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Question

In closure tag wiki page , it reads "jQuery itself is one big closure."

But is promise a closure as well? Could you please explain why or why not? This is how I understand closure: assign a function to a variable and reuse it with different environments. Promise does that with $.ajax() , but I could not find anywhere in stackoverflow where promise is introduced as a closure. Maybe because there are other features of promise like $.Deferred() , resolve() , and fail() to expand its functionality beyond a simple function passing?

Answer 1

Closures

This is how I understand closure: assign a function to a variable and reuse it with different environments.

That's not a strictly accurate definition of a closure.

A closure is a function that has access to a referencing-environment. In Javascript, that means a function that is returned by another function and has access to the original functions scope. there are other SO questions that describe this very well

Closure's are general purpose structures that can be used in a variety of ways. One of their biggest benefits is that they protect private scope, which is why libraries like jQuery are often written as closures, so that they don't need to expose all their functions globally.

Promises

Promises are a different concept. They are a way of structuring asynchronous code to make it easier to follow the flow. A promise object in particular is an object that provides functions to chain operations in a clear and easy to read way. A promise might be implemented using closures, but it does not have to be. For instance here is an implementation that does not use closures:

https://gist.github.com/814052/690a6b41dc8445479676b347f1ed49f4fd0b1637

whereas jQuery's implementation uses at least one closure, but isn't really based on them

http://james.padolsey.com/jquery/#v=1.10.2&fn=jQuery.Deferred

Conclusion

Promises and Closures aren't directly related concepts. Closure's are a programming technique that might be used in a Promise implementation. In the end it is neither impossible or necessary to implement it like that.

Answer 2

You wouldn't ask if a birdhouse was a 2x4, even if you used one to make it. The same can be said of promises and closures. Promises make use of closures to retain references to state, callbacks and other such things.

Answer 3

Because of the nature of JavaScript, being asynchronous that is, we are provide much power by the language and it's runtimes. First off, a Promise in jQuery, although it is not unique to jQuery, is an object that will as the documentation puts it, observe when all actions of a certain type bound to the collection, queued or not, have finished . This means you can use this object to know when to continue after a set or queue of items has finished some behavior. Now a Closure on the other hand is not unique to jQuery, but is rather a JavaScript construct, one that combines two things: a function, and the environment in which that function was created . This means that not only executing a function but doing so in possibly an entirely different context.

Answer 4

Closures and Promise are different concepts. Closures refers to scope of variables where as promise are used to 'promise' that an act on something will occur when it is done on an asynchronous action. Since Javascript is non-blocking (not asynchronous --edit), it will not wait for a function to get a response if it needs to access the internet or disk, that said you can have a promise execute after something is done.