Are promises resolved in an asynchronous or synchronous manner?

Question

Are promises resolved in an asynchronous or synchronous manner?

Lately, I've been delving into JavaScript promises and stumbled upon a scenario that really intrigued me:

let combinedArray = [];

function getArrayOne() {
    $http.post(arrayOnePath).then(function(arr) {
        combinedArray = combinedArray.concat(arr);
    }) // Additional code preventing the use of Promise.all(...)
}

function getArrayTwo() {
    $http.post(arrayTwoPath).then(function(arr) {
        combinedArray = combinedArray.concat(arr);
    }) // More code preventing me from using Promise.all(...)    
}

function getAllArrays() {
    getArrayOne();
    getArrayTwo();
}

As I was working on this logic, it occurred to me that there might be a potential race condition if both promises resolve simultaneously (due to them accessing a shared resource). Upon further contemplation, I realized that the then(..) resolutions are triggered after the post request returns, indicating that this code operates within JavaScript's synchronous execution environment.

I'd appreciate some insight on whether the two instances of combinedArray.concat(arr); could pose an issue if both promises are resolved concurrently.

[Edit] In response to some of the feedback, I want to clarify that I'm not concerned about the order in which the arrays are concatenated into combinedArray.

javascript angularjs asynchronous promise

Answer 1

Answer №1

It is important to note that JavaScript operates as a single-threaded language, which helps prevent race conditions especially when dealing with asynchronous calls.

In certain scenarios, JavaScript may utilize additional threads in the background, such as with node's I/O functions. Additionally, the web worker API enables the creation of isolated yet separate threads that can communicate by passing messages.

The original design of JavaScript was based on its single-threaded nature, making it challenging to introduce multi-threading without risking potential race conditions. This fundamental constraint ensures that existing code continues to function as expected without breaking everything. Therefore, promises are sequentially added to a queue and resolved one by one for consistent and safe execution.

Even within web workers or their node counterparts, each "thread" maintains a distinct memory space where direct access to variables from other threads is restricted. Web workers specifically rely on the postMessage method to serialize objects and transmit them securely between threads.

Answer 2

It is important to note that JavaScript operates as a single-threaded language, which helps prevent race conditions especially when dealing with asynchronous calls.

In certain scenarios, JavaScript may utilize additional threads in the background, such as with node's I/O functions. Additionally, the web worker API enables the creation of isolated yet separate threads that can communicate by passing messages.

The original design of JavaScript was based on its single-threaded nature, making it challenging to introduce multi-threading without risking potential race conditions. This fundamental constraint ensures that existing code continues to function as expected without breaking everything. Therefore, promises are sequentially added to a queue and resolved one by one for consistent and safe execution.

Even within web workers or their node counterparts, each "thread" maintains a distinct memory space where direct access to variables from other threads is restricted. Web workers specifically rely on the postMessage method to serialize objects and transmit them securely between threads.

Answer 3

Answer №2

Important notes about functions used in the Promise API:

When a function fn is passed to a Promise executor (new Promise(fn)), it is executed immediately.
On the other hand, when a function fn is passed to a handler (.then(fn)), it is executed asynchronously.

In a JavaScript environment, two functions will not execute simultaneously unless Web Workers are being used. This does not align with the concept of asynchrony.

The scenario described does not result in a race condition, which typically indicates an issue within the implementation. The order in which the functions execute does not affect the program's functionality unless specific operations depend on their sequence.

Learn more about "Race condition" on Wikipedia:

A race condition occurs when the output of a system relies on uncontrolled event sequence or timing. It only becomes a problem when events do not follow the intended order set by the programmer.

Answer 4

Important notes about functions used in the Promise API:

When a function fn is passed to a Promise executor (new Promise(fn)), it is executed immediately.
On the other hand, when a function fn is passed to a handler (.then(fn)), it is executed asynchronously.

In a JavaScript environment, two functions will not execute simultaneously unless Web Workers are being used. This does not align with the concept of asynchrony.

The scenario described does not result in a race condition, which typically indicates an issue within the implementation. The order in which the functions execute does not affect the program's functionality unless specific operations depend on their sequence.

Learn more about "Race condition" on Wikipedia:

A race condition occurs when the output of a system relies on uncontrolled event sequence or timing. It only becomes a problem when events do not follow the intended order set by the programmer.

Answer 5

Answer №3

It has been mentioned before that your user-level Javascript operates in a single-threaded manner, with the exception of webWorkers not relevant to this scenario.

Since no two pieces of your Javascript run simultaneously, there is no risk of a race condition. However, the issue in your code arises when trying to determine when both async operations have completed in order to utilize the results. One approach to handle this is by implementing the following structure:

function getArrayOne() {
    return $http.post(arrayOnePath).then(function(arr) {
        // perform any necessary processing on arr
        return arr;
    })
}

function getArrayTwo() {
    return $http.post(arrayTwoPath).then(function(arr) {
        // perform any necessary processing on arr
        return arr;
    })
}

function getAllArrays() {
    return Promise.all(getArrayOne(), getArrayTwo()).then(function(results) {
        // results[0] references the first array
        // results[1] references the second array
        return results[0].concat(results[1]);
    });
}

getAllArrays().then(function(results) {
     // access to all combined results
}, function(err) {
     // error handling for any issues during async operations
});

This method not only notifies you when all async operations have concluded and provides the merged results in sequential order, but also properly handles errors from either operation.

Answer 6

It has been mentioned before that your user-level Javascript operates in a single-threaded manner, with the exception of webWorkers not relevant to this scenario.

Since no two pieces of your Javascript run simultaneously, there is no risk of a race condition. However, the issue in your code arises when trying to determine when both async operations have completed in order to utilize the results. One approach to handle this is by implementing the following structure:

function getArrayOne() {
    return $http.post(arrayOnePath).then(function(arr) {
        // perform any necessary processing on arr
        return arr;
    })
}

function getArrayTwo() {
    return $http.post(arrayTwoPath).then(function(arr) {
        // perform any necessary processing on arr
        return arr;
    })
}

function getAllArrays() {
    return Promise.all(getArrayOne(), getArrayTwo()).then(function(results) {
        // results[0] references the first array
        // results[1] references the second array
        return results[0].concat(results[1]);
    });
}

getAllArrays().then(function(results) {
     // access to all combined results
}, function(err) {
     // error handling for any issues during async operations
});

This method not only notifies you when all async operations have concluded and provides the merged results in sequential order, but also properly handles errors from either operation.

Are promises resolved in an asynchronous or synchronous manner?

Answer №1

Answer №2

Answer №3

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