check all checkboxes javascript

I'm trying to have all checkboxes checked (true) when the user clicks on "All" button. I tried this, firstly just to see if "Anglais" could be checked clicking on "All" :

<input type="checkbox" name="anglais"  id="anglais"  value="Anglais"  /> Anglais
<input type="checkbox" name="allemand" id="allemand" value="Allemand" /> Allemand
<input type="checkbox" name="espagnol" id="espagnol" value="Espagnol" /> Espagnol
<input type="checkbox" name="francais" id="francais" value="Francais" /> Francais
<input type="checkbox" onclick="checkedAll()" name="all"      id="all"      value="Tous"     /> Tous

My Javascript :

function checkedAll () {
var checked = false;
var all = document.getElementById('all');
if (checked == false) {
    checked = true
}
else {
    checked = false
}
var ang = document.getElementById('anglais').checked
if (ang == true) {
    ang.checked = true;
    }

But the button(s) are not checked when I click on All. I think I don't understand exactly how to use the .checked method..

Maybe, some of my code has not logic, because it was from this example : http://ift.tt/1CnsBCe