This article mainly introduces examples of the return values ​​​​of then and catch in ES6 Promise. It has certain reference value. Now I share it with you. Friends in need can refer to it.

一.catch is The syntactic sugar of then

Sothen method and catch method will both return a Promise object(Yes, even if return is a certain value, or throw error, or no value is returned)
Let's take a look at the definition of MDN. It may be a bit messy for the sake of rigor. undefined, also the return value)
, throw error

return Promise 2. Let’s talk about the return value and throw error. 1. Return value situation:

Fulfilled

The return value will

. See the example of the code

var example = new Promise((fulfill, reject)=>{
    let i = 1;
    fulfill(i);
})
example
.then((value)=>{ console.log(value); value++; return value;  })
.then((value) => {console.log(value);                        });

Back to the above question, if
does not return, then undefined

var example = new Promise((fulfill, reject)=>{
    let i = 1;
    fulfill(i);
})
example
.then((value)=>{ console.log(value); value++; })
.then((value) => {console.log(value);});

Return The Promise will become Rejected

in the catch or then’s second callback function parameter
Something that has been confused before appears here. Repeat this sentence again: catch is syntactic sugar for then, which is an alias of then(null, rejection).

So, assuming that the
return value in the catch is , the new Promise object will also be in the accepting state. Look at the example:

var example = new Promise((fulfill, reject)=>{
    let i = 1;
    reject(i);
})
example
.catch(()=>{console.log('我是第一个catch的回调函数'); return 1;})
.then(() =>{console.log('我是第一个then的回调函数');    throw Error    })
.catch(()=>{console.log('我是第二个catch的回调函数')})
.then(() => {console.log('我是第二个then的回调函数')})

executes the first catch The callback function
The callback function of the first catch

, soExecute the callback function of the first then
The callback function of the first then throw Error, so Execute the callback function of the second catch
The callback function of the second catch ruturn undefined (as mentioned above) , so Execute the callback function of the second then
3. Return Promise caseAs for the case of return Promise, the same is true. I have just started to contact Promise. I feel uncomfortable with the syntax: Promise objects are automatically generated for you? ! Later, after reading part of the source code analysis, I roughly understood why it was like this. I will also put the link below.

https://github.com/Joeoeoe/-i...

Related recommendations:

Introduction to ES6 Class inheritance and super

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