What is the d3.js v4.0 equivalent for d3.scale.category10()?

A straight-forward solution is to use the following color scales in version-4 of d3.js :

var colorScale_1 = d3.schemeCategory10;
var colorScale_2 = schemeCategory20;
var colorScale_3 = d3.schemeCategory20b;
var colorScale_4 = d3.schemeCategory20c;

colorScale_1, colorScale_2, colorScale_3, colorScale_4 are the arrays of different colors. So, you can use their different indices to fill the shape. For example

svg.append("rect")
.attr("x", 10)
.attr("y", 10)
.attr("width", 100)
.attr("height", 100)
.style("fill", colorScale_1[2])

For reference, take a look here: http://bl.ocks.org/emmasaunders/f4902478bcfa411c77a412c02087bed4

Hope that helps.

Instead of

d3.scale.category10()

use

d3.scaleOrdinal(d3.schemeCategory10);

Create a color scale like this:

var color = d3.scaleOrdinal(d3.schemeCategory10);

use the color like this in the code same as in V3:

svg.append("rect")
.attr("x", 10)
.attr("y", 10)
.attr("width", 100)
.attr("height", 100)
.style("fill", color(3))

read here

Reference here

working code here