我们来观察以下四段代码的区别：

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// 1 setTimeout throw
const p1 = new Promise(function (resolve, reject) {
  setTimeout(function () {
    throw new Error("test");
  }, 0);
});
p1.catch(function (error) {
  console.log("Catch1: ", error);
});
// Uncaught Error: test

// 2 setTimeout reject
const p2 = new Promise(function (resolve, reject) {
  setTimeout(function () {
    reject(new Error("test"));
  }, 0);
});
p2.catch(function (error) {
  console.log("Catch2: ", error);
});
// Catch2:  Error: test

// 3 throw
const p3 = new Promise(function (resolve, reject) {
  throw new Error("test");
});
p3.catch(function (error) {
  console.log("Catch3: ", error);
});
// Catch3:  Error: test

// 4 reject
const p4 = new Promise(function (resolve, reject) {
  reject(new Error("test"));
});
p4.catch(function (error) {
  console.log("Catch4: ", error);
});
// Catch4:  Error: test

then和catch返回新的Promise对象，finally返回本身。

all的then，catch的调用优先级低于all的元素本身的then，catch。

以下两者代码的区别：(定义 Promise 的时候会执行其中的代码)

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const f = () => console.log("now");
Promise.resolve().then(f);
console.log("next");
// next
// now

const f = () => console.log("now");
(() => new Promise((resolve) => resolve(f())))();
console.log("next");
// now
// next