Promise构造函数的Promise.resolve()方法

Promise.resolve(value)方法返回一个由 给定的value 决议的Promise对象。
如果value的值为promise，返回这个promise；
如果value的值是thenable(即，带有then方法的对象)，返回的Promise对象的最终状态由then方法执行决定；
否则的话(即value为 空，基本类型或者不带then方法的对象),返回的Promise对象状态为fulfilled，并且将该value传递给对应的then方法。

先看看简单的例子:

Promise.resolve("Success").then(function(value) {
  console.log(value); // "Success"
}, function(value) {
  // 不会被调用
});

语法

Promise.resolve(value);

参数：value 是将被Promise对象resolve的参数。value 也可以是一个Promise对象，或者是一个thenable。（Argument to be resolved by this Promise. Can also be a Promise or a thenable to resolve.）

举例一：resolve是另外一个promise对象

let original = Promise.resolve('happy chen');
let cast = Promise.resolve(original);

cast.then((val) => {
    console.log(val);
})

console.log(`original === cast ? ${original === cast}`);
// original === cast ? true
// happy chen

这里由于 异步地 调用then 方法，所以 有一个同步异步先后执行的区别
举例二：Resolve一个thenable对象

let p1 = Promise.resolve({
    then: function (onFulfill, onReject) { onFulfill('fulfilled') }
});
console.log(p1 instanceof Promise);

p1.then(v => {
    console.log(v);// "fulfilled"
},(e)=>{
    // not called 
})

// true
// fulfilled

返回值：返回一个resolve过带着给定值的Promise对象，如果参数是一个Promise对象，则直接返回这个Promise对象。（见上面 举例一）

其它的例子：resolve一个数组

let p = Promise.resolve(['happy','chen',666]);

p.then(function(v){
    console.log(v[2]);
})

// 666

resolve thenable 并抛出错误：

// Resolving a thenable object
var p1 = Promise.resolve({ 
  then: function(onFulfill, onReject) { onFulfill('fulfilled!'); }
});
console.log(p1 instanceof Promise) // true, object casted to a Promise

p1.then(function(v) {
    console.log(v); // "fulfilled!"
  }, function(e) {
    // not called
});

// Thenable throws before callback
// Promise rejects
var thenable = { then: function(resolve) {
  throw new TypeError('Throwing');
  resolve('Resolving');
}};

var p2 = Promise.resolve(thenable);
p2.then(function(v) {
  // not called
}, function(e) {
  console.error(e); // TypeError: Throwing
});

// Thenable throws after callback
// Promise resolves
var thenable = { then: function(resolve) {
  resolve('Resolving');
  throw new TypeError('Throwing');
}};

var p3 = Promise.resolve(thenable);
p3.then(function(v) {
  console.log(v); // "Resolving"
}, function(e) {
  // not called
});