promise

概念

Promise是一个构造函数。
该函数有一个函数参数
函数参数有两个参数 resolve reject ,分别为成功与失败之后的回调函数。

用法

var promise = new Promise(function(resolve,reject){
		...
	})

1.Promise.all()=>用于汇总多个promise的结果。
2.Promise.race()
3.Promise.reject(reason)
4.Promise.resolve(value)
5.Promise.prototype.catch(onRejected)
6.Promise.prototype.then(onFulfilled, onRejected)
7.Promise.prototype.finally(onFinally)

async

1.一个async函数可以包含一个await表达式，暂停异步函数的执行并等待传入Promise的解析，然后恢复async函数的执行并返回解析后的值。
eg:=>

function resolveAfter2Seconds(x) {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}


async function add1(x) {
  const a = await resolveAfter2Seconds(20);
  const b = await resolveAfter2Seconds(30);
  return x + a + b;
}

add1(10).then(v => {
  console.log(v);  // prints 60 after 4 seconds.
});


async function add2(x) {
  const p_a = resolveAfter2Seconds(20);
  const p_b = resolveAfter2Seconds(30);
  return x + await p_a + await p_b;
}

add2(10).then(v => {
  console.log(v);  // prints 60 after 2 seconds.
});

2.函数重写承诺链

function getProcessedData(url) {
  return downloadData(url) // returns a promise
    .catch(e => {
      return downloadFallbackData(url)  // returns a promise
    })
    .then(v => {
      return processDataInWorker(v); // returns a promise
    });
}
它可以用一个async函数重写，如下所示：

async function getProcessedData(url) {
  let v;
  try {
    v = await downloadData(url); 
  } catch(e) {
    v = await downloadFallbackData(url);
  }
  return processDataInWorker(v);
}

Promise.all()

用法：Promise.all(迭代)

迭代：一个可迭代的对象，如Array或者String.

返回值

　异步性

1.同步，当且仅当通过的迭代是空的时，Promise.all同步解析
2.异步，不然就要异步处理
快速性
3.快速行为，如果任何元素被拒绝，则Promise.all被拒绝。

eg：1&2=>

var p = Promise.all([]); // will be immediately resolved
var p2 = Promise.all([1337, "hi"]); // non-promise values will be ignored, but the evaluation will be done asynchronously
console.log(p);
console.log(p2)
setTimeout(function(){
    console.log('the stack is now empty');
    console.log(p2);
});

// logs
// Promise { <state>: "fulfilled", <value>: Array[0] }
// Promise { <state>: "pending" }
// the stack is now empty
// Promise { <state>: "fulfilled", <value>: Array[2] }

eg:3=>

var p1 = new Promise((resolve, reject) => { 
  setTimeout(resolve, 1000, 'one'); 
}); 
var p2 = new Promise((resolve, reject) => { 
  setTimeout(resolve, 2000, 'two'); 
});
var p3 = new Promise((resolve, reject) => {
  setTimeout(resolve, 3000, 'three');
});
var p4 = new Promise((resolve, reject) => {
  setTimeout(resolve, 4000, 'four');
});
var p5 = new Promise((resolve, reject) => {
  reject('reject');
});

Promise.all([p1, p2, p3, p4, p5]).then(values => { 
  console.log(values);
}, reason => {
  console.log(reason)
});

//From console:
//"reject"

//You can also use .catch
Promise.all([p1, p2, p3, p4, p5]).then(values => { 
  console.log(values);
}).catch(reason => { 
  console.log(reason)
});

//From console: 
//"reject"

Promise.race()

用法 Promise.race(迭代)

1.如果通过的迭代是空的，则返回的promise将永远等待。
2.如果迭代包含一个或多个非承诺值和/或已经解决/拒绝的承诺，Promise.race 则将解析为在迭代中找到的这些值中的第一个。
3.也有异步性
eg:2=>

var foreverPendingPromise = Promise.race([]);
var alreadyResolvedProm = Promise.resolve(666);

var arr = [foreverPendingPromise, alreadyResolvedProm, "non-Promise value"];
var arr2 = [foreverPendingPromise, "non-Promise value", Promise.resolve(666)];
var p = Promise.race(arr);
var p2 = Promise.race(arr2);

console.log(p);
console.log(p2);
setTimeout(function(){
    console.log('the stack is now empty');
    console.log(p);
    console.log(p2);
});

// logs, in order:
// Promise { <state>: "pending" } 
// Promise { <state>: "pending" } 
// the stack is now empty
// Promise { <state>: "fulfilled", <value>: 666 }
// Promise { <state>: "fulfilled", <value>: "non-Promise value" }

eg:

var p1 = new Promise(function(resolve, reject) { 
    setTimeout(resolve, 500, 'one'); 
});
var p2 = new Promise(function(resolve, reject) { 
    setTimeout(resolve, 100, 'two'); 
});

Promise.race([p1, p2]).then(function(value) {
  console.log(value); // "two"
  // Both resolve, but p2 is faster
});