关于node.js的模块查找顺序（require.resolve()）

前几天社团群里有人问了阿里秋季前端笔试的一道题，想起来以前在官方文档看到过查找模块的算法，干脆自己写一写……

官方的require.resolve实现在这里. 因为我只是想看看查找过程，所以就直接把会被找的路径console.log出来看看而已。代码放在了我的github gist上：https://gist.github.com/joyeec9h3/74262a250b3e880c7fd4

结果如下

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check /home/somebody/node_modules/othermodule
check /home/somebody/node_modules/othermodule.js
check /home/somebody/node_modules/othermodule.json
check /home/somebody/node_modules/othermodule.node
---------------------------------------
if /home/somebody/node_modules/othermodule/package.json exists
check /home/somebody/node_modules/othermodule/package.json[main]
---------------------------------------
if /home/somebody/node_modules/othermodule/index.js exists
check /home/somebody/node_modules/othermodule/index.js
---------------------------------------
if /home/somebody/node_modules/othermodule/index.node exists
check /home/somebody/node_modules/othermodule/index.node
---------------------------------------
check /home/node_modules/othermodule
check /home/node_modules/othermodule.js
check /home/node_modules/othermodule.json
check /home/node_modules/othermodule.node
---------------------------------------
if /home/node_modules/othermodule/package.json exists
check /home/node_modules/othermodule/package.json[main]
---------------------------------------
if /home/node_modules/othermodule/index.js exists
check /home/node_modules/othermodule/index.js
---------------------------------------
if /home/node_modules/othermodule/index.node exists
check /home/node_modules/othermodule/index.node
---------------------------------------
check /node_modules/othermodule
check /node_modules/othermodule.js
check /node_modules/othermodule.json
check /node_modules/othermodule.node
---------------------------------------
if /node_modules/othermodule/package.json exists
check /node_modules/othermodule/package.json[main]
---------------------------------------
if /node_modules/othermodule/index.js exists
check /node_modules/othermodule/index.js
---------------------------------------
if /node_modules/othermodule/index.node exists
check /node_modules/othermodule/index.node
---------------------------------------

for each $PATH in $NODE_PATH

---------------------------------------
if $PATH/package.json exists
check $PATH/package.json[main]
---------------------------------------
if $PATH/index.js exists
check $PATH/index.js
---------------------------------------
if $PATH/index.node exists
check $PATH/index.node
---------------------------------------
if $HOME/.node_modules/package.json exists
check $HOME/.node_modules/package.json[main]
---------------------------------------
if $HOME/.node_modules/index.js exists
check $HOME/.node_modules/index.js
---------------------------------------
if $HOME/.node_modules/index.node exists
check $HOME/.node_modules/index.node
---------------------------------------
if $HOME/.node_libraries/package.json exists
check $HOME/.node_libraries/package.json[main]
---------------------------------------
if $HOME/.node_libraries/index.js exists
check $HOME/.node_libraries/index.js
---------------------------------------
if $HOME/.node_libraries/index.node exists
check $HOME/.node_libraries/index.node
---------------------------------------
if $PREFIX/lib/node/package.json exists
check $PREFIX/lib/node/package.json[main]
---------------------------------------
if $PREFIX/lib/node/index.js exists
check $PREFIX/lib/node/index.js
---------------------------------------
if $PREFIX/lib/node/index.node exists
check $PREFIX/lib/node/index.node

简单来说，如果是require('x')这样开头不是相对or绝对地址符号，尾巴也没说是.js或者.json的，就当做模块来找。先找是不是core module，然后一级一级向上看node_modules文件夹，每一级的node_modules先看里面是否有basename为所找的文件，再看是否有模块名文件夹下package.json的main标明的文件，然后不死心地看看模块名文件夹下有没有index.js和index.node。最后找不到的话，还要搜一遍全局环境，比如$HOME/.node_modules/什么的。