Validating and formatting for SSN field with hyphens: 123-45-6789

Question

I am currently working on a simple form to practice Javascript validation and am having difficulty in adding the dashes in-between the numbers (like this: 123-45-6789). I need to keep the validation that displays whether or not it is valid (9 digits only) as the user is typing. This is what I have so far

function ssnFunction()
{
    let x = document.getElementById("ssn").value;
    let text;
    if (isNaN(x) || x.toString().length != 9)
        {
            text = "\tMust be 9 digits";
            ssnValidation = 0;
        }
    else
        {
            text = "Valid";
            ssnValidation = 1;
        }
    document.getElementById("ssnvalid").innerHTML = text;
    return ssnValidation;
}

<label for="ssn">Social Security: </label>
<input type="text" 
name="ssn" 
id="ssn" 
value="" 
required="required" 
onkeyup="ssnFunction()" size="20" />
<span id="ssnvalid"></span>

Any help and explanation is greatly appreciated