D3.js get data index of shape on mouseover event - d3.js

Question

I am attempting to access the data index of a shape on mouseover so that I can control the behavior of the shape based on the index.
Lets say that this block of code lays out 5 rect in a vertical line based on some data.
var g_box = svg
.selectAll("g")
.data(controls)
.enter()
.append("g")
.attr("transform", function (d,i){
return "translate("+(width - 100)+","+((controlBoxSize+5)+i*(controlBoxSize+ 5))+")"
})
.attr("class", "controls");
g_box
.append("rect")
.attr("class", "control")
.attr("width", 15)
.attr("height", 15)
.style("stroke", "black")
.style("fill", "#b8b9bc");
When we mouseover rect 3, it transitions to double size.
g_box.selectAll("rect")
.on("mouseover", function(d){
d3.select(this)
.transition()
.attr("width", controlBoxSize*2)
.attr("height", controlBoxSize*2);
var additionalOffset = controlBoxSize*2;
g_box
.attr("transform", function (d,i){
if( i > this.index) { // want to do something like this, what to use for "this.index"?
return "translate("+(width - 100)+","+((controlBoxSize+5)+i*(controlBoxSize+5)+additionalOffset)+")"
} else {
return "translate("+(width - 100)+","+((controlBoxSize+5)+i*(controlBoxSize+5))+")"
}
})
})
What I want to do is move rect 4 and 5 on mouseover so they slide out of the way and do not overlap rect 3 which is expanding.
So is there a way to detect the data index "i" of "this" rect in my mouseover event so that I could implement some logic to adjust the translate() of the other rect accordingly?

Answer 1

You can easily get the index of any selection with the second argument of the anonymous function.
The problem here, however, is that you're trying to get the index in an anonymous function which is itself inside the event handler, and this won't work.
Thus, get the index in the event handler...
selection.on("mouseover", function(d, i) {
//index here ---------------------^
... and, inside the inner anonymous function, get the index again, using different parameter name, comparing them:
innerSelection.attr("transform", function(e, j) {
//index here, with a different name -----^
This is a simple demo (full of magic numbers), just to show you how to do it:
var svg = d3.select("svg");
var data = d3.range(5);
var groups = svg.selectAll("foo")
.data(data)
.enter()
.append("g");
var rects = groups.append("rect")
.attr("y", 10)
.attr("x", function(d) {
return 10 + d * 20
})
.attr("width", 10)
.attr("height", 100)
.attr("fill", "teal");
groups.on("mouseover", function(d, i) {
d3.select(this).select("rect").transition()
.attr("width", 50);
groups.transition()
.attr("transform", function(e, j) {
if (i < j) {
return "translate(40,0)"
}
})
}).on("mouseout", function() {
groups.transition().attr("transform", "translate(0,0)");
rects.transition().attr("width", 10);
})
<script src="https://d3js.org/d3.v4.min.js"></script>
<svg></svg>
PS: don't do...
g_box.selectAll("rect").on("mouseover", function(d, i){
... because you won't get the correct index that way (which explain your comment). Instead of that, attach the event to the groups, and get the rectangle inside it.

Answer 2

I'm pretty sure d3 passes in the index as well as the data in the event listener.
So try
.on("mouseover", function(d,i)
where i is the index.
Also you can take a look at a fiddle i made a couple months ago, which is related to what you're asking.
https://jsfiddle.net/guanzo/h1hdet8d/1/

Answer 3

You can find the index usign indexOf(). The second argument in the event mouseover it doesn't show the index in numbers, it shows the data info you're working, well, you can pass this info inside indexOf() to find the number of the index that you need.
.on("mouseover", (event, i) => {
let index = data.indexOf(i);
console.log(index); // will show the index number
})