I have a function that selects all elements with tag a and removes the parentīs parent node if a class is applied to them however I am getting an error in firebug that states it is not a function

Code JavaScript:
function groupSelectedElements() {
    document.getElementById(form_values[0]).getElementsByTagName('a').map(function (value) {
		if (value.className === 'checkbox checkbox_c') {

firebug says

document.getElementById(form_values[0]).getElementsByTagName("a").map is not a function
groupSelectedElements()form_create.php (line 440)
onclick()4 (line 3)
event = click clientX=748, clientY=182