要求中的双括号是什么意思 [英] What does double parentheses mean in a require

问题描述

我知道这个 require 语句的作用.

I know what this require statement does.

var express = require('express');
var app = express();

但有时我会在 require 之后看到两个括号.

But sometimes i have seen two parentheses after the require.

var routes = require('./routes')(app);

问)这是什么意思，它是如何工作的?

Q) What does this mean, and how does it work?

推荐答案

这是一种模式，其中 module.exports 您需要的模块被设置为一个函数.要求该模块返回一个函数，并且 require 后面的括号用参数对函数求值.

This is a pattern in which the module.exports of the module you are requiring is set to a function. Requiring that module returns a function, and the parentheses after the require evaluates the function with an argument.

在上面的示例中，您的 ./routes/index.js 文件将类似于以下内容:

In your example above, your ./routes/index.js file would look something like the following:

module.exports = function(app) {
  app.get('/', function(req, res) {

  });
  // ...
};

这种模式通常用于将变量传递给模块，如上面的 app 变量所示.

This pattern is often used to pass variables to modules, as can bee seen above with the app variable.

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