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Promises In JavaScript

May 14, 2020

Table Of Contents

    Promise - it is an object that produces some value in the future. This object represents the result of an asynchronous operation.

    Promise has 3 states:

    • pending - the initial state of the promise
    • fulfilled - represents successful operation
    • rejected - represents failed operation

    Once a promise has been fulfilled or rejected, it is immutable which means it can't be changed.

    Motivation

    In JavaScript, callback functions were initially used to handle asynchronous operations.

    However, managing callbacks tends to be tricky when you want to do some complex operations and you may end up in callback hell.

    That's what is solved by promises.

    How To Create A Promise?

    Promise is created using a constructor that takes a callback function with 2 arguments:

    const promise = new Promise((resolve, reject) => {
      // Your code
    });

    In the function body, we write some logic that if being executed successfully, results in resolving a promise.

    If something has failed, a promise is rejected.

    After resolving, then method is called, after rejecting - catch:

    const promise = new Promise((resolve, reject) => { 
      const success = true;
      if(success) { 
        resolve("Success"); 
      } else { 
        reject("Fail"); 
      } 
    });
    
    promise
      .then(result => console.log(result)) // Prints "Success"
      .catch(error => console.log(error));
    const promise = new Promise((resolve, reject) => { 
      const success = false;
      if(success) { 
        resolve("Success"); 
      } else { 
        reject("Fail"); 
      } 
    });
    
    promise
      .then(result => console.log(result))
      .catch(error => console.log(error)); // Prints "Fail"

    Promise Consumers

    Promises can be consumed by registering the following methods: then and catch.

    then

    For simplicity's sake, we are used to say that then is fired only in case if the promise is resolved.

    But that's not true, then is invoked when a promise is either resolved or rejected. It takes 2 functions as an arguments: the first function is executed if the promise is resolved, the second one - if rejected:

    const promise = new Promise((resolve, reject) => { 
      const success = true;
      if(success) { 
        resolve("Success"); 
      } else { 
        reject("Fail"); 
      } 
    });
    
    promise
      .then(
        result => console.log(result), // Prints "Success
        error => console.log(error),
      );
    const promise = new Promise((resolve, reject) => { 
      const success = false;
      if(success) { 
        resolve("Success"); 
      } else { 
        reject("Fail"); 
      } 
    });
    
    promise
      .then(
        result => console.log(result),
        error => console.log(error), // Prints "Fail"
      );

    Important note: remember, that errors should not be handled in then consumer, there is catch specifically for this case.

    catch

    catch is fired if the promise is rejected or some error occurred at the execution.

    catch takes one function as a parameter. This method is just a shorthand for then(null, errorHandler).

    const promise = new Promise((resolve, reject) => { 
      const success = false;
      if(success) { 
        resolve("Success"); 
      } else { 
        reject("Fail"); 
      } 
    });
    
    promise
      .catch(error => console.log(error)); // Prints "Fail"

    finally

    There is one more promise consumer - finally.

    This method is used to execute the code once the promise is settled, no matter if it was resolved or rejected.

    It can be very useful when you have to clean up the resources no matter what the outcome is:

    const promise = new Promise((resolve, reject) => { 
      // Your code
    });
    
    promise
      .then(result => {
        // handle success
      })
      .catch(error => {
        // Handle error
      })
      .finally(() => {
        // Clean up the resources
      });

    You can also achieve the same result without using finally method, but then you have to deal with the code duplication:

    const promise = new Promise((resolve, reject) => { 
      // Your code
    });
    
    promise
      .then(result => {
        // handle success
        // Clean up the resources
      })
      .catch(error => {
        // Handle error
        // Clean up the resources
      });

    Chaining Promises

    Each of those methods: then, catch, finally returns a newly generated promise, which means that it can be chained:

    const promise = new Promise((resolve, reject) => { 
      const numbers = [1, 2, 3, 5, 8];
      if(numbers.includes(8)) {
        resolve(8);
      }
      reject("No 8 found");
    });
    
    promise
      .then(result => result / 2)
      .then(result => console.log(result)) // Prints "4"
      .catch(error => console.log(error));

    Static Methods

    Promise.all(iterable)

    Waits for all promises to be resolved, or for any to be rejected.

    If the returned promise resolves, it is resolved with an aggregating array of the values from the resolved promises, in the same order as defined in the iterable of multiple promises.

