d3 tick marks on integers only

The proper solution is to retrieve ticks using .ticks() method, filter them to keep only integers and pass them to .tickValues():

const yAxisTicks = yScale.ticks()
    .filter(tick => Number.isInteger(tick));
const yAxis = d3.axisLeft(yScale)
    .tickValues(yAxisTicks)
    .tickFormat(d3.format('d'));

For completeness here is explanation why other solutions are bad:

@BoomShakalaka solution uses .tickSubdivide() method, which no longer exists.

@cssndrx and @kris solutions force you to specify number of ticks, so it will not work in generic case.