Closures

• Function bundled along with it's lexical scope is closure.

• JavaScript has a lexcial scope environment. If a function needs to access a variable, it first goes to its local memory. When it does not find it there, it goes to the memory of its lexical parent. See Below code, Over here function y along with its lexical scope i.e. (function x) would be called a closure.

  function x() {
    var a = 7;
    function y() {
      console.log(a);
    }
    return y;
  }
  var z = x();
  console.log(z); // value of z is entire code of function y.
  • In above code, When y is returned, not only is the function returned but the entire closure (fun y + its lexical scope) is returned and put inside z. So when z is used somewhere else in program, it still remembers var a inside x()

• Another Example

function z() {
  var b = 900;
  function x() {
    var a = 7;
    function y() {
      console.log(a, b);
    }
    y();
  }
  x();
}
z(); // 7 900

• Thus In simple words, we can say:

  • *A closure is a function that has access to its outer function scope even after the function has returned. Meaning, A closure can remember and access variables and arguments reference of its outer function even after the function has returned.*

• Advantages of Closure:

  • Module Design Pattern
  • Currying
  • Memoize
  • Data hiding and encapsulation
  • setTimeouts etc.

• Disadvantages of Closure:

  • Over consumption of memory
  • Memory Leak
  • Freeze browser