As it has been already mentionned, the (currently) top-rated answer does not take into account the possibility to embed an input inside a label.
Since nobody has posted a JQuery-free answer, here is mine :
var labels = form.getElementsByTagName ('label');
var input_label = {};
for (var i = 0 ; i != labels.length ; i++)
{
var label = labels[i];
var input = label.htmlFor
? document.getElementById(label.htmlFor)
: label.getElementsByTagName('input')[0];
input_label[input.outerHTML] =
(label.innerText || label.textContent); // innerText for IE8-
}
In this example, for the sake of simplicity, the lookup table is directly indexed by the input HTML elements. This is hardly efficient and you can adapt it however you like.
You can use a form as base element, or the whole document if you want to get labels for multiple forms at once.
No checks are made for incorrect HTML (multiple or missing inputs inside labels, missing input with corresponding htmlFor id, etc), but feel free to add them.
You might want to trim the label texts, since trailing spaces are often present when the input is embedded in the label.