diff --git a/src/arc.js b/src/arc.js
index 10ae342..2032882 100644
--- a/src/arc.js
+++ b/src/arc.js
@@ -1,6 +1,6 @@
 import {path} from "d3-path";
-import constant from "./constant";
-import {abs, acos, asin, atan2, cos, epsilon, halfPi, max, min, pi, sin, sqrt, tau} from "./math";
+import constant from "./constant.js";
+import {abs, acos, asin, atan2, cos, epsilon, halfPi, max, min, pi, sin, sqrt, tau} from "./math.js";
 
 function arcInnerRadius(d) {
   return d.innerRadius;
diff --git a/src/area.js b/src/area.js
index 9eb795c..a048a01 100644
--- a/src/area.js
+++ b/src/area.js
@@ -1,8 +1,8 @@
 import {path} from "d3-path";
-import constant from "./constant";
-import curveLinear from "./curve/linear";
-import line from "./line";
-import {x as pointX, y as pointY} from "./point";
+import constant from "./constant.js";
+import curveLinear from "./curve/linear.js";
+import line from "./line.js";
+import {x as pointX, y as pointY} from "./point.js";
 
 export default function() {
   var x0 = pointX,
diff --git a/src/areaRadial.js b/src/areaRadial.js
index d8235c4..61e01d7 100644
--- a/src/areaRadial.js
+++ b/src/areaRadial.js
@@ -1,6 +1,6 @@
-import curveRadial, {curveRadialLinear} from "./curve/radial";
-import area from "./area";
-import {lineRadial} from "./lineRadial"
+import curveRadial, {curveRadialLinear} from "./curve/radial.js";
+import area from "./area.js";
+import {lineRadial} from "./lineRadial.js";
 
 export default function() {
   var a = area().curve(curveRadialLinear),
diff --git a/src/curve/basisClosed.js b/src/curve/basisClosed.js
index 522e305..535df90 100644
--- a/src/curve/basisClosed.js
+++ b/src/curve/basisClosed.js
@@ -1,5 +1,5 @@
-import noop from "../noop";
-import {point} from "./basis";
+import noop from "../noop.js";
+import {point} from "./basis.js";
 
 function BasisClosed(context) {
   this._context = context;
diff --git a/src/curve/basisOpen.js b/src/curve/basisOpen.js
index dac5d4e..4f2e5b1 100644
--- a/src/curve/basisOpen.js
+++ b/src/curve/basisOpen.js
@@ -1,4 +1,4 @@
-import {point} from "./basis";
+import {point} from "./basis.js";
 
 function BasisOpen(context) {
   this._context = context;
diff --git a/src/curve/bundle.js b/src/curve/bundle.js
index 10d5307..ac1014e 100644
--- a/src/curve/bundle.js
+++ b/src/curve/bundle.js
@@ -1,4 +1,4 @@
-import {Basis} from "./basis";
+import {Basis} from "./basis.js";
 
 function Bundle(context, beta) {
   this._basis = new Basis(context);
diff --git a/src/curve/cardinalClosed.js b/src/curve/cardinalClosed.js
index 20516af..acef52e 100644
--- a/src/curve/cardinalClosed.js
+++ b/src/curve/cardinalClosed.js
@@ -1,5 +1,5 @@
-import noop from "../noop";
-import {point} from "./cardinal";
+import noop from "../noop.js";
+import {point} from "./cardinal.js";
 
 export function CardinalClosed(context, tension) {
   this._context = context;
diff --git a/src/curve/cardinalOpen.js b/src/curve/cardinalOpen.js
index 69070d5..e736841 100644
--- a/src/curve/cardinalOpen.js
+++ b/src/curve/cardinalOpen.js
@@ -1,4 +1,4 @@
-import {point} from "./cardinal";
+import {point} from "./cardinal.js";
 
