Adopting a dual dist/ bundled UMD and bundled ES Module strategy.

package.json

@@ -3,6 +3,7 @@
   "version": "0.0.2",
   "license": "ISC",
   "main": "src/index",
+  "module": "dist/notebook-stdlib.js",
   "author": {
     "name": "Observable, Inc."
   },
@@ -29,6 +30,7 @@
     "jsdom": "^11.6.2",
     "rollup": "^0.52.0",
     "rollup-plugin-node-resolve": "^3.0.0",
+    "rollup-plugin-uglify": "^3.0.0",
     "tape": "^4.9.0",
     "tape-await": "0.1.0"
   }

rollup.config.js

@@ -1,15 +1,30 @@
 import node from "rollup-plugin-node-resolve";
+import uglify from "rollup-plugin-uglify";
 
-export default {
-  input: "src/index.js",
-  plugins: [
-    node()
-  ],
-  output: {
-    extend: true,
-    banner: `// @observablehq/notebook-stdlib Copyright ${(new Date).getFullYear()} Observable, Inc.`,
-    file: "dist/notebook-stdlib.js",
-    format: "umd",
-    name: "Observable"
-  }
+const defaultOutput = {
+  banner: `// @observablehq/notebook-stdlib Copyright ${(new Date).getFullYear()} Observable, Inc.`,
 };
+
+function config(output) {
+  return {
+    input: "src/index.js",
+    plugins: [
+      node(),
+      uglify()
+    ],
+    output: Object.assign(output, defaultOutput)
+  };
+}
+
+export default [
+  config({
+    format: "es",
+    file: "dist/notebook-stdlib.js"
+  }),
+  config({
+    format: "umd",
+    extend: true,
+    name: "Observable",
+    file: "dist/notebook-stdlib.umd.js"
+  })
+];

yarn.lock

@@ -228,6 +228,10 @@ combined-stream@^1.0.5, combined-stream@~1.0.5:
   dependencies:
     delayed-stream "~1.0.0"
 
+commander@~2.13.0:
+  version "2.13.0"
+  resolved "https://registry.yarnpkg.com/commander/-/commander-2.13.0.tgz#6964bca67685df7c1f1430c584f07d7597885b9c"
+
 [email protected]:
   version "0.0.1"
   resolved "https://registry.yarnpkg.com/concat-map/-/concat-map-0.0.1.tgz#d8a96bd77fd68df7793a73036a3ba0d5405d477b"
@@ -1138,6 +1142,12 @@ rollup-plugin-node-resolve@^3.0.0:
     is-module "^1.0.0"
     resolve "^1.1.6"
 
+rollup-plugin-uglify@^3.0.0:
+  version "3.0.0"
+  resolved "https://registry.yarnpkg.com/rollup-plugin-uglify/-/rollup-plugin-uglify-3.0.0.tgz#a34eca24617709c6bf1778e9653baafa06099b86"
+  dependencies:
+    uglify-es "^3.3.7"
+
 rollup@^0.52.0:
   version "0.52.3"
   resolved "https://registry.yarnpkg.com/rollup/-/rollup-0.52.3.tgz#020d99fffe9619351e47b3894fd397c26f5e1bf6"
@@ -1200,6 +1210,10 @@ source-map@~0.5.6:
   version "0.5.7"
   resolved "https://registry.yarnpkg.com/source-map/-/source-map-0.5.7.tgz#8a039d2d1021d22d1ea14c80d8ea468ba2ef3fcc"
 
+source-map@~0.6.1:
+  version "0.6.1"
+  resolved "https://registry.yarnpkg.com/source-map/-/source-map-0.6.1.tgz#74722af32e9614e9c287a8d0bbde48b5e2f1a263"
+
 sprintf-js@~1.0.2:
   version "1.0.3"
   resolved "https://registry.yarnpkg.com/sprintf-js/-/sprintf-js-1.0.3.tgz#04e6926f662895354f3dd015203633b857297e2c"
@@ -1360,6 +1374,13 @@ typedarray@^0.0.6:
   version "0.0.6"
   resolved "https://registry.yarnpkg.com/typedarray/-/typedarray-0.0.6.tgz#867ac74e3864187b1d3d47d996a78ec5c8830777"
 
+uglify-es@^3.3.7:
+  version "3.3.9"
+  resolved "https://registry.yarnpkg.com/uglify-es/-/uglify-es-3.3.9.tgz#0c1c4f0700bed8dbc124cdb304d2592ca203e677"
+  dependencies:
+    commander "~2.13.0"
+    source-map "~0.6.1"
+
 ultron@~1.1.0:
   version "1.1.1"
   resolved "https://registry.yarnpkg.com/ultron/-/ultron-1.1.1.tgz#9fe1536a10a664a65266a1e3ccf85fd36302bc9c"