javascript - Add li to ul with looking for if exists -

i'm adding thinks getting input field list. adding function works want check if string, user wants add exists. want don't know right functions use

html

first name: <input type="text" id="firstname"> <br> <p>other people's names:</p> <ul id="demo"></ul> <input type='button' onclick='changetext2()' value='submit' />  

javascript

var list = document.getelementbyid('demo');  function changetext2() {     var firstname = document.getelementbyid('firstname').value;     var listelement = list.getelementsbytagname("li");     // here missing code     if (listelement.text not in list){         var entry = document.createelement('li');         entry.appendchild(document.createtextnode(firstname));         list.appendchild(entry);     }     else alert("is in !"); } 

edit*

the code above in jsfiddle.

you create array , push in each name, add first time. each time try insert new item, check in array see if name has been added. in terms of code:

var addednames = []; var list = document.getelementbyid('demo');  function changetext2() {     var firstname = document.getelementbyid('firstname').value;      // array object has method called indexof, 1 argument     // item looking for. if item not in array,      // method returns -1. otherwise, method returns it's position.     // instance, let have defined following array.      // var numbers = [1,2,3,4].     // numbers.indexof(5) returns -1.     // while numbers.indexof(4) returns 3.     if (addednames.indexof(firstname)==-1){         var entry = document.createelement('li');         entry.appendchild(document.createtextnode(firstname));         list.appendchild(entry);         // add inserted name in array later checks.         addednames.push(firstname);     }     else alert("is in !"); } 

please have jsfiddle