ESM import.meta All In One

ESM import.meta All In One

获取 ES Module 的 meta 原数据

import.meta

The import.meta meta-property exposes context-specific metadata to a JavaScript module.
It contains information about the module, such as the module's URL.

import.meta.url

The full URL to the module, includes query parameters and/or hash (following the ? or #).
In browsers, this is either the URL from which the script was obtained (for external scripts), or the URL of the containing document (for inline scripts).
In Node.js, this is the file path (including the file:// protocol).

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import.meta

使用场景

• 使用 ESM 在 Node.js 中实现 __dirname 功能, 获取模块所在的文件夹的绝对路径
• ...

import fs from 'node:fs';
import path from 'node:path';
import { fileURLToPath } from 'node:url';

const __filename = fileURLToPath(import.meta.url);
const __dirname = path.dirname(__filename);
console.log(`✅ import.meta.url =`, import.meta.url, typeof import.meta.url)
console.log(`✅ __filename =`, __filename, typeof __filename)
console.log(`✅ __dirname`, __dirname, typeof __dirname)

errors

solutions

1. 浏览器 <script type="module">

<script type="module">
  console.log(`import.meta.url =`, import.meta.url);
</script>
<!-- 或 -->
<script src="./main.mjs" type="module"></script>

// main.mjs
console.log(`✅ import.meta.url =`, import.meta.url);

2. Node.js import.meta.resolve

const absolutePath = await import.meta.resolve(specifier[, parent]);

// main.mjs
const resolvedPath = await import.meta.resolve('./helper.mjs');

https://nodejs.org/docs/latest-v15.x/api/esm.html#esm_import_meta

https://deno.land/manual@v1.36.3/runtime/import_meta_api

demos

ESM 中实现 __dirname

import fs from 'node:fs';
import path from 'node:path';
import { fileURLToPath } from 'node:url';
// import fs from 'fs';
// import path from 'path';
// import { fileURLToPath } from 'url';

// create one custom `__dirname`, because it doesn't exist in es-module env.
// use `import.meta.url` to get the current module's URL, ✅
// then get the module's folder absolute path 
const __filename = fileURLToPath(import.meta.url);
const __dirname = path.dirname(__filename);
const dir = path.resolve(path.join(__dirname, 'upload');

if (!fs.existsSync(dir)) {
  fs.mkdirSync(dir);
}

// OR
if (!fs.existsSync(dir)) {
  fs.mkdirSync(dir, {
    mode: 0o744, // Not supported on Windows. Default: 0o777
  });
}

CJS __dirname

const fs = require('fs');
const path = require('path');

const dir = path.resolve(path.join(__dirname, 'upload');

if (!fs.existsSync(dir)) {
  fs.mkdirSync(dir);
}

// OR
if (!fs.existsSync(dir)) {
  fs.mkdirSync(dir, {
    mode: 0o744, // Not supported on Windows. Default: 0o777
  });
}

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node:fs vs fs

import fs from 'node:fs';

import fs from 'fs';

node: 类似 https://, file:// 是一种通信协议 ❓

Modules: node:module API

ESM

// module.mjs
// In an ECMAScript module
import { builtinModules as builtin } from 'node:module';

CJS

// module.cjs
// In a CommonJS module
const builtin = require('node:module').builtinModules;

https://nodejs.org/dist/latest-v18.x/docs/api/module.html

refs

https://stackoverflow.com/questions/21194934/how-to-create-a-directory-if-it-doesnt-exist-using-node-js/71735771#71735771

https://stackoverflow.com/questions/67554506/what-are-the-nodefs-nodepath-etc-modules

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