回调地狱的由来和如何利用promise解决回调地狱

var fs =require('fs')
fs.readFile('./a.txt', 'utf8',function(err,data){
  if(err){
    console.log('读取失败')
  }else{
	console.log(data)
	fs.readFile('./b.txt', 'utf8',function(err,data){
		if(err){
		  console.log('读取失败')
		}else{
		  console.log(data)
		  fs.readFile('./c.txt', 'utf8',function(err,data){
			if(err){
			  console.log('读取失败')
			  
			}else{
			  console.log(data)
			}
		  })
		}
	  })
  }
})

 

var fs =require('fs')
var p1 =new Promise(function(resolve,reject){
    fs.readFile('a.txt','utf8',function(error,data){
            if(error){
                reject()
                console.log('读取失败')
            } else{
                resolve(data)
            }
       
       })
})
var p2 =new Promise(function(resolve,reject){
    fs.readFile('./b.txt','utf8',function(err,data){
        if(err){
            reject()
            console.log('读取失败')
        }else{
            resolve(data)
        }
    })
})

var p3 = new Promise(function(resolve,reject){
    fs.readFile('./c.txt','utf8',function(err,data){
        if(err){
            reject()
            console.log('读取失败')
        }else{
            resolve(data)
        }
    })
})
p1.then(function(data){
    console.log(data)
    return p2
},function(err){
    console.log('读取失败')
}).then(function(data){
    console.log(data)
    return p3
},function(err){
    console.log('读取失败')
}).then(function(data){
    console.log(data)
},function(err){
    console.log('读取失败----------')
})
   

　　

var fs =require('fs')
function readFireFun(url){
    return new Promise(function(resolve,reject){
        fs.readFile(url,'utf8',function(err,data){
            if(err){
                console.log('读取失败')
                reject(err)
            }else{
                console.log(data)
                resolve(data)
            }
        })
    })
}

readFireFun('./a.txt').then(function(data){
    return readFireFun('./b.txt')
}).then(function(data){
    return readFireFun('./c.txt')
})