1. fulfilled酒请，rejected甫恩，pending
A Promise is an object that is used as a placeholder for the eventual results of a deferred (and possibly asynchronous) computation.
Any Promise object is in one of three mutually exclusive states: fulfilled, rejected, and pending:
A promise p is fulfilled if
p.then(f, r)will immediately enqueue a Job to call the functionf.
A promise p is rejected ifp.then(f, r)will immediately enqueue a Job to call the function r.
A promise is pending if it is neither fulfilled nor rejected.
一個(gè)promise總共有三種狀態(tài)，pending榛斯，fulfilled/rejected，new Promise構(gòu)造函數(shù)創(chuàng)建的promise搂捧，初始處于pending狀態(tài)驮俗。
pending的時(shí)候不向Job Queue加入任何Job。
當(dāng)一個(gè)promise p被fulfilled/rejected的時(shí)候异旧，會(huì)立即向名字為PromiseJobs的Job Queue加入一個(gè)Job意述，這個(gè)Job做的事情就是調(diào)用p.then(f, r)中的f/r。
2. settled吮蛹，resolved
A promise is said to be settled if it is not pending, i.e. if it is either fulfilled or rejected.
A promise is resolved if it is settled or if it has been “l(fā)ocked in” to match the state of another promise. Attempting to resolve or reject a resolved promise has no effect. A promise is unresolved if it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be pending, fulfilled or rejected.
2.1 settled
一個(gè)promise被settled荤崇，指的是它脫離了pending狀態(tài)，或者被fulfilled或者被rejected潮针。
2.2 resolved
resolve/reject一個(gè)promise术荤，
指的是調(diào)用new Promise((res,rej)=>{...})中的res/rej函數(shù)。
調(diào)用了res函數(shù)之后的promise每篷，稱(chēng)為resolved瓣戚。
2.3 如何理解“l(fā)ocked in”
A promise is resolved if it is settled or if it has been “l(fā)ocked in” to match the state of another promise.
要想理解“l(fā)ocked in”，首先要知道焦读，res函數(shù)可以接受三種類(lèi)型的值作為參數(shù)子库，
或者是一個(gè)promise，或者是一個(gè)帶有then方法的對(duì)象矗晃，或者是其他值仑嗅。
這里“l(fā)ocked in”指的是res接受promise，或者接受帶有then方法對(duì)象的情況，
這種方式進(jìn)行resolve仓技，會(huì)使得當(dāng)前的promise仍然處于pending狀態(tài)鸵贬，
其settled結(jié)果，與其參數(shù)對(duì)象的最終狀態(tài)有關(guān)脖捻。
例子：
（1）res的參數(shù)為promise的例子阔逼，
a = new Promise((res,rej)=>setTimeout(()=>res(1),1000));
b = new Promise((res,rej)=>res(a));
b.then(v=>console.log(v));
（2）res的參數(shù)為帶then方法的對(duì)象的例子，
a = {
then:(res,rej)=>setTimeout(()=>res(1),1000)
};
b = new Promise((res,rej)=>res(a));
b.then(v=>console.log(v));
2.4 多次resolve無(wú)效
Attempting to resolve or reject a resolved promise has no effect.
一個(gè)promise一旦resolved地沮，再次resolve/reject將失效嗜浮。
無(wú)論第一次resolve所使用的參數(shù)是什么，
例如诉濒，即使第一調(diào)用res仍然使promise處于pending狀態(tài)周伦。
a = new Promise((res,rej)=>{
res({
then:(res,rej)=>setTimeout(()=>res(1),1000)
});
res(2);
setTimeout(()=>rej(3),1000);
});
a.then(
v => console.log(v),
e => console.warn(e)
);
3. Enqueue Job
3.1 何時(shí)p.then(f, r)會(huì)Enqueue Job
如果promise p還沒(méi)有被settled，那么p.then(f, r)未荒，會(huì)將f/r放入promise的[[PromiseFulfillReactions]]/[[PromiseRejectReactions]]列表尾部专挪。
如果p已經(jīng)被settled，會(huì)向名為PromiseJobs的Job Queue中添加一個(gè)用來(lái)處理fulfilled/rejected的PromiseReactionJob片排。
因此寨腔，一個(gè)已經(jīng)settled的promise，每次調(diào)用p.then(f, r)會(huì)Enqueue Job率寡。
[[PromiseFulfillReactions]]和[[PromiseRejectReactions]]的作用是迫卢，當(dāng)promise處于pending狀態(tài)時(shí)，保存p.then(f, r)所添加的處理器函數(shù)f和r冶共。
3.2 使用res / rej Enqueue的兩種Job
res會(huì)出現(xiàn)在以下兩個(gè)地方：
（1）帶有then方法的對(duì)象中乾蛤，{then:(res,rej)=>{...}}
用一個(gè)帶有then方法的對(duì)象調(diào)用res，會(huì)向名為PromiseJobs的Job Queue中添加一個(gè)PromiseResolveThenableJob捅僵。
（2）new Promise((res,rej)=>{...})構(gòu)造函數(shù)中
調(diào)用res/rej家卖，會(huì)導(dǎo)致promise被settled，（只能被settle一次）
此時(shí)會(huì)清空promise的[[PromiseFulfillReactions]]和[[PromiseRejectReactions]]列表庙楚，并向名為PromiseJobs的Job Queue中添加一個(gè)用來(lái)處理fulfilled/rejected的 PromiseReactionJob上荡。
因此，調(diào)用res/rej總是會(huì)Enqueue Job馒闷。
當(dāng)promise被settled的時(shí)候酪捡，[[PromiseFulfillReactions]]和[[PromiseRejectReactions]]就沒(méi)有用了，因?yàn)閟ettled狀態(tài)的promise纳账，即使再添加處理器p.then(f, r)逛薇，也是直接Enqueue Job，和[[PromiseFulfillReactions]]和[[PromiseRejectReactions]]無(wú)關(guān)疏虫。
a = new Promise(res=>setTimeout(()=>res(1),1000));
a.then(v=>console.log(v));
setTimeout(()=>a.then(v=>console.warn(v)),2000);
參考
StackOverflow: What is the correct terminology for javascript promises
ECMAScript 2018 Language Specification