 export function CardinalOpen(context, tension) {
   this._context = context;
diff --git a/src/curve/catmullRom.js b/src/curve/catmullRom.js
index fff05e6..643d10f 100644
--- a/src/curve/catmullRom.js
+++ b/src/curve/catmullRom.js
@@ -1,5 +1,5 @@
-import {epsilon} from "../math";
-import {Cardinal} from "./cardinal";
+import {epsilon} from "../math.js";
+import {Cardinal} from "./cardinal.js";
 
 export function point(that, x, y) {
   var x1 = that._x1,
diff --git a/src/curve/catmullRomClosed.js b/src/curve/catmullRomClosed.js
index eb79521..6c6b965 100644
--- a/src/curve/catmullRomClosed.js
+++ b/src/curve/catmullRomClosed.js
@@ -1,6 +1,6 @@
-import {CardinalClosed} from "./cardinalClosed";
-import noop from "../noop";
-import {point} from "./catmullRom";
+import {CardinalClosed} from "./cardinalClosed.js";
+import noop from "../noop.js";
+import {point} from "./catmullRom.js";
 
 function CatmullRomClosed(context, alpha) {
   this._context = context;
diff --git a/src/curve/catmullRomOpen.js b/src/curve/catmullRomOpen.js
index 217ddb8..7e4c5ca 100644
--- a/src/curve/catmullRomOpen.js
+++ b/src/curve/catmullRomOpen.js
@@ -1,5 +1,5 @@
-import {CardinalOpen} from "./cardinalOpen";
-import {point} from "./catmullRom";
+import {CardinalOpen} from "./cardinalOpen.js";
+import {point} from "./catmullRom.js";
 
 function CatmullRomOpen(context, alpha) {
   this._context = context;
diff --git a/src/curve/linearClosed.js b/src/curve/linearClosed.js
index 3e63e63..e25606f 100644
--- a/src/curve/linearClosed.js
+++ b/src/curve/linearClosed.js
@@ -1,4 +1,4 @@
-import noop from "../noop";
+import noop from "../noop.js";
 
 function LinearClosed(context) {
   this._context = context;
diff --git a/src/curve/radial.js b/src/curve/radial.js
index fb359b5..730c989 100644
--- a/src/curve/radial.js
+++ b/src/curve/radial.js
@@ -1,4 +1,4 @@
-import curveLinear from "./linear";
+import curveLinear from "./linear.js";
 
 export var curveRadialLinear = curveRadial(curveLinear);
 
diff --git a/src/index.js b/src/index.js
index b5347cf..aca91a5 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,46 +1,46 @@
-export {default as arc} from "./arc";
-export {default as area} from "./area";
-export {default as line} from "./line";
-export {default as pie} from "./pie";
-export {default as areaRadial, default as radialArea} from "./areaRadial"; // Note: radialArea is deprecated!
-export {default as lineRadial, default as radialLine} from "./lineRadial"; // Note: radialLine is deprecated!
-export {default as pointRadial} from "./pointRadial";
-export {linkHorizontal, linkVertical, linkRadial} from "./link/index";
+export {default as arc} from "./arc.js";
+export {default as area} from "./area.js";
+export {default as line} from "./line.js";
+export {default as pie} from "./pie.js";
+export {default as areaRadial, default as radialArea} from "./areaRadial.js"; // Note: radialArea is deprecated!
+export {default as lineRadial, default as radialLine} from "./lineRadial.js"; // Note: radialLine is deprecated!
+export {default as pointRadial} from "./pointRadial.js";
+export {linkHorizontal, linkVertical, linkRadial} from "./link/index.js";
 
