如何跳过链中的承诺

Question

I am working in a nodejs project and want to skip promise in a chain. Below is my code. In the first promise block, it will resolve a value {success: true} . On the second block I want to check the value of success , if true I want to return the value to the called and skip the rest of the promises in this chain; while continue the chain if the value is false. I know I can throw an error or reject it on the second block but I have to handle error case which it is not an error case. So how can I achieve this in promise chain? I need a solution without bring any other 3rd party library.

new Promise((resolve, reject)=>{
    resolve({success:true});
}).then((value)=>{
    console.log('second block:', value);
    if(value.success){
        //skip the rest of promise in this chain and return the value to caller
        return value;
    }else{
        //do something else and continue next promise
    }
}).then((value)=>{
    console.log('3rd block:', value);
});

Answer 1

Simply nest the part of the chain you want to skip (the remainder in your case):

new Promise(resolve => resolve({success:true}))
.then(value => {
    console.log('second block:', value);
    if (value.success) {
        //skip the rest of this chain and return the value to caller
        return value;
    }
    //do something else and continue
    return somethingElse().then(value => {
        console.log('3rd block:', value);
        return value;
    });
}).then(value => {
    //The caller's chain would continue here whether 3rd block is skipped or not
    console.log('final block:', value);
    return value;
});

Answer 2

If you don't like the idea of nesting, you can factor the remainder of your chain into a separate function:

// give this a more meaningful name
function theRestOfThePromiseChain(inputValue) {
    //do something else and continue next promise
    console.log('3rd block:', value);

    return nextStepIntheProcess()
        .then(() => { ... });
}

function originalFunctionThatContainsThePromise() {
    return Promise.resolve({success:true})
        .then((value)=>{
            console.log('second block:', value);
            if(value.success){
                //skip the rest of promise in this chain and return the value to caller
                return value;
            }

            return theRestOfThePromiseChain(value);
        });
}

Aside from that, there isn't really a way to stop a promise mid-stream.

Answer 3

You could also continue to throw the error down the chain to a final catch block if you like.

const test = () => {
  throw new Error('boo');
}

const errorHandler = (e) => { throw e };

Promise.resolve()
        .then(console.log('then1'))
        .then(() => test())
        .then(f => console.log('then2'), errorHandler)
        .then(f => console.log('then3'), errorHandler)
        .catch((err) => console.log('caught', err));

// whoo
// then1
// caught Error: boo