Break from promise chain early

First of all, congratulations and thank you very much for making and maintaining this library, its been extremely helpful!

I have a chain of promises at certain stages I would like the whole chain to fail but I have catch handlers further on which I would like to ignore. My use case is a file comparison after it has been processed, I do this for all files in a directory.

readdirp({ root: 'data' }).on('data', (entry) => {
    let promise = renderer.execute(entry.fullPath, "chrome");
    promise.then((file) => {
        return Q.nfcall(fs.writeFile, "./result/chrome-"+entry.name, file);
    }).catch((err) => {
        console.error("Failed saving file","./result/chrome-"+entry.name,"skipping comparison.");
        //Break from chain here
    }).then(() =>{
        return renderer.execute(entry.fullPath,"phantom");
    }).catch((err)=>{
        console.error("phantom renderer failed", err);
        //break from chain
    }).then((file)=>{
        return Q.nfcall(fs.writeFile, "./result/phantom-"+entry.name, file);
    }).catch((err) => {
        console.error("Failed saving file","./result/phantom-"+entry.name,"skipping comparison.");
        //Break from chain here
    }).then(() =>{
        return compare("./result/chrome-"+entry.name,"./result/phantom-"+entry.name);
    }).catch((err)=>{
        console.error("Comparison failed");
    }).done();
});

Is there a way to do this?

Thanks for taking the time to read this!

[cdhowie]

If I’m understanding this right, you want to write out a specific error message based on the step that failed and then abort the whole process. You can do this by attaching your catch continuations only to the specific promises they belong to, then re-rejecting with the same error, which will cause the outer chain to fall to the end.

Reading the following code, you should clearly be able to see based on the indentation where the catch continuations will apply. Note that the outer chain doesn’t have any catch continuations at all, which is why a rejection in one of the inner chains will fall to the end of the chain.

(I refactored your arrow functions where possible, and wrapped a few calls in Q.try() since I wasn’t sure if those functions returned a promise or a value.)

readdirp({ root: 'data' }).on('data', (entry) => {
    renderer.execute(entry.fullPath, "chrome")
    .then(file =>
        Q.nfcall(fs.writeFile, "./result/chrome-"+entry.name, file)
        .catch(err => {
            console.error("Failed saving file","./result/chrome-"+entry.name,"skipping comparison.");
            // The following line passes the rejection through.  The outer
            // chain of "then"s will by entirely bypassed.
            return Q.reject(err);
        })
    )
    .then(() =>
        Q.try(() => renderer.execute(entry.fullPath,"phantom"))
        .catch(err => {
            console.error("phantom renderer failed", err);
            return Q.reject(err);
        })
    )
    .then(file =>
        Q.nfcall(fs.writeFile, "./result/phantom-"+entry.name, file)
        .catch(err => {
            console.error("Failed saving file","./result/phantom-"+entry.name,"skipping comparison.");
            return Q.reject(err);
        })
    )
    .then(() =>
        Q.try(() => compare("./result/chrome-"+entry.name,"./result/phantom-"+entry.name))
        .catch(err => {
            console.error("Comparison failed");
            return Q.reject(err);
        })
    )
    .done();
});

This annotated version shows how the first error would flow:

    readdirp({ root: 'data' }).on('data', (entry) => {
        renderer.execute(entry.fullPath, "chrome")
        .then(file =>
            Q.nfcall(fs.writeFile, "./result/chrome-"+entry.name, file)
            .catch(err => {
                console.error("Failed saving file","./result/chrome-"+entry.name,"skipping comparison.");
                // The following line passes the rejection through.  The outer
                // chain of "then"s will by entirely bypassed.
/* /-------- */ return Q.reject(err);
/* | */     })
/* | */ )
/* | */ .then(() =>
/* | */     renderer.execute(entry.fullPath,"phantom")
/* | */     .catch(err => {
/* | */         console.error("phantom renderer failed", err);
/* | */         return Q.reject(err);
/* | */     })
/* | */ )
/* | */ .then(file =>
/* | */     Q.nfcall(fs.writeFile, "./result/phantom-"+entry.name, file)
/* | */     .catch(err => {
/* | */         console.error("Failed saving file","./result/phantom-"+entry.name,"skipping comparison.");
/* | */         return Q.reject(err);
/* | */     })
/* | */ )
/* | */ .then(() =>
/* | */     compare("./result/chrome-"+entry.name,"./result/phantom-"+entry.name)
/* | */     .catch(err => {
/* | */         console.error("Comparison failed");
/* | */         return Q.reject(err);
/* | */     })
/* | */ )
/* \> */.done();
    });

[kriskowal]

Thank you to everyone who helped @pedro93 in this thread.