-export {default as symbol, symbols} from "./symbol";
-export {default as symbolCircle} from "./symbol/circle";
-export {default as symbolCross} from "./symbol/cross";
-export {default as symbolDiamond} from "./symbol/diamond";
-export {default as symbolSquare} from "./symbol/square";
-export {default as symbolStar} from "./symbol/star";
-export {default as symbolTriangle} from "./symbol/triangle";
-export {default as symbolWye} from "./symbol/wye";
+export {default as symbol, symbols} from "./symbol.js";
+export {default as symbolCircle} from "./symbol/circle.js";
+export {default as symbolCross} from "./symbol/cross.js";
+export {default as symbolDiamond} from "./symbol/diamond.js";
+export {default as symbolSquare} from "./symbol/square.js";
+export {default as symbolStar} from "./symbol/star.js";
+export {default as symbolTriangle} from "./symbol/triangle.js";
+export {default as symbolWye} from "./symbol/wye.js";
 
-export {default as curveBasisClosed} from "./curve/basisClosed";
-export {default as curveBasisOpen} from "./curve/basisOpen";
-export {default as curveBasis} from "./curve/basis";
-export {default as curveBundle} from "./curve/bundle";
-export {default as curveCardinalClosed} from "./curve/cardinalClosed";
-export {default as curveCardinalOpen} from "./curve/cardinalOpen";
-export {default as curveCardinal} from "./curve/cardinal";
-export {default as curveCatmullRomClosed} from "./curve/catmullRomClosed";
-export {default as curveCatmullRomOpen} from "./curve/catmullRomOpen";
-export {default as curveCatmullRom} from "./curve/catmullRom";
-export {default as curveLinearClosed} from "./curve/linearClosed";
-export {default as curveLinear} from "./curve/linear";
-export {monotoneX as curveMonotoneX, monotoneY as curveMonotoneY} from "./curve/monotone";
-export {default as curveNatural} from "./curve/natural";
-export {default as curveStep, stepAfter as curveStepAfter, stepBefore as curveStepBefore} from "./curve/step";
+export {default as curveBasisClosed} from "./curve/basisClosed.js";
+export {default as curveBasisOpen} from "./curve/basisOpen.js";
+export {default as curveBasis} from "./curve/basis.js";
+export {default as curveBundle} from "./curve/bundle.js";
+export {default as curveCardinalClosed} from "./curve/cardinalClosed.js";
+export {default as curveCardinalOpen} from "./curve/cardinalOpen.js";
+export {default as curveCardinal} from "./curve/cardinal.js";
+export {default as curveCatmullRomClosed} from "./curve/catmullRomClosed.js";
+export {default as curveCatmullRomOpen} from "./curve/catmullRomOpen.js";
+export {default as curveCatmullRom} from "./curve/catmullRom.js";
+export {default as curveLinearClosed} from "./curve/linearClosed.js";
+export {default as curveLinear} from "./curve/linear.js";
+export {monotoneX as curveMonotoneX, monotoneY as curveMonotoneY} from "./curve/monotone.js";
+export {default as curveNatural} from "./curve/natural.js";
+export {default as curveStep, stepAfter as curveStepAfter, stepBefore as curveStepBefore} from "./curve/step.js";
 
