What is closure in Swift? Is it similar to callback in Javascript?

When are closures the best way to go in JavaScript?

• I am looking for examples where closures are the best solution in JavaScript. It will be great if you provide a code snippet of a closure and the non-closure solution and explain why closure is better(for any reason)

• Answer:

  Closures are great. Closures are one of the greatest features of JavaScript. The most basic thing to understand about  a closure is that a closure is a function defined in an another scope that has all the access to all the variables within that scope. That says it all one needs to understand about closures. Some of the cases where I prefer to use Closures are: 1. Events and Callbacks Closures help to write non-blocking event driven code. At the beginner level, we go about writing a simple event-driven callback using setTimeout() function call like this: function handleEvent(){ alert(id); } function someEvent(id, timeOut){ setTimeout(handleEvent, timeOut); } someEvent(1,1000); We expect the code to work but it doesn't work because the handleEvent() function doesn't know about the existence of the 'id' variable. So, we get over this using closures like this: function someEvent(id, timeOut) { function handleEvent(){ alert(id); } setTimeout(handleEvent, timeOut); } Another beautiful application of closures is when assigning events. Non-closure way: function assignEvents(arrayofObjects){ for(var i=0;i<arrayofObjects.length;++i){ arrayofObjects[i].onclick = function(i){ alert("You clicked on the object number "+i); } } } In the above implementation, the last value of i is exposed to all the event handler functions, so the code doesn't work the way we expected it to work. So, we solve this problem using closures: function assignEvents( arrayofObjects){ function assignEventHandler(index){ return function(){ alert("You clicked on the object number "+index); } } for(var i=0;i<arrayofObjects.length;++i){ arrayofObjects[i].onclick = assignEventHandler(i); } } In the above code, we used assignEventHandler() function as the outer scope of the closure function() which assigns the event handler to the object. 2. Assigning unique ids to elements Sometimes I often land into situations where I have to assign ids to elements, so to be able to uniquely identify elements, I need to be able to assign them unique ids. I usually solve this problem by using closure to write a helper function to generate ids for elements: var uniqueID = function(){ var id = 0; return function(element){ if(!element.id){ element.id = 'unique-id-' + id; id++; } } } 3. Object reuse This is a clever use of closures to help save run time and memory while cloning objects, i.e. creating objects inheriting from another object. The simplest object cloning code without use of closures: function clone(parent){ function F(){ F.prototype = parent; return new F; } } The above code is slow and consumes too much memory. The function 'F()' is called every time the clone() function is invoked. We don't have to call the function everytime we need to clone the parent, rather we can do it once and reuse it using closure: function clone(parent){ function F(){ return function(parent){ F.prototype = parent; return new F; } } } Instead of the new F object, we are returning the anonymous function inside F() which is reused and no new F is calculated everytime, it is done only once and reused for every other call to clone(). 4. Hiding state and methods in objects This is the most basic use of closures. Read about this http://howtonode.org/why-use-closure One of the best read on closures is http://msdn.microsoft.com/en-us/magazine/ff696765.aspx