$q.all() rejecting due to one failed promise

1. Understanding $q.all() Behavior

In AngularJS, $q.all() is used to execute multiple promises in parallel and wait for all of them to resolve. However, if any one of the promises fails, $q.all() immediately rejects, ignoring other promises.

Example of $q.all() Rejecting Due to One Failed Promise

$q.all([
    $http.get('/api/data1'),  // Success
    $http.get('/api/data2'),  // Fails (404 Not Found)
    $http.get('/api/data3')   // Never executes due to rejection
]).then(function(results) {
    console.log("All API calls successful:", results);
}).catch(function(error) {
    console.error("One of the requests failed:", error);
});

• If /api/data2 fails, $q.all() rejects immediately and skips the success responses.
• This can be problematic when you need partial success handling.

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2. How to Fix $q.all() Rejecting Due to One Failed Promise?

Fix 1: Use .catch() for Each Promise

Handle errors inside each individual promise before passing them to $q.all().

$q.all([
    $http.get('/api/data1').catch(err => ({ error: true, message: err })), 
    $http.get('/api/data2').catch(err => ({ error: true, message: err })), 
    $http.get('/api/data3').catch(err => ({ error: true, message: err }))
]).then(function(results) {
    console.log("Partial success:", results);
});

• Instead of rejecting $q.all(), each request returns a resolved object with an error message.
• You can then check for errors in results without the entire chain failing.

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Fix 2: Use $q.allSettled() (For AngularJS 1.5+)

AngularJS 1.5+ introduced $q.allSettled(), which waits for all promises, even if some fail.

$q.allSettled([
    $http.get('/api/data1'),
    $http.get('/api/data2'),
    $http.get('/api/data3')
]).then(function(results) {
    console.log("All responses received, including errors:", results);
});

• Unlike $q.all(), $q.allSettled() never rejects, even if some promises fail.

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Fix 3: Create a Wrapper Function for $q.all()

If you are using AngularJS below 1.5, manually implement $q.allSettled().

function allSettled(promises) {
    return $q.all(promises.map(p => 
        p.then(
            result => ({ status: "fulfilled", value: result }),
            error => ({ status: "rejected", reason: error })
        )
    ));
}

allSettled([
    $http.get('/api/data1'),
    $http.get('/api/data2'),
    $http.get('/api/data3')
]).then(results => console.log("Handled all promises:", results));

• This ensures all promises complete without causing $q.all() to reject.

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Fix 4: Split Requests and Handle Failures Separately

Instead of using $q.all(), handle successes and failures separately.

let successRequests = [];
let failedRequests = [];

$q.all([
    $http.get('/api/data1'),
    $http.get('/api/data2'),
    $http.get('/api/data3')
]).then(results => {
    results.forEach(response => successRequests.push(response));
}).catch(error => {
    failedRequests.push(error);
}).finally(() => {
    console.log("Success:", successRequests);
    console.log("Failures:", failedRequests);
});

• This separates failed requests from successful ones.

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3. Summary & Best Practices

Issue	Solution
$q.all() rejects if one promise fails	Use .catch() on each promise
Need all results (including failures)	Use $q.allSettled() (AngularJS 1.5+)
Older AngularJS versions	Implement custom allSettled() function
Want separate success & failure handling	Use .then(), .catch(), and .finally()

By implementing these fixes, you ensure robust API handling while using $q.all() in AngularJS.