[英]Difference between import X and import * as X in node.js (ES6 / Babel)?
I have a node.js library lib
written in ES6 (compiled with Babel ) in which I export the following submodules:我有一个用 ES6 编写的 node.js 库lib
(用Babel编译),我在其中导出以下子模块:
"use strict";
import * as _config from './config';
import * as _db from './db';
import * as _storage from './storage';
export var config = _config;
export var db = _db;
export var storage = _storage;
If from my main project I include the library like this如果从我的主项目中包含这样的库
import * as lib from 'lib';
console.log(lib);
I can see the proper output and it work as expected { config: ... }
.我可以看到正确的输出,它按预期工作{ config: ... }
。 However, if I try to include the library like this:但是,如果我尝试包含这样的库:
import lib from 'lib';
console.log(lib);
it will be undefined
.它将是undefined
。
Can someone explain what is happening here?有人可以解释这里发生了什么吗? Aren't the two import methods supposed to be equivalent?这两种导入方法不应该是等效的吗? If not, what difference am I missing?如果没有,我缺少什么区别?
import * as lib from 'lib';
is asking for an object with all of the named exports of 'lib'.正在请求一个对象,其中包含所有命名为“lib”的导出。
export var config = _config;
export var db = _db;
export var storage = _storage;
are named exports, which is why you end up with an object like you did.被命名为导出,这就是为什么你最终得到一个像你一样的对象。
import lib from 'lib';
is asking for the default
export of lib
.要求default
导出lib
。 eg例如
export default 4;
would make lib === 4
.将使lib === 4
。 It does not fetch the named exports.它不获取命名的导出。 To get an object from the default export, you'd have to explicitly do要从默认导出中获取对象,您必须明确地执行
export default {
config: _config,
db: _db,
storage: _storage
};
Just adding to Logan's solution because understanding import with brackets, * and without solved a problem for me.只是添加到Logan 的解决方案中,因为理解带括号的导入,* 并且没有为我解决问题。
import * as lib from 'lib';
is the equivalent of:相当于:
import {config, db, storage} as lib from 'lib';
Where the * is similar to a wildcard that imports all of the export var
from lib.其中 * 类似于从 lib 导入所有export var
的通配符。
export var config;
export var db;
export var storage;
Alternatively, using:或者,使用:
import lib from 'lib';
Allows you to only access the default export:允许您仅访问默认导出:
// lib.js
export default storage;
Using {} also only imports specific components from the module, which reduces the footprint with bundlers like Webpack.使用 {} 也仅从模块导入特定组件,这减少了 Webpack 等打包程序的占用空间。
While:尽管:
import storage, { config, db } from './lib'
would import all modules including export default storage;
将导入所有模块,包括export default storage;
See Dan Abramov's answer: When should I use curly braces for ES6 import?请参阅 Dan Abramov 的回答: 何时应使用花括号进行 ES6 导入?
import X from Y;
is a syntax sugar.是一种语法糖。
import lib from 'lib';
is equal to等于
import { default as lib } from 'lib';
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.