the thing after reject must enter the second callback in then. If the second callback is not written in then, enter catch

 var p1=new Promise((resolve,rej) => {
    console.log('resolve')
    //throw new Error('')
    rej('')

 })

 p1.then(data =>{
    console.log('data::',data);
 },err=> {
    console.log('err::',err)
 }).catch(
    res => {
    console.log('catch data::', res)
 })

 VM367054:2 resolve
 VM367054:11 err:: 

• cases where there is no second callback in then

 var p1=new Promise((resolve,rej) => {
    console.log('resolve')
    //throw new Error('')
    rej('')

 })

 p1.then(data =>{
    console.log('data::',data);
 }).catch(
    res => {
    console.log('catch data::', res)
 })

 VM367054:2 resolve
 VM367054:11 catch data:: 

• if you don't have then, you can enter catch directly

 var p1=new Promise((resolve,rej) => {
    console.log(' resolve')
    //throw new Error('')
    rej('')

 })

 p1.catch(
    res => {
    console.log('catch data::', res)
 })
VM367087:2 resolve
VM367087:9 catch data:: 

II. Resolve will definitely enter the first callback of then, but certainly will not enter catch

 var p1=new Promise((resolve,rej) => {
    console.log('resolve')
    //throw new Error('')
    resolve('')

 })

 p1.then(data =>{
    console.log('data::',data);
 }).catch(
    res => {
    console.log('catch data::', res)
 })
VM367087:2 resolve
VM367087:9 data:: 

• does not enter catch

 var p1=new Promise((resolve,rej) => {
    console.log('resolve')
    //throw new Error('')
    resolve('')

 })

 p1.catch(
    res => {
    console.log('catch data::', res)
 })
VM367087:2 resolve

throw new Error is the same as rej, but only one of them will occur
in addition, a network exception (such as network outage) will directly enter catch instead of the second callback of then

reject function changes the state of the Promise object from "incomplete" to "failed" (that is, from pending to rejected),) when the asynchronous operation fails, and passes the error reported by the asynchronous operation as an argument.

for catch, if the asynchronous operation throws an error, the state changes to rejected, and the callback function specified by the catch method is called to handle the error. In addition, the callback function specified by the then method will also be caught by the catch method if an error is thrown during the run.

similar to a throw error and a catch error reject, you can also use then not to catch

Promise.reject ('error') .catch (err= > console.log (err)) / / output 'error'
execute catch (if you write)