Use Closure compiler for smallest possible bundle.

.gitignore

@@ -1 +1,5 @@
 node_modules
+*.swp
+package-lock.json
+sharedb-client-browser*
+

build.mjs

@@ -0,0 +1,61 @@
+import { rollup } from 'rollup';
+import { nodeResolve } from '@rollup/plugin-node-resolve';
+import commonjs from '@rollup/plugin-commonjs';
+import nodePolyfills from 'rollup-plugin-node-polyfills';
+import { terser } from "rollup-plugin-terser";
+import compiler from '@ampproject/rollup-plugin-closure-compiler';
+
+// Inspired by
+// https://rollupjs.org/guide/en/#rolluprollup
+const buildBundle = async ({ inputOptions, outputOptions }) => {
+  const bundle = await rollup(inputOptions);
+  const { output } = await bundle.generate(outputOptions);
+  await bundle.write(outputOptions);
+  await bundle.close();
+};
+
+// Build the client bundle, non-optimized.
+const buildClient = async () => {
+  await buildBundle({
+    inputOptions: {
+      input: 'client.js',
+      plugins: [
+        commonjs(),
+        nodePolyfills(),
+        nodeResolve(),
+      ],
+    },
+    outputOptions: {
+      file: 'sharedb-client-browser.js',
+      format: 'iife',
+    },
+  });
+};
+
+// Optimize the size of the bundle.
+const buildOptimized = async () => {
+  await buildBundle({
+    inputOptions: {
+      input: 'client.js',
+      // Results:
+      //  - Terser alone           --> 86.2 kB
+      //  - Closure Compiler alone --> 68.0 kB
+      //  - Both --> 67.2 kB
+      // Went with both.
+      plugins: [
+        commonjs(),
+        nodePolyfills(),
+        nodeResolve(),
+        compiler({ compilation_level: 'ADVANCED' }),
+        terser(),
+      ],
+    },
+    outputOptions: {
+      file: 'sharedb-client-browser.min.js',
+      format: 'iife'
+    },
+  });
+};
+
+await buildClient();
+await buildOptimized();