Promise error propagation

Question

I have this function which will if it fails retry, and if it fails x times it should finally reject the Promise. I have implemented this as follows:

examplefunction(retries = -1) {
    return new Promise(async (resolve, reject) => {
        try {
            const url = `example.org`;
            const json = await this._sendRequest(url);
            resolve(json);
        } catch (e) {
            if( retries > 0 ) {
                return this.examplefunction(retries - 1);
            }
            else {
                reject("Unable to communicate with Server. Please try again later!");
            }
        }
    });
}

the function is beeing called like this:

backend.examplefunction(3).then(
    (json) => {
        console.log(json);
    },
    (reason) => {
        console.log.(reason);
    }
)

This code is executed in a React context, so it is also running through a babel transpilitaion.

My problem is that when it will finally reject after x retries this results in a Uncaught Promise Error:

Uncaught (in promise) Unable to communicate with Server. Please try again later!!

Could anybody explain to me why this is happening?

does it log the reason ("error") before throwing the exception? — Thomas Powell, May 11 '18 at 15:36

score 2 · Accepted Answer · answered May 11 '18 at 15:48

2

Avoid the Promise constructor antipattern, and never pass an async function to it! You should write

async function example(retries = -1) { /*
^^^^^ */
    try {
        const url = `example.org`;
        const json = await this._sendRequest(url);
        return json;
//      ^^^^^^
    } catch (e) {
        if (retries > 0) {
            return this.examplefunction(retries - 1);
//          ^^^^^^ this works as expected now
        } else {
            throw new Error("Unable to communicate with Server. Please try again later!");
//          ^^^^^
        }
    }
}

You were never resolveing the promise in the retry case, and when the recursive call failed eventually then that promise was completely ignored.

answered May 11 '18 at 15:48

Bergi

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Thanks this cleared it. One thing I don't quite understand however is the following: When using the Antipattern approach and rejecting, the onRejected Function will receive a String with the error message, in this approach however it will receive an error object. – Jakob Abfalter May 11 '18 at 16:01
1

Yes, I just changed it (without a comment) because [you should always reject with errors, not strings](https://stackoverflow.com/q/26020578/1048572). You could of course `throw` the string literal if you insisted. – Bergi May 11 '18 at 16:05

Promise error propagation

1 Answers1

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