block-hoist-plugin.js

// @flow

import sortBy from "lodash.sortby";

import loadConfig, { type Plugin } from "../config";

let LOADED_PLUGIN: Plugin | void;

export default function loadBlockHoistPlugin(): Plugin {
  if (!LOADED_PLUGIN) {
    // Lazy-init the internal plugin to remove the init-time circular
    // dependency between plugins being passed @babel/core's export object,
    // which loads this file, and this 'loadConfig' loading plugins.
    const config = loadConfig.sync({
      babelrc: false,
      configFile: false,
      plugins: [blockHoistPlugin],
    });
    LOADED_PLUGIN = config ? config.passes[0][0] : undefined;
    if (!LOADED_PLUGIN) throw new Error("Assertion failure");
  }

  return LOADED_PLUGIN;
}

const blockHoistPlugin = {
  /**
   * [Please add a description.]
   *
   * Priority:
   *
   *  - 0 We want this to be at the **very** bottom
   *  - 1 Default node position
   *  - 2 Priority over normal nodes
   *  - 3 We want this to be at the **very** top
   *  - 4 Reserved for the helpers used to implement module imports.
   */

  name: "internal.blockHoist",

  visitor: {
    Block: {
      exit({ node }) {
        let hasChange = false;
        for (let i = 0; i < node.body.length; i++) {
          const bodyNode = node.body[i];
          if (bodyNode?._blockHoist != null) {
            hasChange = true;
            break;
          }
        }
        if (!hasChange) return;

        node.body = sortBy(node.body, function (bodyNode) {
          let priority = bodyNode?._blockHoist;
          if (priority == null) priority = 1;
          if (priority === true) priority = 2;

          // Higher priorities should move toward the top.
          return -1 * priority;
        });
      },
    },
  },
};