-export {default as stack} from "./stack";
-export {default as stackOffsetExpand} from "./offset/expand";
-export {default as stackOffsetDiverging} from "./offset/diverging";
-export {default as stackOffsetNone} from "./offset/none";
-export {default as stackOffsetSilhouette} from "./offset/silhouette";
-export {default as stackOffsetWiggle} from "./offset/wiggle";
-export {default as stackOrderAppearance} from "./order/appearance";
-export {default as stackOrderAscending} from "./order/ascending";
-export {default as stackOrderDescending} from "./order/descending";
-export {default as stackOrderInsideOut} from "./order/insideOut";
-export {default as stackOrderNone} from "./order/none";
-export {default as stackOrderReverse} from "./order/reverse";
+export {default as stack} from "./stack.js";
+export {default as stackOffsetExpand} from "./offset/expand.js";
+export {default as stackOffsetDiverging} from "./offset/diverging.js";
+export {default as stackOffsetNone} from "./offset/none.js";
+export {default as stackOffsetSilhouette} from "./offset/silhouette.js";
+export {default as stackOffsetWiggle} from "./offset/wiggle.js";
+export {default as stackOrderAppearance} from "./order/appearance.js";
+export {default as stackOrderAscending} from "./order/ascending.js";
+export {default as stackOrderDescending} from "./order/descending.js";
+export {default as stackOrderInsideOut} from "./order/insideOut.js";
+export {default as stackOrderNone} from "./order/none.js";
+export {default as stackOrderReverse} from "./order/reverse.js";
diff --git a/src/line.js b/src/line.js
index 16816ce..3ba2ab3 100644
--- a/src/line.js
+++ b/src/line.js
@@ -1,7 +1,7 @@
 import {path} from "d3-path";
-import constant from "./constant";
-import curveLinear from "./curve/linear";
-import {x as pointX, y as pointY} from "./point";
+import constant from "./constant.js";
+import curveLinear from "./curve/linear.js";
+import {x as pointX, y as pointY} from "./point.js";
 
 export default function() {
   var x = pointX,
diff --git a/src/lineRadial.js b/src/lineRadial.js
index 92697b9..beaf577 100644
--- a/src/lineRadial.js
+++ b/src/lineRadial.js
@@ -1,5 +1,5 @@
-import curveRadial, {curveRadialLinear} from "./curve/radial";
-import line from "./line";
+import curveRadial, {curveRadialLinear} from "./curve/radial.js";
+import line from "./line.js";
 
 export function lineRadial(l) {
   var c = l.curve;
diff --git a/src/link/index.js b/src/link/index.js
index 71f2786..b3075d3 100644
--- a/src/link/index.js
+++ b/src/link/index.js
@@ -1,8 +1,8 @@
 import {path} from "d3-path";
-import {slice} from "../array";
-import constant from "../constant";
-import {x as pointX, y as pointY} from "../point";
-import pointRadial from "../pointRadial";
+import {slice} from "../array.js";
+import constant from "../constant.js";
+import {x as pointX, y as pointY} from "../point.js";
+import pointRadial from "../pointRadial.js";
 
 function linkSource(d) {
   return d.source;
diff --git a/src/offset/expand.js b/src/offset/expand.js
index 3aa0389..965bea1 100644
--- a/src/offset/expand.js
+++ b/src/offset/expand.js
@@ -1,4 +1,4 @@
-import none from "./none";
+import none from "./none.js";
 
 export default function(series, order) {
   if (!((n = series.length) > 0)) return;
diff --git a/src/offset/silhouette.js b/src/offset/silhouette.js
index 31afd25..87829be 100644
--- a/src/offset/silhouette.js
+++ b/src/offset/silhouette.js
@@ -1,4 +1,4 @@
-import none from "./none";
+import none from "./none.js";
 
 export default function(series, order) {
   if (!((n = series.length) > 0)) return;
diff --git a/src/offset/wiggle.js b/src/offset/wiggle.js
index 0f8c12b..8db717c 100644
--- a/src/offset/wiggle.js
+++ b/src/offset/wiggle.js
@@ -1,4 +1,4 @@
-import none from "./none";
+import none from "./none.js";
 
 export default function(series, order) {
   if (!((n = series.length) > 0) || !((m = (s0 = series[order[0]]).length) > 0)) return;
diff --git a/src/order/appearance.js b/src/order/appearance.js
index 731d076..e052924 100644
--- a/src/order/appearance.js
+++ b/src/order/appearance.js
@@ -1,4 +1,4 @@
-import none from "./none";
+import none from "./none.js";
 
 export default function(series) {
   var peaks = series.map(peak);
diff --git a/src/order/ascending.js b/src/order/ascending.js
index c97e118..e0d28e3 100644
--- a/src/order/ascending.js
+++ b/src/order/ascending.js
@@ -1,4 +1,4 @@
-import none from "./none";
+import none from "./none.js";
 
