Auto Format Phone Number in Jquery

As far as I can work out, all you really need to do is this:

$('#ssn').keyup(function()
{
    this.value = this.value.replace(/(\d{3})\-?/g,'$1-');
});

but this will only work when people enter digits, so I'd suggest an introducing an extra check:

$('#ssn').keyup(function(e) {
  if ((e.keyCode > 47 && e.keyCode < 58) || (e.keyCode < 106 && e.keyCode > 95)) {
    this.value = this.value.replace(/(\d{3})\-?/g, '$1-');
    return true;
  }
  
  //remove all chars, except dash and digits
  this.value = this.value.replace(/[^\-0-9]/g, '');
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<input type="text" id="ssn">

A little more on the regex /(\d{3})\-?/g:
This replaces group of 3 digits with itself, followed by a dash. The brackets create a back reference to the matched digits, that is used in the replacement string ($1- -> $1 being the back reference).
Note that an optional dash is replaced, too, but not included in the back reference. if the input is 123, and the replace pattern would be something like /(\d{3})/g, or /(\d{3}\-?)/g the value would become 123-4, 123--45, 123---456 and so on, doubling the dashes each time.

Warning:
This will give the user some grief, since the arrow keys and such won't work.Luckily, that's an easy fix: Just add the following code at the top of your function:

if (e.keyCode > 36 && e.keyCode < 41)
{
    return true;
}

And the arrows work just fine. for other keys (such as delete, backspace, shift etc...) check this page.

For a full example: here's the fiddle

Since you're using jQuery you can try jquery masked-input-plugin.

There's a jsFiddle for you here where you can see how it works.

The source code for the project on GitHub can be found here.

The implementation is more than simple:

HTML:

<input id="ssn"/>

javascript:

$("#ssn").mask("999-999-999");

UPDATE:

Another good one can be found here.

For the formatting XXX-XXX-XXXX here is a simple solution:

if(this.value.trim().length <= 8){
    this.value = this.value.replace(/(\d{3})\-?/g,'$1-');
 }

Here is an example and you not need any framework or library or npm:

this.phone.addEventListener('keyup', event => {

        if(this.phone.value.trim().length <= 8){
            this.phone.value = this.phone.value.replace(/(\d{3})\-?/g,'$1-');
        }
    }

You could also do this if you like:

 this.phone.addEventListener('keyup', event => {

        if(this.phone.value.trim().length <= 8){
            this.phone.value = this.phone.value.replace(/(\d{3})\-?/g,'$1-');
        }else{
            this.phone.value = this.phone.value.replace(/(\d{4})\-?/g,'$1');
        }
    }

Replace this this.phone with your value. I hope this helps.