Can a promise be safely resolved more than once?

Question

Can a promise be safely resolved more than once?

In my application, there is an i18n service that includes the following code snippet:

var i18nService = function() {
  this.ensureLocaleIsLoaded = function() {
    if( !this.existingPromise ) {
      this.existingPromise = $q.defer();

      var deferred = this.existingPromise;
      var userLanguage = $( "body" ).data( "language" );
      this.userLanguage = userLanguage;

      console.log( "Loading locale '" + userLanguage + "' from server..." );
      $http( { method:"get", url:"/i18n/" + userLanguage, cache:true } ).success( function( translations ) {
        $rootScope.i18n = translations;
        deferred.resolve( $rootScope.i18n );
      } );
    }

    if( $rootScope.i18n ) {
      this.existingPromise.resolve( $rootScope.i18n );
    }

    return this.existingPromise.promise;
  };

The purpose of the ensureLocaleIsLoaded function is to ensure that the locale is loaded. Therefore, it is designed so that users can call it multiple times without any issue.

Currently, I am storing a single promise and resolving it when the function is called again after successfully retrieving the locale from the server.

It seems like this approach is functioning correctly, but I'm curious if there might be a better way to handle this.

javascript angularjs promise

Answer 1

Answer №1

From my current understanding of promises, this situation should be completely acceptable. The key thing to grasp is that once a promise is resolved (or rejected), the deferred object is finalized.

If you attempt to call then(...) on the promise again, you will immediately receive the originally resolved/rejected result.

Subsequent calls to resolve() will not produce any impact.

Check out the runnable example below which illustrates these scenarios:

var p = new Promise((resolve, reject) => {
  resolve(1);
  reject(2);
  resolve(3);
});

p.then(x => console.log('resolved to ' + x))
 .catch(x => console.log('never called ' + x));

p.then(x => console.log('one more ' + x));
p.then(x => console.log('two more ' + x));
p.then(x => console.log('three more ' + x));

Answer 2

From my current understanding of promises, this situation should be completely acceptable. The key thing to grasp is that once a promise is resolved (or rejected), the deferred object is finalized.

If you attempt to call then(...) on the promise again, you will immediately receive the originally resolved/rejected result.

Subsequent calls to resolve() will not produce any impact.

Check out the runnable example below which illustrates these scenarios:

var p = new Promise((resolve, reject) => {
  resolve(1);
  reject(2);
  resolve(3);
});

p.then(x => console.log('resolved to ' + x))
 .catch(x => console.log('never called ' + x));

p.then(x => console.log('one more ' + x));
p.then(x => console.log('two more ' + x));
p.then(x => console.log('three more ' + x));

Answer 3

Answer №2

Resolving promises multiple times can be a tricky situation as once it's resolved, it's considered complete. One effective solution is to implement the observer-observable pattern. Below is an example of code I wrote that observes events from a socket client. Feel free to modify this code according to your requirements.

const invokeMethodWithArguments = (methodName, args) => (source) => source[methodName].apply(null, args);
    const hasMethod = (name) => (target = {}) => typeof target[name] === 'function';
    const Observable = function (fn) {
        const subscribers = [];
        this.subscribe = subscribers.push.bind(subscribers);
        const observer = {
            next: (...args) => subscribers.filter(hasMethod('next')).forEach(invokeMethodWithArguments('next', args))
        };
        setTimeout(() => {
            try {
                fn(observer);
            } catch (e) {
                subscribers.filter(hasMethod('error')).forEach(invokeMethodWithArguments('error', e));
            }
        });

    };

    const fromEvent = (target, eventName) => new Observable((obs) => target.on(eventName, obs.next));

    fromEvent(client, 'document:save').subscribe({
        async next(document, docName) {
            await writeFilePromise(resolve(dataDirectory, `${docName}`), document);
            client.emit('document:save', document);
        }
    });

Answer 4

Resolving promises multiple times can be a tricky situation as once it's resolved, it's considered complete. One effective solution is to implement the observer-observable pattern. Below is an example of code I wrote that observes events from a socket client. Feel free to modify this code according to your requirements.

const invokeMethodWithArguments = (methodName, args) => (source) => source[methodName].apply(null, args);
    const hasMethod = (name) => (target = {}) => typeof target[name] === 'function';
    const Observable = function (fn) {
        const subscribers = [];
        this.subscribe = subscribers.push.bind(subscribers);
        const observer = {
            next: (...args) => subscribers.filter(hasMethod('next')).forEach(invokeMethodWithArguments('next', args))
        };
        setTimeout(() => {
            try {
                fn(observer);
            } catch (e) {
                subscribers.filter(hasMethod('error')).forEach(invokeMethodWithArguments('error', e));
            }
        });

    };

    const fromEvent = (target, eventName) => new Observable((obs) => target.on(eventName, obs.next));

    fromEvent(client, 'document:save').subscribe({
        async next(document, docName) {
            await writeFilePromise(resolve(dataDirectory, `${docName}`), document);
            client.emit('document:save', document);
        }
    });

Answer 5

Answer №3

Not long ago, I encountered a similar situation where I discovered that a promise can only be resolved once. Subsequent attempts to resolve it will not result in any errors or warnings, nor will the then method be invoked.

