SOURCE

var arr = [];
function sel(v) {
    var n = arr.indexOf(v);
  console.log(n)
    if (n==-1) {
        arr.push(v);
    } else {
        arr.splice(n,1);
    }
    document.getElementById("text").value = arr;
}

<input type="checkbox" value="a" onclick="sel(this.value)" /> a
<input type="checkbox" value="b" onclick="sel(this.value)" /> b
<input type="checkbox" value="c" onclick="sel(this.value)" /> c
<input type="checkbox" value="d" onclick="sel(this.value)" /> d
<input type="text" id="text" />