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Promise nested then execute order All In One

Promise nested then

nested Promise

1. not return new Promise

遇到 Promise 立即执行，Promise 后面紧跟着的第一个 then 也马上执行;
后面的 then 按照 Promise 创建的先后顺序，依次执行;

"use strict";

/**
 *
 * @author xgqfrms
 * @license MIT
 * @copyright xgqfrms
 * @created 2020-10-01
 * @modified
 *
 * @description
 * @difficulty Easy Medium Hard
 * @complexity O(n)
 * @augments
 * @example
 * @link
 * @solutions
 *
 * @best_solutions
 *
 */

const log = console.log;


const promise = new Promise((resolve, reject) =>{
  log(1);
  resolve(1);
});

// then((json) => log(3, json), (err) => log(err));
promise
.then((v) => {
  // not new Promise ⚠️
  // return new Promise((ok, err) => {
  new Promise((ok, err) => {
    log(2);
    log(`v =`, v);
    // resolve callback, ok
    ok(`✅`);
    // reject callback, err
    err(`❌`)
  }).then((v) => {
    log(3);
    log(`v =`, v, `
`);
    return v;
  }).then((v) => {
    log(4);
    log(`v =`, v, `
`);
    return v;
  });
  // 遇到 Promise 立即执行，Promise 后面紧跟着的第一个 then 也马上执行，后面的 then 按照 Promise 创建的先后顺序，依次执行 then!
  return v;
})
.then((v) => {
  log(5);
  log(`v =`, v);
  return v;
})
.then((v) => {
  log(6);
  log(`v =`, v);
  return v;
});


/*

1
2
v = 1
3
v = ✅

5
v = 1
4
v = ✅

6
v = 1
*/

2. return new Promise

"use strict";

/**
 *
 * @author xgqfrms
 * @license MIT
 * @copyright xgqfrms
 * @created 2020-10-01
 * @modified
 *
 * @description
 * @difficulty Easy Medium Hard
 * @complexity O(n)
 * @augments
 * @example
 * @link
 * @solutions
 *
 * @best_solutions
 *
 */

const log = console.log;


const promise = new Promise((resolve, reject) =>{
  log(1);
  resolve(1);
});

// then((json) => log(3, json), (err) => log(err));
promise
.then((v) => {
  // return new Promise
  return new Promise((ok, err) => {
    log(2);
    log(`v =`, v);
    // resolve callback, ok
    ok(`✅`);
    // reject callback, err
    err(`❌`)
  }).then((v) => {
    log(3);
    log(`v =`, v);
    return v;
  }).then((v) => {
    log(4);
    log(`v =`, v);
    return v;
  });
  // return v;
})
.then((v) => {
  log(5);
  log(`v =`, v);
  return v;
})
.then((v) => {
  log(6);
  log(`v =`, v);
  return v;
});


/*

1
2
v = 1
3
v = ✅
4
v = ✅
5
v = ✅
6
v = ✅

*/

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