Expression Regular - Back References

A backreference only matches the same text that was captured with the corresponding capturing group. It does not repeat the pattern, but the already captured value.

In your case, \1 tries to match 16 after the first -.

In JS regex, you can't use (?1) / \g<1> subroutine calls that can be used in PCRE/Onigmo, but you may define the subpattern as a variable and build the pattern dynamically:

var target = "16-10-2017";
var  p1 = "[0123][0-9]";
var rx = new RegExp("(" + p1 + ")-(" + p1 + ")-(\\d{4})", "g");
console.log(target.replace(rx, "$1/$2/$3"));