$q: standard reject handler

Question

$q: standard reject handler

I am looking for a way to ensure that when clients do not specify a reject handler in the 'then' function, a default one is automatically triggered.

For example, let's say the default action is alert(1)

In this scenario, calling

mypromise.then(function(result){...})

will trigger an alert with 1. However, using

mypromise.then(null, function(reason){alert(2)})

will instead show an alert with 2.

javascript angularjs promise angular-promise

Answer 1

Answer №1

Imagine if there existed a method to accomplish that.

Picture a fantastical machine capable of always determining whether .then is invoked with a function as the second argument for a specific promise.

Why does it matter?

This means you can identify if someone called:

myPromise.then(..., function(){ F(); });

At any given moment from any location. And have G() act as the default outcome.

And then what?

You could take an entire program consisting of plenty of code P ₍₁₎ and transform that code into:

var myPromise = $q.reject();
P; // incorporate the program's code
myPromise.then(null, function(){}); // add a handler

Okay, so I can achieve that, but why?

Now our miraculous machine can analyze an arbitrary program P and determine if myPromise had a rejection handler appended to it. This occurs only if and when P does not include an infinite loop (i.e., it stops). Consequently, detecting the addition of a catch handler is equivalent to solving the halting problem, which is impossible. ₍₂₎

In essence - it is unfeasible to determine whether a .catch handler is ever attached to a promise.

No more complicated explanations, give me a solution!

Excellent reaction! While this problem is theoretically unsolvable, it is quite manageable in practical scenarios. The key lies in a rule of thumb:

If an error handler isn't added within a microtask (digest in Angular) – no error handlers will be attached and the default handler can be executed instead.

In short: You don't usually attach .then(null, function(){}) asynchronously. Although promises are resolved asynchronously, handlers are typically attached synchronously, making this technique effective.

// maintaining library independence as much as possible.
var p = myPromiseSource(); // obtain a promise from the source
var then = p.then; // starting 1.3+, you can grab the constructor and use prototype instead
var t = setTimeout(function(){ // in angular utilize $timeout, not a microtask but acceptable
    defaultActionCall(p);// execute the default action!
});
// .catch delegates to `.then` in almost every library, hence simply `then`
p.then = function then(onFulfilled, onRejected){
    // delegate, disregarding progression since it is rarely used anyway.
    if(typeof onRejected === "function"){ // eliminate default action
        clearTimeout(t); // `timeout.cancel(t)` in Angular     
    }
    return then.call(this, onFulfilled, onRejected);
};

Is that all?

I just want to emphasize that situations necessitating such an extreme approach are uncommon. When considering implementing rejection tracking in io – numerous individuals proposed that if a promise is rejected without a catch, the entire application should probably cease operation. So proceed with caution :)

^{(1) assume P does not contain a variable myPromise, if it does rename myPromise to something P does not contain.}

^{(2) Admittedly - one might argue that inspecting the code of P without executing it can reveal whether myPromise receives a rejection handler. Formally, we alter every return in P and other termination forms to a}

return myPromise.then(null, function(){})

rather than placing it at the end. By doing this, we capture the "conditional aspect."

Answer 2

Imagine if there existed a method to accomplish that.

Picture a fantastical machine capable of always determining whether .then is invoked with a function as the second argument for a specific promise.

Why does it matter?

This means you can identify if someone called:

myPromise.then(..., function(){ F(); });

At any given moment from any location. And have G() act as the default outcome.

And then what?

You could take an entire program consisting of plenty of code P ₍₁₎ and transform that code into:

var myPromise = $q.reject();
P; // incorporate the program's code
myPromise.then(null, function(){}); // add a handler

Okay, so I can achieve that, but why?

Now our miraculous machine can analyze an arbitrary program P and determine if myPromise had a rejection handler appended to it. This occurs only if and when P does not include an infinite loop (i.e., it stops). Consequently, detecting the addition of a catch handler is equivalent to solving the halting problem, which is impossible. ₍₂₎

In essence - it is unfeasible to determine whether a .catch handler is ever attached to a promise.

No more complicated explanations, give me a solution!

Excellent reaction! While this problem is theoretically unsolvable, it is quite manageable in practical scenarios. The key lies in a rule of thumb:

If an error handler isn't added within a microtask (digest in Angular) – no error handlers will be attached and the default handler can be executed instead.

In short: You don't usually attach .then(null, function(){}) asynchronously. Although promises are resolved asynchronously, handlers are typically attached synchronously, making this technique effective.

// maintaining library independence as much as possible.
var p = myPromiseSource(); // obtain a promise from the source
var then = p.then; // starting 1.3+, you can grab the constructor and use prototype instead
var t = setTimeout(function(){ // in angular utilize $timeout, not a microtask but acceptable
    defaultActionCall(p);// execute the default action!
});
// .catch delegates to `.then` in almost every library, hence simply `then`
p.then = function then(onFulfilled, onRejected){
    // delegate, disregarding progression since it is rarely used anyway.
    if(typeof onRejected === "function"){ // eliminate default action
        clearTimeout(t); // `timeout.cancel(t)` in Angular     
    }
    return then.call(this, onFulfilled, onRejected);
};

Is that all?

I just want to emphasize that situations necessitating such an extreme approach are uncommon. When considering implementing rejection tracking in io – numerous individuals proposed that if a promise is rejected without a catch, the entire application should probably cease operation. So proceed with caution :)

^{(1) assume P does not contain a variable myPromise, if it does rename myPromise to something P does not contain.}

^{(2) Admittedly - one might argue that inspecting the code of P without executing it can reveal whether myPromise receives a rejection handler. Formally, we alter every return in P and other termination forms to a}

return myPromise.then(null, function(){})

rather than placing it at the end. By doing this, we capture the "conditional aspect."

$q: standard reject handler

Answer №1

Imagine if there existed a method to accomplish that.

Why does it matter?

And then what?

Okay, so I can achieve that, but why?

No more complicated explanations, give me a solution!

Is that all?

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