记一个Promise问题

我们来观察以下四段代码的区别：

// 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)

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