) return arr.join(` ${conjunction} `);
    arr = arr.slice();
    arr[l-1] = `${conjunction} ${arr[l-1]}`;
    return arr.join(", ");
}

So it seems better to let this problem in userland.

May I suggest:

['tom', 'dick', 'harry'].join(', ').replace(/, ([^,]*)$/, ' and $1')
> "tom, dick and harry"

No, this is specific enough that you will have to write a custom function. The good news is, as you said, once you use Array.join to take care of all the separators, the last one will be easy enough to update.