 export default function(series) {
   var sums = series.map(sum);
diff --git a/src/order/descending.js b/src/order/descending.js
index c65b9a4..dd27201 100644
--- a/src/order/descending.js
+++ b/src/order/descending.js
@@ -1,4 +1,4 @@
-import ascending from "./ascending";
+import ascending from "./ascending.js";
 
 export default function(series) {
   return ascending(series).reverse();
diff --git a/src/order/insideOut.js b/src/order/insideOut.js
index 7b8d1b9..b0b2abd 100644
--- a/src/order/insideOut.js
+++ b/src/order/insideOut.js
@@ -1,5 +1,5 @@
-import appearance from "./appearance";
-import {sum} from "./ascending";
+import appearance from "./appearance.js";
+import {sum} from "./ascending.js";
 
 export default function(series) {
   var n = series.length,
diff --git a/src/order/reverse.js b/src/order/reverse.js
index b93af7c..8380ca0 100644
--- a/src/order/reverse.js
+++ b/src/order/reverse.js
@@ -1,4 +1,4 @@
-import none from "./none";
+import none from "./none.js";
 
 export default function(series) {
   return none(series).reverse();
diff --git a/src/pie.js b/src/pie.js
index 49cbc78..1108722 100644
--- a/src/pie.js
+++ b/src/pie.js
@@ -1,7 +1,7 @@
-import constant from "./constant";
-import descending from "./descending";
-import identity from "./identity";
-import {tau} from "./math";
+import constant from "./constant.js";
+import descending from "./descending.js";
+import identity from "./identity.js";
+import {tau} from "./math.js";
 
 export default function() {
   var value = identity,
diff --git a/src/stack.js b/src/stack.js
index e36e885..ab4008e 100644
--- a/src/stack.js
+++ b/src/stack.js
@@ -1,7 +1,7 @@
-import {slice} from "./array";
-import constant from "./constant";
-import offsetNone from "./offset/none";
-import orderNone from "./order/none";
+import {slice} from "./array.js";
+import constant from "./constant.js";
+import offsetNone from "./offset/none.js";
+import orderNone from "./order/none.js";
 
 function stackValue(d, key) {
   return d[key];
diff --git a/src/symbol/circle.js b/src/symbol/circle.js
index ff8098d..4653651 100644
--- a/src/symbol/circle.js
+++ b/src/symbol/circle.js
@@ -1,4 +1,4 @@
-import {pi, tau} from "../math";
+import {pi, tau} from "../math.js";
 
 export default {
   draw: function(context, size) {
diff --git a/src/symbol/star.js b/src/symbol/star.js
index 7d3c19a..c3560c3 100644
--- a/src/symbol/star.js
+++ b/src/symbol/star.js
@@ -1,4 +1,4 @@
-import {pi, tau} from "../math";
+import {pi, tau} from "../math.js";
 
 var ka = 0.89081309152928522810,
     kr = Math.sin(pi / 10) / Math.sin(7 * pi / 10),
diff --git a/src/symbol.js b/src/symbol.js
index bd147ce..74530de 100644
--- a/src/symbol.js
+++ b/src/symbol.js
@@ -1,12 +1,12 @@
 import {path} from "d3-path";
-import circle from "./symbol/circle";
-import cross from "./symbol/cross";
-import diamond from "./symbol/diamond";
-import star from "./symbol/star";
-import square from "./symbol/square";
-import triangle from "./symbol/triangle";
-import wye from "./symbol/wye";
-import constant from "./constant";
+import circle from "./symbol/circle.js";
+import cross from "./symbol/cross.js";
+import diamond from "./symbol/diamond.js";
+import star from "./symbol/star.js";
+import square from "./symbol/square.js";
+import triangle from "./symbol/triangle.js";
+import wye from "./symbol/wye.js";
+import constant from "./constant.js";
 
 export var symbols = [
   circle,