Promise 物件

提供一個機制來排程對尚未計算的值進行的工作。它是管理與非同步 API 互動的抽象概念。

語法

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

promise.then(onCompleted, onRejected);
promise.catch(onRejected)

Promise.resolve(someValue) // fulfilled
Promise.reject("failure reason"); //rejected

參數

promise
必要項。要對其指派承諾的變數名稱。
resolve
必要項。執行以表示承諾已順利完成的函式。
reject
選擇項。執行以表示已拒絕承諾並發生錯誤的函式。

promise
必要項。Promise 物件
onCompleted
必要項。承諾順利完成時要執行的「履行」處理常式函式。
onRejected
選擇項。要在拒絕承諾時執行的錯誤處理常式函式。

備註

承諾必須完成並具有值，或必須有原因的拒絕。完成或拒絕 (視何者先發生) 承諾時，會執行 Promise 物件的 then 方法。如果承諾順利完成，則會執行 then 方法的履行處理常式函式。如果已拒絕承諾，則會執行 then 方法 (或 catch 方法) 的錯誤處理常式函式。

下列範例示範如何呼叫會傳回承諾的函式 (timeout)。在 5000 毫秒逾時到期之後，會執行 then 方法的履行處理常式。

let timeout = duration => {
  // function timeout(duration) {
  return new Promise((resolve, reject) => {
    // return new Promise(function(resolve, reject) {
    setTimeout(resolve, duration);
  });
};

// Note: This code uses arrow function syntax
var m = timeout(5000).then(() => {
  console.log("done!");
});

// Output (after 5 seconds):
// done!

let timeout = duration => {
  return new Promise((resolve, reject) => {
    setTimeout(reject, duration);
  });
};

var p = timeout(5000)
  .then(() => {
    console.log("done!");
  })
  .catch(err => {
    console.log("error!");
  });

// Output (after five seconds):
// error!

var p = Promise.resolve("success");
p.then(result => {
  console.log(result);
});

// Output:
// success

var p = Promise.reject("failure");
p.catch(result => {
  console.log(result);
});

// Output:
// failure

• pending 擱置中: initial state, neither fulfilled nor rejected.
• fulfilled 已經完成: meaning that the operation completed successfully.
• rejected 已拒絕: meaning that the operation failed.

resolve 解析

const myFirstPromise = new Promise((resolve, reject) => {
  // do something asynchronous which eventually calls either:
  //
  //   resolve(someValue); // fulfilled
  // or
  //   reject("failure reason"); // rejected
});

let myFirstPromise = new Promise((resolve, reject) => {
  setTimeout(function(){
    resolve("Success!");
  }, 250);
});

myFirstPromise.then((successMessage) => {
  console.log("Yay! " + successMessage);
});

// var b = true;
var b = false;

var p1 = new Promise(function (resolve, reject) {
  if (b) {
    resolve('Success!');
  } else {
    reject(Error('oh, no!'));
  }
});

var myFunc = function() {
  p1.then(function(value){
    console.log(value);
  })
  .catch(function(e) {
    console.log(e);
  })
};

myFunc();

var doSomething = function(value) {
  return new Promise(function (resolve, reject) {
    resolve('Success!');
  });
};

var doSomething = function(value) {
  return Promise.resolve('Success!');
};

var p1 = Promise.resolve(3);
var p2 = 1337;
var p3 = new Promise((resolve, reject) => {
  setTimeout(resolve, 100, 'foo');
}); 

Promise.all([p1, p2, p3]).then(values => { 
  console.log(values); // [3, 1337, "foo"] 
});

參考網站：

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/all
• https://msdn.microsoft.com/zh-tw/library/dn802826(v=vs.94).aspx

"use strict";

const promisify = require("es6-promisify");
const fs = require("fs");

const stat = promisify(fs.stat);

stat("example.txt")
  .then(stats => {
    console.log("Got stats", stats);
  })
  .catch(err => {
    console.error("Yikes!", err);
  });

參考網站：

• https://www.npmjs.com/package/es6-promisify