CODEWITHSUNDEEP
javascriptangularjspromiseq
Bryce Johnson
Bryce Johnson

Reputation: 6901

Why would you execute $q.when() without passing it a promise/value?

According to the Angular docs on $q, $q.when() expects a promise/value to passed in.

But I just came across someone else's code where it's called without passing in any params. Here's a simplified version of what I see: 

var modal = false;

if (modalOpen) {
    return $q.when()
}
modalOpen = true;

modal = newModal({
    template: opts.template,
});

modal.result.finally(function(){ 
    modalOpen = false;
});
}

Upvotes: 6

Views: 2546

Answers (2)

Nelu
Nelu

Reputation: 18740

If you're wondering what $q.when() returns - it returns undefined.

If you need it to return null, then do $q.when(null).

This is useful when you have a type system like TypeScript and need to know what the return type is for your method when you're unable to resolve the promise with the type requested.

const getUser = async (id: number): Promise<User | undefined> =>
    someCondition ? Api.getUser(id) : $q.when();

Upvotes: 1

Benjamin Gruenbaum
Benjamin Gruenbaum

Reputation: 276306

Methods should either return synchronously or return asynchronously to remain consistent. If a method returns synchronously sometimes and still wants to keep the fact sometimes it is already resolved transparent - it returns an empty resolved promise. Having APIs that sometimes return promises and sometimes synchronously is a recipe for trouble.

Using $q.when is the simplest way to get an empty resolved promise in Angular.

Upvotes: 7

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