To address this issue, I came up with the following workaround:

getUsers(users => showThem(users));

getUsers(callback){
    callback(getCachedUsers())
    api.getUsers().then(users => callback(users))
}

You can simply pass your function as a callback and call it multiple times as needed! I hope this solution is clear and helpful.

Answer 6

Not long ago, I encountered a similar situation where I discovered that a promise can only be resolved once. Subsequent attempts to resolve it will not result in any errors or warnings, nor will the then method be invoked.

To address this issue, I came up with the following workaround:

getUsers(users => showThem(users));

getUsers(callback){
    callback(getCachedUsers())
    api.getUsers().then(users => callback(users))
}

You can simply pass your function as a callback and call it multiple times as needed! I hope this solution is clear and helpful.

Answer 7

Answer №4

To modify the return value of a promise, simply return a new value in the then function and continue chaining more then/catch functions.

var p1 = new Promise((resolve, reject) => { resolve(1) });
    
var p2 = p1.then(v => {
  console.log("Updated value after first then:", v);
  return 2;
});
    
p2.then(v => {
  console.log("New value after second then:", v);
});

Answer 8

To modify the return value of a promise, simply return a new value in the then function and continue chaining more then/catch functions.

var p1 = new Promise((resolve, reject) => { resolve(1) });
    
var p2 = p1.then(v => {
  console.log("Updated value after first then:", v);
  return 2;
});
    
p2.then(v => {
  console.log("New value after second then:", v);
});

Answer 9

Answer №5

If you want to ensure the behavior, it's vital to write tests.

Running the test below will help you reach a conclusion:

The resolve()/reject() call does not throw an error.

Once settled (rejected), the resolved value (rejected error) will remain unchanged regardless of subsequent resolve() or reject() calls.

For more details, feel free to check out my blog post.

/* eslint-disable prefer-promise-reject-errors */
const flipPromise = require('flip-promise').default

describe('promise', () => {
    test('error catch with resolve', () => new Promise(async (rs, rj) => {
        const getPromise = () => new Promise(resolve => {
            try {
                resolve()
            } catch (err) {
                rj('error caught in unexpected location')
            }
        })
        try {
            await getPromise()
            throw new Error('error thrown out side')
        } catch (e) {
            rs('error caught in expected location')
        }
    }))
    test('error catch with reject', () => new Promise(async (rs, rj) => {
        const getPromise = () => new Promise((_resolve, reject) => {
            try {
                reject()
            } catch (err) {
                rj('error caught in unexpected location')
            }
        })
        try {
            await getPromise()
        } catch (e) {
            try {
                throw new Error('error thrown out side')
            } catch (e){
                rs('error caught in expected location')
            }
        }
    }))
    test('await multiple times resolved promise', async () => {
        const pr = Promise.resolve(1)
        expect(await pr).toBe(1)
        expect(await pr).toBe(1)
    })
    test('await multiple times rejected promise', async () => {
        const pr = Promise.reject(1)
        expect(await flipPromise(pr)).toBe(1)
        expect(await flipPromise(pr)).toBe(1)
    })
    test('resolve multiple times', async () => {
        const pr = new Promise(resolve => {
            resolve(1)
            resolve(2)
            resolve(3)
        })
        expect(await pr).toBe(1)
    })
    test('resolve then reject', async () => {
        const pr = new Promise((resolve, reject) => {
            resolve(1)
            resolve(2)
            resolve(3)
            reject(4)
        })
        expect(await pr).toBe(1)
    })
    test('reject multiple times', async () => {
        const pr = new Promise((_resolve, reject) => {
            reject(1)
            reject(2)
            reject(3)
        })
        expect(await flipPromise(pr)).toBe(1)
    })

    test('reject then resolve', async () => {
        const pr = new Promise((resolve, reject) => {
            reject(1)
            reject(2)
            reject(3)
            resolve(4)
        })
        expect(await flipPromise(pr)).toBe(1)
    })
test('constructor is not async', async () => {
    let val
    let val1
    const pr = new Promise(resolve => {
        val = 1
        setTimeout(() => {
            resolve()
            val1 = 2
        })
    })
    expect(val).toBe(1)
    expect(val1).toBeUndefined()
    await pr
    expect(val).toBe(1)
    expect(val1).toBe(2)
})

})

Answer 10

If you want to ensure the behavior, it's vital to write tests.

Running the test below will help you reach a conclusion:

The resolve()/reject() call does not throw an error.

Once settled (rejected), the resolved value (rejected error) will remain unchanged regardless of subsequent resolve() or reject() calls.

For more details, feel free to check out my blog post.

