浏览器加载react/vue组件时，遇到es6转es5，jsx转js...时，一种方法是用webpack离线编译，一种方法是用babel在线热编译（为了效率部署前可以预热）。

我比较喜欢在线热编译，好处是发布时快，不经过build直接源码发布，并可以避免忘记编译步骤导致bug。

为了提供效率，把热编译结果保存为文件缓存起来。先检查是否有编译后缓存文件，有就直接读取，否者编译，再读取。

let fs=require('fs');
let babel = require("@babel/core");
let babel_preset_env=require('@babel/preset-env');

function transform(f,cb){
  let fc=f+'.js';
  fs.access(fc,function(err){
    if (err){
      fs.readFile(f,'utf8',function(err,code){
        if (err){cb(err)}
        else{
          let r=babel.transformSync(code,{presets:[babel_preset_env]});
          code=r.code;
          cb(null,code);
          fs.writeFile(fc,code,function(err){
          });
        }
      });
    }
    else{
      fs.readFile(f,'utf8',function(err,code){
        cb(err,code);
      });
    }
  });
}
transform('a.jsx',function(err,code){console.log(err,code});

但在多并发时，问题来了：多个并发任务可能会同时都检查到缓存不存在，然后开始编译.......很浪费，其实只需要一个任务来编译，其它任务等待编译结束后再读取缓存。

其实有点复杂，涉及到文件锁机制，阿里“通义千问”建议用proper-lockfile，我没用。模拟一下：

/**
 * 测试多进程下，判断一个文件是否存在，不存在才生成内容创建文件
 * 难点：
 * （1）避免多个进程在生成文件内容。
 * （2）一个进程如何等待正在生成文件的进程生成完成再读取。
 * （3）等待的效率，降低cpu占用
 */
let path=require('path');
let fs=require('fs');
let fn=path.join(__dirname,'a.txt');

function rw5(task){
  let fnlock=fn+'.lock';
  //递归获取锁
  function lock(cb){
    fs.open(fnlock,'wx',function(err,fhandle){
      if (err){
        console.log(task,' locked,try again...');
        setTimeout(function(){
          lock(cb);
        },1);
      }
      else{
        fs.close(fhandle,function(err){
          if (err) console.log(err);
          console.log(task,' got lock');
          cb();
        });
      }
    });
  }
  function unlock(){
    fs.unlink(fnlock,function(err){if (err) console.log(err)});
  }
  function read(cb){
    fs.readFile(fn,'utf8',function(err,data){
      //读不到正在写入的内容
      if (err) console.log(err);
      else console.log(task,' readed:',data);
      if (cb) cb();
    });
  }
  function write(cb){
    let content='hello';
    //用setTimeout模拟一个长时间的content计算过程，比如babel转码
    setTimeout(function(){
      fs.writeFile(fn,content,function(err,data){
        console.log(task,' writed:',content);
        if (cb) cb();
      });
    }, 1000);
  }
  fs.access(fn,function(err,data){
    if (err){
      console.log(task,' not exists');
      lock(function(){
        fs.access(fn,function(err,data){
          if (err){
            console.log(task,' not exists');
            write(function(){
              unlock();
            });
          }
          else{
            read(function(){
              unlock();
            });
          }
        });
      });
    }
    else{
      read();
    }
  });
}
rw5(1);
rw5(2);

看到的运行记录，可能如下：

D:\work\Source\yujiang.Foil.Node\test\filerw>node multirw.js
1  not exists
2  not exists
1  got lock
2  locked,try again...
1  not exists
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
2  locked,try again...
1  writed: hello
2  locked,try again...
2  got lock
2  readed: hello