不能调用或构造名称空间样式的导入，并且将在运行时导致失败 [英] A namespace-style import cannot be called or constructed, and will cause a failure at runtime

问题描述

在这里，发生奇怪的事情，使用TypeScript 2.7.2，在VSCode版本1.21中带有@ types/express及其后继的代码，在某些情况下，VSCode引发错误，指出无法调用或构造名称空间样式的导入，并且将在运行时导致失败.".但是，在具有类似设置和类似tsconfig.json文件的其他计算机上，代码也可以正常工作.

Strange stuff happening here, with TypeScript 2.7.2, in VSCode version 1.21 with @types/express and the code that follows, in some cases VSCode throws errors stating that "A namespace-style import cannot be called or constructed, and will cause a failure at runtime.". However on other machines with similar setups and similar tsconfig.json files the code just works.. What is happening here:

import { Bank } from './Bank';
import * as Express from 'express';  <== errors here..

let app: Express.Express;
this.app = Express();                <== and here

为什么会这样?

TIA，

约翰.

推荐答案

该错误仅在esModuleInterop: true时发生.也许VSCode忽略了您的tsconfig.json或使用了另一个esModuleInterop: true的.

The error should only happen with esModuleInterop: true. Maybe VSCode is ignoring your tsconfig.json or another one with esModuleInterop: true is used.

要进行最终修复，请将编译器选项esModuleInterop设置为true并使用import express而不是import * as express.

For a definitive fix set the compiler option esModuleInterop to true and use import express instead of import * as express.

问题:

ES6规范定义了命名空间对象"的概念.在此语句中，名称空间对象为namespaceObject:import * as namespaceObject from 'a-module'. typeof这个对象是object，您不应调用它.

The ES6 specification defines the notion of "namespace object". A namespace object is namespaceObject in this statement : import * as namespaceObject from 'a-module'. The typeof this object is object, you are not supposed to be able to call it.

到目前为止，您一直在使用import * as express，因为express是用于Node.js的CommonJS模块，它是使用module.exports导出的.但是，这在ES6规范中是非法的，TypeScript现在会警告您.

You were using import * as express until now because express is a CommonJS module, for Node.js, it is exported using module.exports. However it is illegal in the ES6 spec, and TypeScript now warns you.

解决方案:

将esModuleInterop设置为true将使TypeScript包装您的import调用，以检查该模块是ES6模块还是CommonJS.如果它是CommonJS模块，而您正在使用import default from 'module'，那么TypeScript将找出并返回正确的CommonJS模块.

Setting esModuleInterop to true will make TypeScript wrap your import calls to check if the module is an ES6 module or a CommonJS. If it's a CommonJS module and you are using import default from 'module' TypeScript will find out and return the correct CommonJS module.

从 TypeScript发行说明中:

注意:新行为将添加到标志下，以避免不必要的行为 破坏现有的代码库.我们强烈建议将两者都应用于 新的和现有的项目.对于现有项目，名称空间导入 (从* express中导入*为express； express();)必须为 转换为默认导入(来自"express"的导入快递； express();).

Note: The new behavior is added under a flag to avoid unwarranted breaks to existing code bases. We highly recommend applying it both to new and existing projects. For existing projects, namespace imports (import * as express from "express"; express();) will need to be converted to default imports (import express from "express"; express();).

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