有关node.js导出的问题

Question

I am reading some other's code, I saw a piece of code like this

if ( typeof module === 'object' ) {
    module.exports = BBB;
}

I am wondering 1) why using if statement here 2) when using module.exports, does it mean all BBB namespace is exported

BTW, BBB is a namespace defined like

var BBB = {};

Answer 1

It's likely the module is intended to work in both the browser and on the server side (presumably using node.js).

Creating a sort of wrapper for your module allows it to be used in a variety of javascript loaders such as AMD or RequireJS or CommonJS (used by node)

I'd also recommend looking at umdjs/umd (Universal Module Definition). This repo documents how you can create a wrapper for your module so that it will work in every environment you target.

---

Lastly, you can sort of think of module.exports like a return value of a function. When someone imports the module, the export is what is given to them.

If this is used

// bbb.js
module.exports = BBB;

When the module is required using (for example)

// otherfile.js
var BBB = require('./bbb');

BBB will match the exported object.

Check out the node.js module docs for more general help.

Answer 2

它是要检测您所处的环境，模块存在于节点中，而不存在于没有浏览器或类似功能的浏览器中

Answer 3

Most likely the author is creating a module that may not necessarily be consumed in a CommonJS enviorment, but still is offering support for it.

The CommonJS standard defines a few free variables, require , exports , and module . module must be an Object.

So when the author is checking that module is of type object , they are essentialy checking for CommonJS support, they are then assigning module.exports to BBB , so that when a consumer require 's their module BBB is returned. I won't go into the details of CommonJS but you can check out the standard for more info.

Why would I check for CommonJS support?

Because your code is intended for use in multiple enviornments/packagers. For example Browserify, and Webpack use the CommonJS standard to package code for use in the browser. But when creating an API for the browser, one should assume that consumers may not be using CommonJS, therefore module will not be defined and assigning a value to module.exports will throw an error.

Answer 4

1) This checks if you are using this code on the server side (NodeJS).

2) Yes, all BBB namespace is exported

Here are all necessary information: http://www.sitepoint.com/understanding-module-exports-exports-node-js/