index.js

import { declare } from "@babel/helper-plugin-utils";
import {
  isModule,
  rewriteModuleStatementsAndPrepareHeader,
  isSideEffectImport,
  buildNamespaceInitStatements,
  ensureStatementsHoisted,
  wrapInterop,
} from "@babel/helper-module-transforms";
import simplifyAccess from "@babel/helper-simple-access";
import { template, types as t } from "@babel/core";

export default declare((api, options) => {
  api.assertVersion(7);

  const {
    loose,

    // 'true' for non-mjs files to strictly have .default, instead of having
    // destructuring-like behavior for their properties.
    strictNamespace = false,

    // 'true' for mjs files to strictly have .default, instead of having
    // destructuring-like behavior for their properties.
    mjsStrictNamespace = true,
    allowTopLevelThis,
    strict,
    strictMode,
    noInterop,
    lazy = false,
    // Defaulting to 'true' for now. May change before 7.x major.
    allowCommonJSExports = true,
  } = options;

  if (
    typeof lazy !== "boolean" &&
    typeof lazy !== "function" &&
    (!Array.isArray(lazy) || !lazy.every(item => typeof item === "string"))
  ) {
    throw new Error(`.lazy must be a boolean, array of strings, or a function`);
  }

  if (typeof strictNamespace !== "boolean") {
    throw new Error(`.strictNamespace must be a boolean, or undefined`);
  }
  if (typeof mjsStrictNamespace !== "boolean") {
    throw new Error(`.mjsStrictNamespace must be a boolean, or undefined`);
  }

  const getAssertion = localName => template.expression.ast`
    (function(){
      throw new Error(
        "The CommonJS '" + "${localName}" + "' variable is not available in ES6 modules." +
        "Consider setting setting sourceType:script or sourceType:unambiguous in your " +
        "Babel config for this file.");
    })()
  `;

  const moduleExportsVisitor = {
    ReferencedIdentifier(path) {
      const localName = path.node.name;
      if (localName !== "module" && localName !== "exports") return;

      const localBinding = path.scope.getBinding(localName);
      const rootBinding = this.scope.getBinding(localName);

      if (
        // redeclared in this scope
        rootBinding !== localBinding ||
        (path.parentPath.isObjectProperty({ value: path.node }) &&
          path.parentPath.parentPath.isObjectPattern()) ||
        path.parentPath.isAssignmentExpression({ left: path.node }) ||
        path.isAssignmentExpression({ left: path.node })
      ) {
        return;
      }

      path.replaceWith(getAssertion(localName));
    },

    AssignmentExpression(path) {
      const left = path.get("left");
      if (left.isIdentifier()) {
        const localName = path.node.name;
        if (localName !== "module" && localName !== "exports") return;

        const localBinding = path.scope.getBinding(localName);
        const rootBinding = this.scope.getBinding(localName);

        // redeclared in this scope
        if (rootBinding !== localBinding) return;

        const right = path.get("right");
        right.replaceWith(
          t.sequenceExpression([right.node, getAssertion(localName)]),
        );
      } else if (left.isPattern()) {
        const ids = left.getOuterBindingIdentifiers();
        const localName = Object.keys(ids).filter(localName => {
          if (localName !== "module" && localName !== "exports") return false;

          return (
            this.scope.getBinding(localName) ===
            path.scope.getBinding(localName)
          );
        })[0];

        if (localName) {
          const right = path.get("right");
          right.replaceWith(
            t.sequenceExpression([right.node, getAssertion(localName)]),
          );
        }
      }
    },
  };

  return {
    name: "transform-modules-commonjs",

    visitor: {
      Program: {
        exit(path, state) {
          if (!isModule(path)) return;

          // Rename the bindings auto-injected into the scope so there is no
          // risk of conflict between the bindings.
          path.scope.rename("exports");
          path.scope.rename("module");
          path.scope.rename("require");
          path.scope.rename("__filename");
          path.scope.rename("__dirname");

          // Rewrite references to 'module' and 'exports' to throw exceptions.
          // These objects are specific to CommonJS and are not available in
          // real ES6 implementations.
          if (!allowCommonJSExports) {
            simplifyAccess(path, new Set(["module", "exports"]));
            path.traverse(moduleExportsVisitor, {
              scope: path.scope,
            });
          }

          let moduleName = this.getModuleName();
          if (moduleName) moduleName = t.stringLiteral(moduleName);

          const { meta, headers } = rewriteModuleStatementsAndPrepareHeader(
            path,
            {
              exportName: "exports",
              loose,
              strict,
              strictMode,
              allowTopLevelThis,
              noInterop,
              lazy,
              esNamespaceOnly:
                typeof state.filename === "string" &&
                /\.mjs$/.test(state.filename)
                  ? mjsStrictNamespace
                  : strictNamespace,
            },
          );

          for (const [source, metadata] of meta.source) {
            const loadExpr = t.callExpression(t.identifier("require"), [
              t.stringLiteral(source),
            ]);

            let header;
            if (isSideEffectImport(metadata)) {
              if (metadata.lazy) throw new Error("Assertion failure");

              header = t.expressionStatement(loadExpr);
            } else {
              const init =
                wrapInterop(path, loadExpr, metadata.interop) || loadExpr;

              if (metadata.lazy) {
                header = template.ast`
                  function ${metadata.name}() {
                    const data = ${init};
                    ${metadata.name} = function(){ return data; };
                    return data;
                  }
                `;
              } else {
                header = template.ast`
                  var ${metadata.name} = ${init};
                `;
              }
            }
            header.loc = metadata.loc;

            headers.push(header);
            headers.push(
              ...buildNamespaceInitStatements(meta, metadata, loose),
            );
          }

          ensureStatementsHoisted(headers);
          path.unshiftContainer("body", headers);
        },
      },
    },
  };
});