Element

// checked
document.getElementById('myRadio').checked = true;
document.getElementById('myRadio').checked = "checked";

// unchecked
document.getElementById('myRadio').checked = false;

let select_element = document.querySelector('#mySelect');
// Get
let selected_value = select_element.value;
// Set
select_element.value = 'KJ';

let select_element = document.querySelector('#mySelect');
select_element.disabled = true;
select_element.disabled = false;

<select onChange="getSelectOptionText(this);">
  <option value="1">Dog</option>
  <option value="2">Cat</option>
  <option value="3">Bird</option>
</select>

function getSelectOptionText(select_element) {
    let select_element_options = select_element.options;
    let select_element_selected_index = select_element.selectedIndex;
    let select_element_option_text = select_element_options[select_element_selected_index].text;

    alert(select_element_option_text);
}