Implementing Interceptor in AngularJS

Question

Looking at this AuthInterceptor from this helpful answer, how are the request and response key's used in the returned JSON object?

Also, what's the meaning of return config || $q.when(config)? I understand that the second part is returned if config is null or undefined, but what does $q.when(config) mean in this code?

myApp.factory('AuthInterceptor', function ($window, $q) {
    return {
        'request': function(config) {
            config.headers = config.headers || {};
            if ($window.sessionStorage.getItem('token')) {
                config.headers.Authorization = $window.sessionStorage.getItem('token');
            }
            return config || $q.when(config);
        },
        'response': function(response) {
            if(response.status === 401) {
                $location('/login');
            }
            return response || $q.when(response);
        }
    };
});

I typed out the above linked answer. It worked for me, but I don't understand how it's used.

Community · Accepted Answer · 2017-05-23 12:05:08Z

1

When registering an $http interceptor, you should essentially pass an object with the following (optional) keys: request, requestError, response, responseError.
For more info on when each interceptor function is called, take a look at this answer.

$q.when() turns any JS object into a promise (that resolved immediatelly with the passed in object as each resolved value). But, frankly (although I've seen it a lot) I don't understand why one would need to return config || $q.when(config), instead of just config.

edited May 23, 2017 at 12:05

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answered Jun 9, 2014 at 17:13

gkalpak

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2 Comments

Kevin Meredith Over a year ago

could you please say a bit more: I don't understand why one would need to return config || $q.when(config), instead of just config.? I'd like to know what you're thinking, please.

gkalpak Over a year ago

Actually, there nothing moe to say other than what I said in my comment to Matt Way's answer (which is essentially what you said in your comment).

Matt Way · Accepted Answer · 2014-06-09 17:26:21Z

1

request and response are simple used as intermediaries. In other words, the request function is run prior to a request (after requesting), and the response function is run prior to handling a request result.

$q.when() ensures that the return value is able to be handled as a promise. Essentially it wraps the value in a promise, so that functions like returnedValue.then() work.

See: https://docs.angularjs.org/api/ng/service/$q

Kevin asked why you would want to use $q.when(). This is to ensure that any outside function that consumes this one receives a promise, if it is expecting one. For example, imagine the situation where you call a function expecting a promise:

someThirdPartyFunc(someArg).then(function(result){
    // do somethign with result
});

If you ran this and someThirdPartyFunc returned a simple value (say 100), then this would obviously fail. If the implementor wrapped the return value in $q.when(100), then the function call above would be valid, and result would contain the value 100.

edited Jun 9, 2014 at 17:26

answered Jun 9, 2014 at 17:08

Matt Way

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4 Comments

Kevin Meredith Over a year ago

why would you want to handle a return variable as a promise?

Matt Way Over a year ago

Updated answer for you.

Kevin Meredith Over a year ago

so let's say that my regular, non-intercepting code executes: $http.post("foo/bar.com"), expecting a promise back. This HTTP POST returns an object: {count: 100}. The above response property of my AuthInterceptor would return the object as response is truthy. Wouldn't this implementation then break the expected promise result of the caller of the $http.post(...)? Or perhaps it's up to the implementor to decide whether to return response or response wrapped in a promise?

gkalpak Over a year ago

This is why I don't see the purpose of returning a promise. Normally (i.e. if config is defined) the function will return an object (not a promise), but if config is not defined then it will return a promise (wrapped around the undefined config). It doesn't make sense and indeed it seems like breaking your own implementation (but again I might be missing something).

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