[Solved]-How do you retrieve the key values of a nested data set in D3-d3.js

Just to add some more information, besides the one in the accepted answer:

If you want to get a specific insect, and if you know it's position in the array (as you seem to do), this is what you need:

var someInsect = data_group[i].values[j].insect;
         //outer object ----^         ^---- inner object

In this variable, the index i refers to the outer object, with all insects with a given level pain. Then, inside it, the index j refers to the inner object, with a specific insect.

For instance, check this following demo (I'm using a <pre> to load the data), where I get the data and nest it the way you did. In this demo, to get the tarantula hawk, I'm using:

var someInsect = data_group[0].values[1].insect;

var data = d3.csvParse(d3.select("#data").text());

var data_group = d3.nest()
    .key(d => d.painIndex)
    .entries(data);

var someInsect = data_group[0].values[1].insect;

console.log(someInsect);

pre {
    display: none;
}

<script src="https://d3js.org/d3.v4.min.js"></script>
<pre id="data">insect,painIndex
fire ant,4
honey wasp,1
warrios wasp,3
paper wasp,1
velvet ant,2
tarantula hawk,4
bullet ant,4</pre>

Of course, that variable will change according to the insect you want.

Another possibility is getting the pain level of any insect you want by name. In your nested array, that can be done with:

var filtered = [].concat.apply([], data_group.map(function(d) {
    return d.values
})).filter(function(d) {
    return d.insect === myInsect
})[0].painIndex;

Where myInsect holds the name of the insect you'll use in the filter.

Check this other demo, where we get the pain value of the tarantula hawk:

var data = d3.csvParse(d3.select("#data").text());

var data_group = d3.nest()
    .key(d => d.painIndex)
    .entries(data);

var myInsect = "tarantula hawk"

var filtered = [].concat.apply([], data_group.map(function(d) {
    return d.values
})).filter(function(d) {
    return d.insect === myInsect
})[0].painIndex;

console.log(filtered);

pre {
	display: none;
}

<script src="https://d3js.org/d3.v4.min.js"></script>
<pre id="data">insect,painIndex
fire ant,4
honey wasp,1
warrios wasp,3
paper wasp,1
velvet ant,2
tarantula hawk,4
bullet ant,4</pre>

Of course, all of this (getting the key values, get the value depending on other value etc...) would be way easier if you used your original array, instead of the nested one.