There are es2015 in the final package file

[zhubin1992]

d3-geo-projection@2.8.0/dist/d3-geo-projection.js (:445)

function solve2d(f, MAX_ITERATIONS = 40, eps = epsilon2) {
  return function(x, y, a = 0, b = 0) {
    ...
  };
}

Default function parameters are ECMAScript 2015.That will cause some errors.

[zhubin1992]

We need use Babel to use the latest JavaScript features that aren't yet supported by browsers and Node.js.

[Fil]

My mistake. I always forget. Thank you!

[mbostock]

You can add an eslint rule to remind you if you like.