Convert build scripts to ESM (#38984)

build/rollup.config.mjs

@@ -0,0 +1,59 @@
+import path from 'node:path'
+import process from 'node:process'
+import { fileURLToPath } from 'node:url'
+import { babel } from '@rollup/plugin-babel'
+import { nodeResolve } from '@rollup/plugin-node-resolve'
+import replace from '@rollup/plugin-replace'
+import banner from './banner.mjs'
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url))
+
+const BUNDLE = process.env.BUNDLE === 'true'
+const ESM = process.env.ESM === 'true'
+
+let destinationFile = `bootstrap${ESM ? '.esm' : ''}`
+const external = ['@popperjs/core']
+const plugins = [
+  babel({
+    // Only transpile our source code
+    exclude: 'node_modules/**',
+    // Include the helpers in the bundle, at most one copy of each
+    babelHelpers: 'bundled'
+  })
+]
+const globals = {
+  '@popperjs/core': 'Popper'
+}
+
+if (BUNDLE) {
+  destinationFile += '.bundle'
+  // Remove last entry in external array to bundle Popper
+  external.pop()
+  delete globals['@popperjs/core']
+  plugins.push(
+    replace({
+      'process.env.NODE_ENV': '"production"',
+      preventAssignment: true
+    }),
+    nodeResolve()
+  )
+}
+
+const rollupConfig = {
+  input: path.resolve(__dirname, `../js/index.${ESM ? 'esm' : 'umd'}.js`),
+  output: {
+    banner: banner(),
+    file: path.resolve(__dirname, `../dist/js/${destinationFile}.js`),
+    format: ESM ? 'esm' : 'umd',
+    globals,
+    generatedCode: 'es2015'
+  },
+  external,
+  plugins
+}
+
+if (!ESM) {
+  rollupConfig.output.name = 'bootstrap'
+}
+
+export default rollupConfig