JavaScript promise的使用和原理分析

const promise = new Promise((resolve, reject) => {
  resolve("hahaha")
})
// 1.同一个Promise可以被多次调用then方法
// 当我们的resolve方法被回调时, 所有的then方法传入的回调函数都会被调用
promise.then(res => {
  console.log("res1:", res)    //hahaha
})
promise.then(res => {
  console.log("res2:", res)    //hahaha
})
promise.then(res => {
  console.log("res3:", res)    //hahaha
})

Promise.resolve()
// 等同于
new Promise((resolve)=>{resolve()})

Promise.resolve().then(() => {
  console.log(0);
  return Promise.resolve(4)
}).then(res => {
  console.log(res)
})
Promise.resolve().then(() => {
  console.log(1);
}).then(() => {
  console.log(2);
}).then(() => {
  console.log(3);
}).then(() => {
  console.log(5);
}).then(() =>{
  console.log(6);
})
// 0,1,2,3,4,5,6

(
  async function() {
    return Promise.resolve()
  }
)().then(()=>{
  console.log(1)
})
new Promise((resolve) => {
  resolve()
}).then(()=>{
  console.log(2)
}).then(()=>{
  console.log(3)
})
/// 2 3 1

const promise1 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('Promise 1 resolved');
      console.log(0)
  }, 1000);
});
const promise2 = new Promise((resolve, reject) => {
  setTimeout(() => {
      console.log(2)
    resolve('Promise 2 resolved');
  }, 2000);
});
Promise.race([promise1, promise2]).then(result => {
  console.log(result); // "Promise 1 resolved"
});
// 0
//Promise 1 resolved
//2