The D3 v4 API is here. According to the changelog:
D3 4.0 provides default styles and shorter syntax. In place of d3.svg.axis and axis.orient, D3 4.0 now provides four constructors for each orientation: d3.axisTop, d3.axisRight, d3.axisBottom, d3.axisLeft.
Therefore, those lines should be:
var xAxis = d3.axisBottom(xRange).tickFormat(function(d){ return d.x;});
var yAxis = d3.axisLeft(yRange);
PS: I’m assuming that you want the ticks to be below the axis, which is normally the case, since you didn’t show the orient in your original lines.
PPS: At the time of the writing the linked documentation applies to D3 v4. Caveat, lector: that can change at any time (see comments below).