Complete a promise using the then() method and return the result

Question

Complete a promise using the then() method and return the result

I'm working with a JavaScript code snippet that looks like this:

function justTesting() {
  promise.then(function(output) {
    return output + 1;
  });
}

var test = justTesting();

Every time I check the value of the test variable, it's always undefined. I suspect that this is because the promises haven't been resolved yet. Is there a way to return a value from a promise?

javascript promise

Answer 1

Answer №1

When a result is returned from a callback function within a then() statement, there's a touch of magic involved. If a value is returned, the subsequent then() function will be triggered with that value. On the other hand, if something resembling a promise is returned, the following then() function will pause until that promise is resolved or rejected.

Source: https://web.dev/promises/#queuing-asynchronous-actions

Answer 2

When a result is returned from a callback function within a then() statement, there's a touch of magic involved. If a value is returned, the subsequent then() function will be triggered with that value. On the other hand, if something resembling a promise is returned, the following then() function will pause until that promise is resolved or rejected.

Source: https://web.dev/promises/#queuing-asynchronous-actions

Answer 3

Answer №2

If you want to utilize a promise, you'll need to either invoke a function that generates a promise or craft one yourself. It's important to understand the underlying issue you're attempting to address, but here's an example of how you can create a promise on your own:

function customPromiseExample(input) {
    return new Promise(function(resolve, reject) {
        // perform some asynchronous operation here
        setTimeout(function() {
           // fulfill the promise with a certain value
           resolve(input + 10);
        }, 500);
    });
}

customPromiseExample(29).then(function(value) {
   // retrieve the value from the promise here
   log(value);
});

// output displayed within the snippet
function log(x) {
    document.write(x);
}

Alternatively, if you already have a function that yields a promise, you can leverage that function and return its promise:

// function that yields a promise
function delayTimer(t) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      resolve();
    }, t);
  });
}

function customPromiseExample(input) {
  return delayTimer(100).then(function() {
    return input + 10;
  });
}

customPromiseExample(29).then(function(value) {
  // retrieve the value from the promise here
  log(value);
});

// output displayed within the snippet
function log(x) {
    document.write(x);
}

Answer 4

If you want to utilize a promise, you'll need to either invoke a function that generates a promise or craft one yourself. It's important to understand the underlying issue you're attempting to address, but here's an example of how you can create a promise on your own:

function customPromiseExample(input) {
    return new Promise(function(resolve, reject) {
        // perform some asynchronous operation here
        setTimeout(function() {
           // fulfill the promise with a certain value
           resolve(input + 10);
        }, 500);
    });
}

customPromiseExample(29).then(function(value) {
   // retrieve the value from the promise here
   log(value);
});

// output displayed within the snippet
function log(x) {
    document.write(x);
}

Alternatively, if you already have a function that yields a promise, you can leverage that function and return its promise:

// function that yields a promise
function delayTimer(t) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      resolve();
    }, t);
  });
}

function customPromiseExample(input) {
  return delayTimer(100).then(function() {
    return input + 10;
  });
}

customPromiseExample(29).then(function(value) {
  // retrieve the value from the promise here
  log(value);
});

// output displayed within the snippet
function log(x) {
    document.write(x);
}

Answer 5

Answer №3

In this code snippet, I have made some changes to the justTesting function to ensure that a promise is returned. This allows you to process the result once the function is resolved.

// updated answer

function justTesting() {
  return new Promise((resolve, reject) => {
    if (true) {
      return resolve("testing");
    } else {
      return reject("promise failed");
   }
 });
}

justTesting()
  .then(res => {
     let test = res;
     // perform actions with the output here :)
  })
  .catch(err => {
    console.log(err);
  });

I hope this explanation is useful!

// previous answer

function justTesting() {
  return promise.then(function(output) {
    return output + 1;
  });
}

justTesting().then((res) => {
     var test = res;
    // perform actions with the output here :)
    }

Answer 6

In this code snippet, I have made some changes to the justTesting function to ensure that a promise is returned. This allows you to process the result once the function is resolved.

// updated answer

function justTesting() {
  return new Promise((resolve, reject) => {
    if (true) {
      return resolve("testing");
    } else {
      return reject("promise failed");
   }
 });
}

justTesting()
  .then(res => {
     let test = res;
     // perform actions with the output here :)
  })
  .catch(err => {
    console.log(err);
  });

I hope this explanation is useful!

