
<input id="demoText" type="text" value="texte...">

document.getElementById('demoText').value

texte...


<input name="demoRadio" type="radio" value="1" checked>Un
<input name="demoRadio" type="radio" value="2" >Deux

Un Deux
document.getElementsByName('demoRadio')[i].value;
document.getElementsByName('demoRadio')[i].checked;


<input type="checkbox" id="demoCheckboxId" name="demoCheckbox" value="admin">Administrateur?

Administrateur?
document.getElementsByName('demoCheckbox')[0].checked;
document.getElementById('demoCheckboxId').checked;
document.getElementById('demoCheckboxId').value;

value = admin, checked = false


<select id="demoSelectId" name="demoSelect">
    <option value="1">Un</option>
    <option value="2">Deux</option>
    <option value="3">Trois</option>
</select>

let select = document.getElementById('demoSelectId');