javascript - Replace input text value by text in a string in REALTIME -

what need : when press key on keyboard in input text, replace content of string.

this have : https://jsfiddle.net/ypcxu27o/

<input id="edvalue" type="text" onkeyup="edvaluekey()">   function edvaluekey() {     //var moninput = document.getelementbyid("edvalue").value;     var montexte = 'this test';     var cpttexte = document.getelementbyid("edvalue").value;     cpttexte = cpttexte.length;     //alert(cpttexte);     document.getelementbyid("edvalue").value = montexte.substr(0, cpttexte);     //document.getelementbyid("edvalue").innerhtml = 'yuty'; } 

my problem see key press before script replace it. is-it possible hide ?

thank you.

this should it:

document.getelementbyid('edvalue').addeventlistener('keydown', function(ev) {    var montexte = 'this test';    if(ev.which !== 8) {  //everything except backspace      this.value= montexte.substr(0,this.value.length+1);      ev.preventdefault();    }  });

<input id="edvalue" type="text">