slice D3 update - arrays

Question

I have created a dynamic dropdown who enables to select a country name to see data for this country. When a country is selected a function is invoked that will update the chart with data for this country.
It works well unless I select a country I have already previously selected, in which case I have the following error message:
"Uncaught TypeError: t.slice is not a function"
The dropdown is created and updated with the following code
var countryList = d3.map();
data.forEach(function(d) {
countryList.set(d.country,d.country); });
var select = d3.select(dropdown)
.selectAll("option")
.data(countryList.values().sort())
.enter().append("option")
.attr("value", function(d) {
return [d]; })
.text(function(d) { return [d]; });
d3.selectAll("select")
.on("change",function(){
var selectedCountry = document.getElementById("dropdown");
selectedCountry.options[selectedCountry.options.selectedIndex].selected = true;
var countryName = countryList.get(this.value)
//Calling the function countryChart
countryChart(data,countryName);
});
The function starts with the following code
function countryChart(data,countryName){
dataCou=data.filter(function(d) {
return(d.country == countryName)});
dataCou.forEach(function(d) {
d.monthYear = format.parse(d.monthYear);
d.count =+ d.count;
});
The part of the code that seems to be causing some problem is:
dataCou.forEach(function(d) {
d.monthYear = format.parse(d.monthYear);
d.count =+ d.count;
});
My data are formatted this way:
highest,monthYear,count,country
Community,2011-05-01,1116,Total
Community,2011-06-01,338,Total
Community,2011-07-01,56,Total
Community,2011-08-01,46,Total
Community,2011-09-01,52,Total
Community,2011-10-01,137,Total
Education,2011-05-01,1116,Total
Education,2011-06-01,338,Total
Education,2011-07-01,56,Total
Education,2011-08-01,46,Total
Education,2011-09-01,52,Total
Education,2011-10-01,137,Total
Community,2011-05-01,1116,USA
Community,2011-06-01,338,USA
Community,2011-07-01,56,USA
Community,2011-08-01,46,USA
Community,2011-09-01,52,USA
Community,2011-10-01,137,USA
Education,2011-05-01,1116,USA
Education,2011-06-01,338,USA
Education,2011-07-01,56,USA
Education,2011-08-01,46,USA
Education,2011-09-01,52,USA
Education,2011-10-01,137,USA
Do you have any idea what it is I am doing wrong?

Answer 1

Your d.monthYear is the number for some reason, but must be a string.
d.monthYear = format.parse(d.monthYear); // <-- here is the problem
When I do d3.time.format("%Y-%m-%d").parse(5), I get Uncaught TypeError: t.slice is not a function(…)
But when I do d3.time.format("%Y-%m-%d").parse('2011-05-01'), I get Sun May 01 2011 00:00:00 GMT-0700 (Pacific Daylight Time)