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https://github.com/luoyjx/promise-cookbook
:book: Learn how to cook promise in Javascript
https://github.com/luoyjx/promise-cookbook
promise
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:book: Learn how to cook promise in Javascript
- Host: GitHub
- URL: https://github.com/luoyjx/promise-cookbook
- Owner: luoyjx
- License: mit
- Created: 2017-04-21T07:20:07.000Z (over 7 years ago)
- Default Branch: master
- Last Pushed: 2017-04-28T02:36:10.000Z (over 7 years ago)
- Last Synced: 2024-11-05T18:53:10.689Z (2 months ago)
- Topics: promise
- Language: JavaScript
- Homepage:
- Size: 11.7 KB
- Stars: 2
- Watchers: 2
- Forks: 2
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- License: LICENSE
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README
# promise-cookbook
learn how to cook promise
## Promise 标准描述
标准中规定：
1. Promise对象初始状态为 `Pending`，在被 `resolve` 或 `reject` 时，状态变为 `Fulfilled` 或 `Rejected`
1. resolve接收成功的数据，reject接收失败或错误的数据
1. Promise对象必须有一个 `then` 方法，且只接受两个可函数参数 `onFulfilled`、`onRejected`
## then方法
标准中规定：
1. then 方法必须返回一个新的 `Promise实例`（ES6中的标准，Promise/A+中没有明确说明）
1. 为了保证 then中回调的执行顺序，`onFulfilled` 或 `onRejected` 必须异步调用
## 扩展方法实现
### `Promise.all`
```javascript
Promise.all = function(iterable){
var _this = this;
return new this(function(resolve, reject){
if(!iterable || !Array.isArray(iterable)) return reject( new TypeError('must be an array') );
var len = iterable.length;
if(!len) return resolve([]);
var res = Array(len), counter = 0, called=false;
iterable.forEach(function(v, i){
(function(i){
_this.resolve(v).then(function(value){
res[i]=value;
if(++counter===len && !called){
called = true;
return resolve(res)
}
}, function(err){
if(!called){
called = true;
return reject(err);
}
})
})(i)
})
})
}
```
## `Promise.race`
用于并行执行promise组成的数组（数组中可以不是Promise对象，
在调用过程中会使用 `Promise.resolve(value)` 转换成Promise对象），如果某个promise的状态率先改变，
就获得改变的结果，返回一个新的Promise对象
```javascript
Promise.race = function(iterable){
var _this = this;
return new this(function(resolve, reject){
if(!iterable || !Array.isArray(iterable)) return reject( new TypeError('must be an array') );
var len = iterable.length;
if(!len) return resolve([]);
var called = false;
iterable.forEach(function(v, i){
_this.resolve(v).then(function(res){
if(!called){
called = true;
return resolve(res);
}
}, function(err){
if(!called){
called = true;
return reject(err);
}
})
})
})
}
```
## `Promise.resolve`
用于包装任意对象为promise对象，返回一个新的promise,并且状态是resolved
```javascript
Promise.resolve = function(value){
if(value instanceof this) return value;
return executeCallback.bind(new this())('resolve', value);
}
```
## `Promise.reject`
用于包装任意对象为promise对象，返回一个新的promise,并且状态是rejected
```javascript
Promise.reject = function(value){
if(value instanceof this) return value;
return executeCallback.bind(new this())('reject', value);
}
```