unpkg/modules/plugins/unpkgRewrite.js

import URL from 'whatwg-url';
import warning from 'warning';

const bareIdentifierFormat = /^((?:@[^/]+\/)?[^/]+)(\/.*)?$/;

function isValidURL(value) {
  return URL.parseURL(value) != null;
}

function isProbablyURLWithoutProtocol(value) {
  return value.substr(0, 2) === '//';
}

function isAbsoluteURL(value) {
  return isValidURL(value) || isProbablyURLWithoutProtocol(value);
}

function isBareIdentifier(value) {
  return value.charAt(0) !== '.' && value.charAt(0) !== '/';
}

function rewriteValue(/* StringLiteral */ node, origin, dependencies) {
  if (isAbsoluteURL(node.value)) {
    return;
  }

  if (isBareIdentifier(node.value)) {
    // "bare" identifier
    const match = bareIdentifierFormat.exec(node.value);
    const packageName = match[1];
    const file = match[2] || '';

    warning(
      dependencies[packageName],
      'Missing version info for package "%s" in dependencies; falling back to "latest"',
      packageName
    );

    const version = dependencies[packageName] || 'latest';

    node.value = `${origin}/${packageName}@${version}${file}?module`;
  } else {
    // local path
    node.value = `${node.value}?module`;
  }
}

export default function unpkgRewrite(origin, dependencies = {}) {
  return {
    manipulateOptions(opts, parserOpts) {
      parserOpts.plugins.push(
        'dynamicImport',
        'exportDefaultFrom',
        'exportNamespaceFrom',
        'importMeta'
      );
    },

    visitor: {
      CallExpression(path) {
        if (path.node.callee.type !== 'Import') {
          // Some other function call, not import();
          return;
        }

        rewriteValue(path.node.arguments[0], origin, dependencies);
      },
      ExportAllDeclaration(path) {
        rewriteValue(path.node.source, origin, dependencies);
      },
      ExportNamedDeclaration(path) {
        if (!path.node.source) {
          // This export has no "source", so it's probably
          // a local variable or function, e.g.
          // export { varName }
          // export const constName = ...
          // export function funcName() {}
          return;
        }

        rewriteValue(path.node.source, origin, dependencies);
      },
      ImportDeclaration(path) {
        rewriteValue(path.node.source, origin, dependencies);
      }
    }
  };
}