JSFiddle - React, Tailwind, and code Playground

HTML

<form id="formulario">
    <input type="checkbox" name="checkbox" value="a1"/>
    <input type="checkbox" name="checkbox" value="a2"/>
    <input type="checkbox" name="checkbox" value="a3"/>
    <input type="checkbox" name="checkbox" value="a4"/>
</form>
<a href="javascript:recuperarCheckbox()">mostrar</a>

JavaScript

function recuperarCheckbox(){
    valores=new Array();
    var checkboxes = document.getElementById("formulario").checkbox;
    for (var i=0; i < checkboxes.length; i++){
        if (checkboxes[i].checked){
            valores.push(checkboxes[i].value);
        }
    }
    for(var i=0; i<valores.length;i++){
        alert(valores[i]);
    }
}