// previous answer

function justTesting() {
  return promise.then(function(output) {
    return output + 1;
  });
}

justTesting().then((res) => {
     var test = res;
    // perform actions with the output here :)
    }

Answer 7

Answer №4

To eliminate confusion with promises, my preference lies in utilizing the "await" command and async functions.

For this scenario, I would establish an asynchronous function initially, which will replace the anonymous function invoked within the "promise.then" section of this query:

async function SubFunction(output){

   // Request made to database, resulting in a promise, similar to an Ajax call:

   const response = await axios.get( GetApiHost() + '/api/some_endpoint')

   // Response:
   return response;

}

Subsequently, I would invoke this function within the main function:

async function justTesting() {
   const lv_result = await SubFunction(output);

   return lv_result + 1;
}

Note that both the main function and sub function have been transformed into async functions in this context.

Answer 8

To eliminate confusion with promises, my preference lies in utilizing the "await" command and async functions.

For this scenario, I would establish an asynchronous function initially, which will replace the anonymous function invoked within the "promise.then" section of this query:

async function SubFunction(output){

   // Request made to database, resulting in a promise, similar to an Ajax call:

   const response = await axios.get( GetApiHost() + '/api/some_endpoint')

   // Response:
   return response;

}

Subsequently, I would invoke this function within the main function:

async function justTesting() {
   const lv_result = await SubFunction(output);

   return lv_result + 1;
}

Note that both the main function and sub function have been transformed into async functions in this context.

Answer 9

Answer №5

In my understanding, the primary goal of the author is to retrieve a value directly from a promise without the need to return another promise. As far as my knowledge goes, achieving this without using a then() or async/await environment seems to be unattainable. It appears that a promise will always be returned regardless of the circumstances.

Answer 10

In my understanding, the primary goal of the author is to retrieve a value directly from a promise without the need to return another promise. As far as my knowledge goes, achieving this without using a then() or async/await environment seems to be unattainable. It appears that a promise will always be returned regardless of the circumstances.

Answer 11

Answer №6

When using Promises, values are not technically "returned"; instead, they are passed to a callback function specified by using .then().

The concept being communicated is likely that you should include resolve(someData); within the promise implementation.

Then, in the code following the then method, you can access the data stored in someData to perform additional operations.

Answer 12

When using Promises, values are not technically "returned"; instead, they are passed to a callback function specified by using .then().

The concept being communicated is likely that you should include resolve(someData); within the promise implementation.

Then, in the code following the then method, you can access the data stored in someData to perform additional operations.

Answer 13

Answer №7

To effectively solve this problem, utilizing reference data types such as arrays or objects is necessary.

function bar(x, y){
  let output = [];
  const userCategories = new Promise((resolve, reject) => {
                        resolve(['category A', 'category C']);
                      })
  
  userCategories.then((uc) => {
    return new Promise((resolve, reject) => {
      resolve([...uc, 'category D', 'category E']);
    })
  }).then(response => {
    return output.push(...response);
  });
  return output;
};
bar();

Answer 14

To effectively solve this problem, utilizing reference data types such as arrays or objects is necessary.

function bar(x, y){
  let output = [];
  const userCategories = new Promise((resolve, reject) => {
                        resolve(['category A', 'category C']);
                      })
  
  userCategories.then((uc) => {
    return new Promise((resolve, reject) => {
      resolve([...uc, 'category D', 'category E']);
    })
  }).then(response => {
    return output.push(...response);
  });
  return output;
};
bar();

Answer 15

Answer №8

When a promise is resolved, you should not try to return a value directly. Instead, you can call a separate function to handle the resolved promise value:

function handlePromise() {
    promise.then(function(data) {
        // Call another function to handle the resolved value
        processValue(data + 1);
    });
}

function processValue(value) {
    // Perform actions with the resolved value
}

var result = handlePromise();

Answer 16

When a promise is resolved, you should not try to return a value directly. Instead, you can call a separate function to handle the resolved promise value:

function handlePromise() {
    promise.then(function(data) {
        // Call another function to handle the resolved value
        processValue(data + 1);
    });
}

function processValue(value) {
    // Perform actions with the resolved value
}

var result = handlePromise();

Complete a promise using the then() method and return the result

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

Answer №6

Answer №7

Answer №8

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