inconsistentRead.js

const fs = require('fs');
const cache = {};
function inconsistentRead(filename, callback) {
     if(cache[filename]) {
       //invoked synchronously
       callback(cache[filename]);
     } else {
      //asynchronous function
      fs.readFile(filename, 'utf8', (err, data) => {
        cache[filename] = data;
        callback(data);
      });
    } 
}


function createFileReader(filename) { 
const listeners = []; inconsistentRead(filename, value => {
  listeners.forEach(listener => listener(value)); });
  return {
    onDataReady: listener => listeners.push(listener)
  };
}


const reader1 = createFileReader('data.txt'); 
reader1.onDataReady(data => {
  console.log('First call data: ' + data);
  //...sometime later we try to read again from the same file
  const reader2 = createFileReader('data.txt'); 
  reader2.onDataReady( data => {
    console.log('Second call data: ' + data); });
  });
});


// 考察输出结果，涉及知识点异步、时间循环等