<!-- This is a Better practice in JavaScript! -->
var Saya = (function() {
    "use strict";
    var hai = document.getElementById("hai"),
    juki = document.getElementById("juki");

    return hai.value = juki.value;
});

<!-- Html code -->
<select name="juki" id="juki" onchange="Saya();">
<option value="">please select</option>
<option value="a">a</option>
<option value="b">b</option>
<option value="c">c</option>
</select>
<input name="hai" id="hai" type="text">
<!-- //End -->