Does using catch require throw

Question

Newbie question here - I have a program making API calls via promises/fetch. I want to add a catch, but I'm wondering do I need to also then have a throw statement in there? Or can I just use the catch alone?

Answer 1

I want to add a catch, but I'm wondering do I need to also then have a throw statement in there?

No, you do not need a throw unless that's the programming result you want.

A .catch() is used to "handle" a rejected promise. You, yourself, do not have to use a throw in order to handle a rejected promise.

A throw might be used in conjunction with a .catch() if you want to execute some code when there's a rejected promise, but then return a rejected promise to the caller. In that case, you'd do something like this:

function someFunction() {
    return fn().then(val => {
       // do something here when promise resolves
       return someVal;
    }).catch(err => {
       // do something here when promise rejects
       console.log(err);
       // throw the error to keep the returned promise rejected
       throw err;
    });
}

If you just have a .catch() without throwing inside it, then the rejected promise will be considered "handled" and will become resolved, not rejected. Note, this is pretty much the same as a try/catch works for synchronous code.

Here's a perfectly fine example where you do not need a throw with the .catch().

const rp = require('request-promise');
const defaultConfig = {...};

function readRemoteConfig(url) {
    return rp({url, json: true}).catch(err => {
        // when remote config is not available, substitute default config
        return defaultConfig;
    });
}

---

FYI, even if you want to keep the promise rejected, you don't have to use a throw to accomplish that. You can also return a rejected promise which will accomplish the same result:

function someFunction() {
    return fn().then(val => {
       // do something here when promise resolves
       return someVal;
    }).catch(err => {
       // do something here when promise rejects
       console.log(err);
       // return rejected promise to keep the returned promise rejected
       return Promise.reject(err);
    });
}

It is a personal preference whether you want to do it this way or use throw.