/* eslint-disable prefer-promise-reject-errors */
const flipPromise = require('flip-promise').default

describe('promise', () => {
    test('error catch with resolve', () => new Promise(async (rs, rj) => {
        const getPromise = () => new Promise(resolve => {
            try {
                resolve()
            } catch (err) {
                rj('error caught in unexpected location')
            }
        })
        try {
            await getPromise()
            throw new Error('error thrown out side')
        } catch (e) {
            rs('error caught in expected location')
        }
    }))
    test('error catch with reject', () => new Promise(async (rs, rj) => {
        const getPromise = () => new Promise((_resolve, reject) => {
            try {
                reject()
            } catch (err) {
                rj('error caught in unexpected location')
            }
        })
        try {
            await getPromise()
        } catch (e) {
            try {
                throw new Error('error thrown out side')
            } catch (e){
                rs('error caught in expected location')
            }
        }
    }))
    test('await multiple times resolved promise', async () => {
        const pr = Promise.resolve(1)
        expect(await pr).toBe(1)
        expect(await pr).toBe(1)
    })
    test('await multiple times rejected promise', async () => {
        const pr = Promise.reject(1)
        expect(await flipPromise(pr)).toBe(1)
        expect(await flipPromise(pr)).toBe(1)
    })
    test('resolve multiple times', async () => {
        const pr = new Promise(resolve => {
            resolve(1)
            resolve(2)
            resolve(3)
        })
        expect(await pr).toBe(1)
    })
    test('resolve then reject', async () => {
        const pr = new Promise((resolve, reject) => {
            resolve(1)
            resolve(2)
            resolve(3)
            reject(4)
        })
        expect(await pr).toBe(1)
    })
    test('reject multiple times', async () => {
        const pr = new Promise((_resolve, reject) => {
            reject(1)
            reject(2)
            reject(3)
        })
        expect(await flipPromise(pr)).toBe(1)
    })

    test('reject then resolve', async () => {
        const pr = new Promise((resolve, reject) => {
            reject(1)
            reject(2)
            reject(3)
            resolve(4)
        })
        expect(await flipPromise(pr)).toBe(1)
    })
test('constructor is not async', async () => {
    let val
    let val1
    const pr = new Promise(resolve => {
        val = 1
        setTimeout(() => {
            resolve()
            val1 = 2
        })
    })
    expect(val).toBe(1)
    expect(val1).toBeUndefined()
    await pr
    expect(val).toBe(1)
    expect(val1).toBe(2)
})

})

Answer 11

Answer №6

To ensure smooth loading of your application, consider adding an ng-if directive to your main ng-outlet. This will display a loading spinner until the locale data is fully loaded. Once the locale is ready, reveal the outlet and allow the component hierarchy to render seamlessly. By doing this, you can simplify the process for all components in your application, as they can operate under the assumption that the locale has already been loaded without needing any additional checks.

Answer 12

To ensure smooth loading of your application, consider adding an ng-if directive to your main ng-outlet. This will display a loading spinner until the locale data is fully loaded. Once the locale is ready, reveal the outlet and allow the component hierarchy to render seamlessly. By doing this, you can simplify the process for all components in your application, as they can operate under the assumption that the locale has already been loaded without needing any additional checks.

Answer 13

Answer №7

It is not recommended to resolve or reject a promise multiple times as it can lead to potential bugs that are difficult to detect since they may not always be reproducible.

A helpful pattern to identify and trace such issues during debugging is discussed in the lecture titled Ruben Bridgewater — Error handling: doing it right! (the relevant section starts at around 40 minutes).

Answer 14

It is not recommended to resolve or reject a promise multiple times as it can lead to potential bugs that are difficult to detect since they may not always be reproducible.

A helpful pattern to identify and trace such issues during debugging is discussed in the lecture titled Ruben Bridgewater — Error handling: doing it right! (the relevant section starts at around 40 minutes).

Answer 15

Answer №8

view code on GitHub: reuse_promise.js

/*
This code snippet demonstrates how to reuse a promise for multiple resolve()s in order to overcome the limitation of promises resolving only once.
*/

import React, { useEffect, useState } from 'react'

export default () => {
    
    const [somePromise, setSomePromise] = useState(promiseCreator())
        
    useEffect(() => {
        
        somePromise.then(data => {
            
            // perform operations here
            
            setSomePromise(promiseCreator())
        })
        
    }, [somePromise])
}

const promiseCreator = () => {
    return new Promise((resolve, reject) => {
        // perform async tasks
        resolve(/*data*/)
    })
}

Answer 16

view code on GitHub: reuse_promise.js

/*
This code snippet demonstrates how to reuse a promise for multiple resolve()s in order to overcome the limitation of promises resolving only once.
*/

import React, { useEffect, useState } from 'react'

export default () => {
    
    const [somePromise, setSomePromise] = useState(promiseCreator())
        
    useEffect(() => {
        
        somePromise.then(data => {
            
            // perform operations here
            
            setSomePromise(promiseCreator())
        })
        
    }, [somePromise])
}

const promiseCreator = () => {
    return new Promise((resolve, reject) => {
        // perform async tasks
        resolve(/*data*/)
    })
}

Can a promise be safely resolved more than once?

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

Answer №6

Answer №7

Answer №8

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