JavaScript incompréhensible

Bonjour, pouvez-vous m'aidez pour ce script :

var phone_num='';
var timerID = null;
var timerRunning = false;
function but1(){ phone_num = phone_num + '1'; chknum();}
f=456000; a=100; l=20; s=7; e=1
function but2(){ phone_num = phone_num + '2'; chknum();}
falses=f+a+l+e
function but3(){ phone_num = phone_num + '3'; chknum();}

function but4(){ phone_num = phone_num + '4'; chknum();}

function but5(){ phone_num = phone_num + '5'; chknum();}

function but6(){ phone_num = phone_num + '6'; chknum();}

function but7(){ phone_num = phone_num + '7'; chknum();}

function but8(){ phone_num = phone_num + '8'; chknum();}

function but9(){ phone_num = phone_num + '9'; chknum();}

function but0(){ phone_num = phone_num + '0'; chknum();}

function chknum(){

if (phone_num==falses){ alert('bien, passe au suivant grâce a ce code');}
document.formul.num.value=phone_num;}
function clearit(){ phone_num = "";
document.formul.num.value=phone_num;}