While most answers are a copy of SolutionYogi's answer, they all miss an important check to see if 'cell' is not null which will return an error if clicking on the headers. So, here is the answer with the check included:
function addRowHandlers() {
var table = document.getElementById("tableId");
var rows = table.getElementsByTagName("tr");
for (i = 0; i < rows.length; i++) {
var currentRow = table.rows[i];
var createClickHandler = function(row) {
return function() {
var cell = row.getElementsByTagName("td")[0];
// check if not null
if(!cell) return; // no errors!
var id = cell.innerHTML;
alert("id:" + id);
};
};
currentRow.onclick = createClickHandler(currentRow);
}
}