What is a 'Closure'?

Question

I asked a question about Currying and closures were mentioned. What is a closure? How does it relate to currying?

Answer 1

A simple example in Groovy for your reference:

def outer() {
    def x = 1
    return { -> println(x)} // inner
}
def innerObj = outer()
innerObj() // prints 1

Answer 2

Here's a real world example of why Closures kick ass... This is straight out of my Javascript code. Let me illustrate.

Function.prototype.delay = function(ms /*[, arg...]*/) {
  var fn = this,
      args = Array.prototype.slice.call(arguments, 1);

  return window.setTimeout(function() {
      return fn.apply(fn, args);
  }, ms);
};

And here's how you would use it:

var startPlayback = function(track) {
  Player.play(track);  
};
startPlayback(someTrack);

Now imagine you want the playback to start delayed, like for example 5 seconds later after this code snippet runs. Well that's easy with delay and it's closure:

startPlayback.delay(5000, someTrack);
// Keep going, do other things

When you call delay with 5000ms, the first snippet runs, and stores the passed in arguments in it's closure. Then 5 seconds later, when the setTimeout callback happens, the closure still maintains those variables, so it can call the original function with the original parameters.
This is a type of currying, or function decoration.

Without closures, you would have to somehow maintain those variables state outside the function, thus littering code outside the function with something that logically belongs inside it. Using closures can greatly improve the quality and readability of your code.

Answer 3

Closure is very easy. We can consider it as follows : Closure = function + its lexical environment

Consider the following function:

function init() {
    var name = “Mozilla”;
}

What will be the closure in the above case ? Function init() and variables in its lexical environment ie name. Closure = init() + name

Consider another function :

function init() {
    var name = “Mozilla”;
    function displayName(){
        alert(name);
}
displayName();
}

What will be the closures here ? Inner function can access variables of outer function. displayName() can access the variable name declared in the parent function, init(). However, the same local variables in displayName() will be used if they exists.

Closure 1 : init function + ( name variable + displayName() function) --> lexical scope

Closure 2 : displayName function + ( name variable ) --> lexical scope

Answer 4

Closure is a feature in JavaScript where a function has access to its own scope variables, access to the outer function variables and access to the global variables.

Closure has access to its outer function scope even after the outer function has returned. This means a closure can remember and access variables and arguments of its outer function even after the function has finished.

The inner function can access the variables defined in its own scope, the outer function’s scope, and the global scope. And the outer function can access the variable defined in its own scope and the global scope.

Example of Closure:

var globalValue = 5;

function functOuter() {
  var outerFunctionValue = 10;

  //Inner function has access to the outer function value
  //and the global variables
  function functInner() {
    var innerFunctionValue = 5;
    alert(globalValue + outerFunctionValue + innerFunctionValue);
  }
  functInner();
}
functOuter();  

Output will be 20 which sum of its inner function own variable, outer function variable and global variable value.

Answer 5

Here is another real life example, and using a scripting language popular in games - Lua. I needed to slightly change the way a library function worked to avoid a problem with stdin not being available.

local old_dofile = dofile

function dofile( filename )
  if filename == nil then
    error( 'Can not use default of stdin.' )
  end

  old_dofile( filename )
end

The value of old_dofile disappears when this block of code finishes it's scope (because it's local), however the value has been enclosed in a closure, so the new redefined dofile function CAN access it, or rather a copy stored along with the function as an 'upvalue'.