Get value returned from Promise in JavaScript

Question

I´m new to JavaScript and a little bit confused about Promises, this is what I have:

export const testFunction = () => dispatch => {
    otherFunction().then(( response ) => {
        //do something...
        return response;
    }).catch(( error ) => {
        //do something...
        return error;
    });
}

In another file I'm trying to get the value returned from the then like this:

let result = this.props.testFunction()

And like this:

let result = this.props.testFunction ().then(( result ) => {
  console.log(result);
}).catch(( error ) => {
  console.log(result); // undefined
});

But I get undefined as the result, what is the correct way of getting that value?

Notice that .then takes a function parameter. This is where you are returning and is why it's not getting to the outer function's return value. Instead/to fix, just return the promise — ControlAltDel
– ControlAltDel, Commented Feb 25, 2021 at 20:40
Yet it returns undefined, my bet is he's missing the return on the second line. Also, if you want to catch the error at the end don't catch it in your testFunction or rethrow it then. — MinusFour
– MinusFour, Commented Feb 25, 2021 at 20:40

Kubwimana Adrien · Accepted Answer · 2021-02-25 20:46:53Z

1

testFunction is not returning a Promise so you can't use then or catch and returns undefined because well, it's not returning any thing. Try to return a promise like the example below however I am not sure what the dispatch argument supposed to do so I have removed it and hopefully this'll help:

export const testFunction = () => {
    return new Promise((resolve, reject) => {
        otherFunction().then(( response ) => {
            //do something...
            resolve(response);
        }).catch(( error ) => {
            //do something...
            reject(error);
        });
    });
}

answered Feb 25, 2021 at 20:46

Kubwimana Adrien

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Comments

Ramsés Ariel · Accepted Answer · 2021-02-25 20:45:28Z

when you are trying to return a promise to use it in another file, you must use the following syntax:

const testFunction = () => {
    return new Promise((resolve, reject) => {
        if (error) {
            return reject(error); // this is the value sent to .catch(error)
        }
        return resolve(valueToReturn); // this is the value sent to .then(result)
    });
}

This is how you create a promise to use where you want, if it has an error it will sent to catch block, otherwise you should see the console.log(result) value.

And in your external file you can use the syntax that you are using, try it in that way to see the console.log value.

Here is a link to see more information: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise

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