如何从一个文件中获取变量到node.js中的另一个文件 [英] how to get a variable from a file to another file in node.js

问题描述

这是我的第一个文件:

var self=this;
var config ={
    'confvar':'configval'
};

我希望将此配置变量保存在另一个文件中，所以我在另一个文件中所做的是:

I want this config variable in another file, so what I have done in another file is:

conf = require('./conf');
    url=conf.config.confvar;

但这给我一个错误.

TypeError: Cannot read property 'confvar' of undefined

请提出我该怎么办?

推荐答案

您需要的是 module.exports

导出

在当前模块的所有实例之间共享的对象 并通过require()进行访问.出口与 module.exports对象.有关更多信息，请参见src/node.js.出口商品 实际上不是每个模块的全局变量，而是局部模块.

An object which is shared between all instances of the current module and made accessible through require(). exports is the same as the module.exports object. See src/node.js for more information. exports isn't actually a global but rather local to each module.

例如，如果要在sourceFile.js上使用值为"variableValue"的variableName公开，则可以这样设置整个导出:

For example, if you would like to expose variableName with value "variableValue" on sourceFile.js then you can either set the entire exports as such:

module.exports = { variableName: "variableValue" };

或者您可以使用以下方法设置单个值:

OR you can set the individual value with:

module.exports.variableName = "variableValue";

要在另一个文件中使用该值，则需要先require(...)(具有相对路径):

To consume that value in another file, you need to require(...) it first (with relative pathing):

const sourceFile = require('./sourceFile');
console.log(sourceFile.variableName);

或者，您可以对其进行解构.

Alternatively, you can deconstruct it.

const { variableName } = require('./sourceFile');

如果要从文件中删除的全部是variableName，则

If all you want out of the file is variableName then

const variableName = 'variableValue'
module.exports = variableName

./consumer.js:

const variableName = require('./sourceFile')

编辑(2020):

自Node.js version 8.9.0起，您还可以使用具有不同支持级别的ECMAScript模块. 文档

Edit (2020):

Since Node.js version 8.9.0, you can also use ECMAScript Modules with varying levels of support. The docs

• 对于Node v13.9.0及更高版本，默认情况下启用实验模块
• 对于低于13.9.0版本的Node版本，请使用--experimental-modules

在作为初始输入传递给节点或被ES模块代码中的import语句引用时，Node.js会将以下内容视为ES模块:

Node.js will treat the following as ES modules when passed to node as the initial input, or when referenced by import statements within ES module code:

• 以.mjs结尾的文件.
• 当最近的父级package.json文件包含值为"module"的顶级字段"type"时，文件以.js结尾.
• 作为参数传递给--eval或--print的字符串，或通过带有标志--input-type = module的STDIN通过管道传递给节点的信号.

• Files ending in .mjs.
• Files ending in .js when the nearest parent package.json file contains a top-level field "type" with a value of "module".
• Strings passed in as an argument to --eval or --print, or piped to node via STDIN, with the flag --input-type=module.

设置完成后，就可以使用import和export.

Once you have it setup, you can use import and export.

使用上面的示例，您可以采用两种方法

Using the example above, there are two approaches you can take

// this is a named export of variableName
export const variableName = 'variableValue'
// alternatively, you could have exported it as a default. 
// For sake of explanation, I'm wrapping the variable in an object
// but it is not necessary. 
// you can actually omit declaring what variableName is here. 
// { variableName } is equivalent to { variableName: variableName } in this case. 
export default { variableName: variableName } 

./consumer.js:

// there are three ways of importing. 
// If you need access to a non-default export, then 
// you use { nameOfExportedVariable } 
import { variableName } from './sourceFile'
console.log(variableName) // 'variableValue'

// Otherwise, you simply provide a local variable name 
// for what was exported as default.
import sourceFile from './sourceFile'
console.log(sourceFile.variableName) // 'variableValue'

./sourceFileWithoutDefault.js:

// The third way of importing is for situations where there
// isn't a default export but you want to warehouse everything
// under a single variable. Say you have:
export const a = 'A'
export const b = 'B'

./consumer2.js

// then you can import all exports under a single variable
// with the usage of * as:
import * as sourceFileWithoutDefault from './sourceFileWithoutDefault'

console.log(sourceFileWithoutDefault.a) // 'A'
console.log(sourceFileWithoutDefault.b) // 'B'

// you can use this approach even if there is a default export:
import * as sourceFile from './sourceFile'

// default exports are under the variable default:
console.log(sourceFile.default) // { variableName: 'variableValue' }

// as well as named exports:
console.log(sourceFile.variableName) // 'variableValue

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