Understanding how D3.js binds data to nodes

Question

I\'m reading through the D3.js documentation, and am finding it hard to understand the selection.data method from the documentation.

This is the example code given

Answer 1

Use the counter i to show the index of the data being used.

var tr = d3.select("body").append("table").selectAll("tr")
.data(matrix)
.enter().append("tr") //create a row for each data entry, first index
.text(function(d, i) { return i}); // show the index i.e. d[0][] then d[1][] etc.

var td = tr.selectAll("td")
.data(function(d) { return d; })
.enter().append("td")
.style("background-color", "yellow") //show each cell
.text(function(d,i) { return i + " " + d; }); // i.e d[from the tr][0] then d[from the tr][1]...