# Promise思想

改善Callback Hell问题

## 什么样的函数需要改写成Promise

例1:

```js
mysql.query('SELECT 1 + 1 AS solution', function (err, rows, fields) {
  if (err) {
    throw err;
  }
  //对查询结果进行操作
  console.log('The solution is: ', rows[0].solution);
});
```

例2:

```js
fs.readFile('myfile.txt', function (err, file) {
  if (err) {
    throw err;
  }
  //对file进行操作
});
```

## Callback Hell

例子:

```js
mysql.query(sql1, function(err1, rows1) {
  if (err1){
    throw err1;
  }
  mysql.query(sql2,function(err2,rows2){
    if(err2){
      throw err2;
    }
    //对rows1和rows2进行操作

  });
});
```

这样写法的问题:

1. 嵌套太多,可读性差
2. 变量名受污染
3. 各个函数之间的干扰,一但错误发生,不能继续向下执行

等等

## 示例代码

```js
'use strict';
/**
 * 生成一个defer对象
 * @return {object} Promise对象
 */
let getDefer = function () {
  let deferred = {};
  deferred.promise = new Promise(function (resolve, reject) {
    deferred.resolve = resolve;
    deferred.reject = reject;
  });
  return deferred;
};

/*
 * 通过getDefer封装Promise
 */
let promiseFunc = function (input) {
  let deferred = getDefer();
  if (parseInt(input, 10) < 10) {
    deferred.resolve(input * input);
  }
  else {
    deferred.reject('Input Number Too Large');
  }
  return deferred.promise;
};

promiseFunc(5).then(function (result) {
  console.log(result);//25
}).catch(function (err) {
  console.log(err);//不报错
});

promiseFunc(15).then(function (result) {
  console.log(result);
}).catch(function (err) {
  console.log(err);//报错
});


/*
 * 链式写法
 */
setTimeout(function () {
  console.log('=============');
  promiseFunc(2).then(function (val) {
    console.log('Step 1:', val); //val===4
    return promiseFunc(val);
  }).then(function (val) {
    console.log('Step 2:', val); //val===16
    return promiseFunc(val);
  }).then(function (val) {
    console.log('step 3:', val); //err 不执行这句,跳到catch,往后的then均不执行
    return promiseFunc(val);
  }).then(function (val) {
    console.log('Step 4:', val);
    return promiseFunc(val);
  }).catch(function (err) {
    console.log(err);
  });
}, 20);

/*
 * Promise.all
 * 全部执行完,再返回
 * (常用)
 */
setTimeout(function () {
  Promise.all([promiseFunc(1), promiseFunc(2), promiseFunc(3)]).then(function (val) {
    console.log('=============');
    console.log(val);//[1,4,9]
  });
}, 100);

/*
 * Promise.race
 * 谁先执行完,返回谁
 * (不常用)
 */
setTimeout(function () {
  Promise.race([promiseFunc(1), promiseFunc(2), promiseFunc(3)]).then(function (val) {
    console.log('=============');
    console.log(val);//1
  });
}, 100);
```

执行结果：

```
25
Input Number Too Large
=============
Step 1: 4
Step 2: 16
Input Number Too Large
=============
[ 1, 4, 9 ]
=============
1
```

## What's Next?

ES7 Async/Await:

```js
(async() => {
  const result = await promiseFN();
  console.log(result);
})();
```

多个异步方法优化:

```js
(async() => {
  const tasks = [];
  tasks.push(promiseFN1());
  tasks.push(promiseFN2());
  tasks.push(promiseFN3());
  const result = await Promise.all(tasks);
  console.log(result[0], result[1], result[2]);
})();
```