CODEWITHSUNDEEP
javascriptnode.jspromisecoding-style
kougami
kougami

Reputation: 904

Proper way to 'recover' from a failed promise?

I'm fairly new to JS development and I've recently discovered the concept of DRY (Don't Repeat Yourself), which has helped me clean up my code a lot.

I have the following type of issue in a few places throughout my project and I'm struggling to think of a way to improve it whilst maintaining the principles of readability and of not repeating code.

    if (something) {
        doPromise().then(() => {
            doSomething()
        }).catch(e => {
            doThisInstead()
        })
    } else {
        doThisInstead()
    }

The crux of it is that I need to execute doThisInstead() or whatever function / in-line code is there whenever either the if statement goes to the else block, or when the promise goes to the catch block, and in this particular instance, I have no way of knowing that the promise will go to the catch block before it is attempted.

Writing code like this can quickly become messy, so I'd appreciate any tips. Many thanks!

Upvotes: 1

Views: 805

Answers (3)

kougami
kougami

Reputation: 904

This can be solved with a Promise, using the following code:

function hypotheticalFunction() {
    const doSomething = () => {
        // stuff
    }
    const doThisInstead = () => {
        // stuff
    }
    const doSomethingHandler = () => {
        return new Promise((resolve,reject) => { 
            if (something) {
                doPromise().then(() => {
                    doSomething();
                    resolve();
                }).catch(() => {
                    reject();
                })
            } else {
                reject();
            }
        })
    }
    doSomethingHandler().catch(doThisInstead);
}
hypotheticalFunction();

Upvotes: 0

Bergi
Bergi

Reputation: 664297

You might be looking for if-else flow in promise (bluebird), just with catch instead of then:

(something
  ? doPromise().then(() => {
      doSomething()
    })
  : Promise.reject()
).catch(e => {
    doThisInstead()
})

Written with async/await, it would be

try {
    if (!something)
        throw new Error("something is wrong")
    await doPromise();
    await doSomething();
} catch(e) {
    await doThisInstead();
}

An alternative that does not rely as much on exceptions would be

if (!something || await doPromise().then(doSomething).then(() => false, () => true)) {
    doThisInstead();
}

Upvotes: 1

Terry Lennox
Terry Lennox

Reputation: 30675

If you use the _async / await _ syntax, you can await doPromise(), then run doThisInstead() if either something is falsey or an error occurred, this means only one call to doThisInstead() in your code.

This example will cause doPromise() to fail 50% of the time.

let something = true;

// Fail 50% of the time
function doPromise() {
    return new Promise((resolve, reject) => setTimeout(Math.random() <= 0.5 ? resolve: () => reject(new Error("Some error")), 100));
}

function doSomething() {
    console.log("doSomething()");
}

function doThisInstead() {
    console.log("doThisInstead()");
}

async function test() {
    errorOccurred = false;
    if (something) {
        try {
            await doPromise();
            doSomething();
        } catch (e) {
            errorOccurred = true;
        }
        console.log("doPromise: " + (errorOccurred ? "error occurred." : "ran successfully"));
    }
    
    // Run doThisInstead() if either an error occurred or something is falsey
    if (!something || errorOccurred) {
        doThisInstead();
    }
}

test()

Upvotes: 0

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