I want to declare a javascript variable by INPUT TEXT

Question

I would like to do the inverse of this script, in this script you click the GENERATE button, it generates 4 digits

What I want to do is the reverse, I want to declare the variable "1298105" (or any other) and click on Generate and it generate below in one <div> the result "1298105XXXX" as declared in input

How to do, some example ?

            function gerar() {

                // numero aleatorios
                var numeros = [];
                for (var i = 0; i < 4; ++i) {
                    var sorteio = Math.floor(Math.random() * 9);

                    if (sorteio == 0)
                        sorteio = 1; // 1 - 9

                    numeros[i] = sorteio; 
                }

                var senha = numeros.join('');

                document.getElementById('txtsenha').value = senha;
            }

        <input type="text" name="txtsenha" id="txtsenha"/>
        <input type="button" value="Gerar" onclick="javascript:gerar()"/>

Answer 1

function gerar() {
                // numero aleatorios
                var numeros = [];
                for (var i = 0; i < 4; ++i) {
                    var sorteio = Math.floor(Math.random() * 9);

                    if (sorteio == 0)
                        sorteio = 1; // 1 - 9

                    numeros[i] = sorteio; 
                }

                var senha = numeros.join('');


    var x = document.getElementById("txtsenha").value;
    document.getElementById("demo").innerHTML = x+senha;
}

<input type="text" name="txtsenha" id="txtsenha"/>
<input type="button" value="Gerar" onclick="javascript:gerar()"/>
<div id="demo"></div>