Babel Transformer that addds __dirname, __filename

inject-dirname.mjs

import {template} from '@babel/core'
import path from 'path'
import fs from 'fs'

const header = `
import { dirname as _INJECT_d } from 'path';
import { fileURLToPath as _INJECT_f } from 'url';
const __filename = _INJECT_f(import.meta.url)
const __dirname = _INJECT_d(__filename)
`

function getPackageType(file, prev) {
    // `url` is only a file path during the first iteration when passed the
    // resolved url from the load() hook
    // an actual file path from load() will contain a file extension as it's
    // required by the spec
    // this simple truthy check for whether `url` contains a file extension will
    // work for most projects but does not cover some edge-cases (such as
    // extensionless files or a url ending in a trailing space)
    const ext = path.extname(file);
    if (ext === "cjs") return "commonjs";
    if (ext === "mjs") return "module";
    // If it is a file path, get the directory it's in
    const dir = ext ? path.dirname(file) : file;
    // Compose a file path to a package.json in the same directory,
    // which may or may not exist
    const packagePath = path.resolve(dir, "package.json");
    // Try to read the possibly nonexistent package.json
    let type
    try {
         type = JSON.parse(fs.readFileSync(packagePath, { encoding: "utf8" })).type
    } catch(err) {
        if (err?.code !== "ENOENT") console.error(err);
    }
    // Ff package.json existed and contained a `type` field with a value, voila
    if (type) return type;
    // Otherwise, (if not at the root) continue checking the next directory up
    // If at the root, stop and return false
    return dir.length > 1 && getPackageType(path.resolve(dir, ".."), dir);
}

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

    const { types: t } = api;

    return {
        name: 'inject',

        visitor: {
            Program(path, state) {
                const filename = state.file.opts.parserOpts.sourceFilename || state.file.opts.filename || '/'
                const type = getPackageType(filename)
                // path.unshiftContainer('body', template(`console.log(${JSON.stringify({s:path.node.sourceType,b:path.scope.hasBinding('__dirname'), t:type,filename})});`, { sourceType: path.node.sourceType })())
                if (!path.scope.hasBinding('__dirname')) {
                    if (path.node.sourceType === 'module' && type === 'module') {
                        path.unshiftContainer('body', template(header, { sourceType: path.node.sourceType })())
                    }
                }
            },
        },
    };
}