d3.js choropleth map example error - d3.js

Question

I'm trying to learn to build a choropleth map using an example provided by d3.js.
I keep getting an error: "TypeError: congress.forEach is not a function" when I switch from the .tsv file in the example to a .json file in my version. Here is the code I'm using:
<script type="text/javascript">
var width = 960,
height = 500;
var path = d3.geo.path();
var svg = d3.select("body").append("svg")
.attr("width", width)
.attr("height", height);
var g = svg.append("g")
.call(d3.behavior.zoom()
.scaleExtent([1, 10])
.on("zoom", zoom));
d3.json("us-10m.json", function (error, us) {
d3.json("https://www.govtrack.us/api/v2/role?current=true", function (error, congress) {
var memberId = {};
congress.forEach(function (d) { //TypeError:congress.forEach is not a function
memberId[d.id] = +d.id;
});
g.append("g")
.attr("class", "states")
.selectAll("path")
.data(topojson.feature(us, us.objects.states).features)
.enter().append("path")
.attr("d", path)
.style("fill", function (d) {
return color(memberId[d.id]); // <-C
});
g.append("path")
.datum(topojson.mesh(us, us.objects.states, function(a, b) { return a !== b; }))
.attr("class", "states")
.attr("d", path);
});
});
function zoom() {
g.attr("transform", "translate("
+ d3.event.translate
+ ")scale(" + d3.event.scale + ")");
}
Any help is appreciated. Sorry I didn't put this on jsfiddle but I couldn't connect to .us-10m.json because of a cross domain issue.
Thank you,
Thom

Answer 1

congress is not an Array — it's an Object:
{
meta: {
limit: 100,
offset: 0,
total_count: 540
},
objects: [ {...}, {...}, {...}, {...} ]
}
Need to iterate over congress.objects:
congress.objects.forEach(function() {...})