    If it rejects, it is rejected with the reason from the first promise in the iterable that was rejected:

    const promise1 = new Promise((resolve, reject) => { 
      resolve(1);
    });
    const promise2 = new Promise((resolve, reject) => { 
      resolve(2);
    });
    const promise3 = new Promise((resolve, reject) => { 
      resolve(3);
    });
    
    Promise.all([promise1, promise2, promise3])
      .then(result => console.log(result)); // Prints "[1, 2, 3]"
    const promise1 = new Promise((resolve, reject) => { 
      resolve(1);
    });
    const promise2 = new Promise((resolve, reject) => { 
      reject("Error");
    });
    const promise3 = new Promise((resolve, reject) => { 
      resolve(3);
    });
    
    Promise.all([promise1, promise2, promise3])
      .then(result => console.log(result))
      .catch(error => console.log(error)); // Prints "Error"

    Promise.allSettled(iterable)

    Waits until all promises have settled (each may resolve or reject).

    Returns a promise that resolves after all of the given promises have either resolved or rejected, with an array of objects that each describe the outcome of each promise:

    const promise1 = new Promise((resolve, reject) => { 
      resolve(1);
    });
    const promise2 = new Promise((resolve, reject) => { 
      reject("Error");
    });
    const promise3 = new Promise((resolve, reject) => { 
      resolve(3);
    });
    
    Promise.allSettled([promise1, promise2, promise3])
      /*
        Prints:
        [
          { status: "fulfilled", value: 1 },
          { status: "rejected", reason: "Error" },
          { status: "fulfilled", value: 3 }
        ]
      */
      .then(result => console.log(result));

    Promise.race(iterable)

    Waits until any of the promises is resolved or rejected.

    If the returned promise resolves, it is resolved with the value of the first promise in the iterable that resolved.

    If it rejects, it is rejected with the reason from the first promise that was rejected:

    const promise1 = new Promise((resolve, reject) => {
      resolve(1);
    });
    const promise2 = new Promise((resolve, reject) => { 
      reject("Error");
    });
    const promise3 = new Promise((resolve, reject) => { 
      resolve(3);
    });
    
    Promise.race([promise1, promise2, promise3])
      .then(result => console.log(result)) // Prints "1"
      .catch(error => console.log(error));
    const promise1 = new Promise((resolve, reject) => {
      reject("Error");
    });
    const promise2 = new Promise((resolve, reject) => { 
      resolve(2);
    });
    const promise3 = new Promise((resolve, reject) => { 
      resolve(3);
    });
    
    Promise.race([promise1, promise2, promise3])
      .then(result => console.log(result))
      .catch(error => console.log(error)); // Prints "Error"

    Promise.resolve(value)

    Returns a new Promise object that is resolved with the given value.

    If the value is a thenable (i.e. has a then method), the returned promise will "follow" that thenable, adopting its eventual state; otherwise, the returned promise will be fulfilled with the value.

    Generally, if you don't know if a value is a promise or not, Promise.resolve(value) it instead and work with the return value as a promise:

    const promise1 = new Promise((resolve, reject) => {
      resolve(1);
    });
    
    promise1.then(result => console.log(result)); // Prints "1"

    Promise.reject(reason)

    Returns a new Promise object that is rejected with the given reason:

    const promise1 = new Promise((resolve, reject) => {
      reject("Error");
    });
    
    promise1.catch(error => console.log(error)); // Prints "Error"

    Callback Hell

    Do you remember that promises were designed in order to avoid using callbacks to handle asynchronous operations?

    That's true, but it does not mean that you can't experience the same callback hell using promises.

    Real-world example:

    fetchUser()
      .then(user => 
        fetchSettings(user.id)
          .then(settings => 
            fetchStatistics(settings.showStatistics)
              .then(statistics => console.log(statistics));
          );
      );

    In the example above we fetch user, then after fetch is successfully completed, we fetch settings, after settings fetched we get the statistics and so on.

    The code gets messy due to a lot of nesting.

    async/await to rescue. It is another alternative for consuming promises, which makes asynchronous code look and behave more like synchronous code.

    But that's the topic for the next article.

    Summary

    • Promise - it is an object that produces some value in the future
    • Promises were created to get rid of the callback hell
    • Promise has 3 states: pending, fulfilled and rejected
    • Promise is consumed using: then, catch and finally
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    Amazing. Very detailed explanation in very clear terms.
    January 11, 2021
    Thanks for sharing.