BREAKING CHANGE: We keep external dynamic imports in CommonJS output by default as import()

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

• resolves Allow preservation of dynamic import with variable in CJS #4610

Description

In order to complete the Node ESM story in Rollup 3, I decided to add this last feature to Rollup 3. This will by default keep external (and only those) dynamic imports in CommonJS output as import(…) to support ESM interop. You can deactivate this behavior via a new option dynamicImportInCjs: false, which will activate the old rewriting behavior.

output.dynamicImportInCjs

Type: boolean
CLI: --dynamicImportInCjs/--no-dynamicImportInCjs
Default: true

While CommonJS output originally supported only require(…) to import dependencies, recent Node versions also started to support import(…), which is the only way to import ES modules from CommonJS files. If this option is true, which is the default, Rollup will keep external dynamic imports as import(…) expressions in CommonJS output. Set this to false to rewrite dynamic imports using require(…) syntax.

// input
import('external').then(console.log);

// cjs output with dynamicImportInCjs: true or not set
'use strict';

import('external').then(console.log);

// cjs output with dynamicImportInCjs: false
'use strict';

function _interopNamespaceDefault(e) {
  var n = Object.create(null);
  if (e) {
    Object.keys(e).forEach(function (k) {
      if (k !== 'default') {
        var d = Object.getOwnPropertyDescriptor(e, k);
        Object.defineProperty(
          n,
          k,
          d.get
            ? d
            : {
                enumerable: true,
                get: function () {
                  return e[k];
                }
              }
        );
      }
    });
  }
  n.default = e;
  return Object.freeze(n);
}

Promise.resolve()
  .then(function () {
    return /*#__PURE__*/ _interopNamespaceDefault(require('external'));
  })
  .then(console.log);