回调函数

• 参数在方法体内不加''()'',则正常使用,加"()"并且括号内有值则会返出去

const fs = require('fs');
const path = require('path');
function getJsonFromConfig(c,filePath) {
  fs.readFile(path.join(__dirname, c), `utf-8`, (err, res) => {
    if (err) {
      // reject(err);
    }
    filePath(JSON.parse(res));
  });
}

getJsonFromConfig(`../config/config.json`,aa => {
  console.log(`-----=====`, aa);
});

• 将回调函数内部值拿出外部

const fs = require('fs');
const path = require('path');

async function getJsonFromConfig() {
     
  var promise=new Promise((resolve, reject) => {
    fs.readFile(path.join(__dirname, `../config/config.json`), `utf-8`, (err, res) => {
      if (err) {
        reject(err);
      }
      resolve(JSON.parse(res));
    });
  });
  var data = await promise;
  console.log(`-----data=====`, data) 
  return data
  
}

var b = async function () {
  var data = await getJsonFromConfig()
  console.log(`-----aaaaaaa=====`,data) 

}

b()

• 最简单阻塞读取

var configPath = path.join(__dirname, `../../config/config.json`);

  var deviceType = JSON.parse(fs.readFileSync(configPath))[`deviceType`];