why is my chaine.charAT not a function and what else is broken in here

[ineedthisnpw]

\[code\]function chainePair(chaine) { var longueur = chaine.length; for (var i = 0; i <= longueur; i += 2) { var result = chaine.charAT(i); document.getElementById("reponse1").innerHTML = result + " "; }}?\[/code\]I am trying to make this function write every 2 letter of my string in the space allocated by reponse1 id. example: the string monster would show: \[code\]m n t r\[/code\]