Added documentation on Closures

Author: avinashyadav16

13_Closures.md

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+# Episode 10 : Closures in JS
+
+### Important Interview Question
+
+**Closure :** Function bundled together with its lexical environment/scope.
+
+JS is a weird language. You can pass functions as parameters to another function, assign a variable to an entire function, or even return a function.
+eg:
+
+```javascript
+function x() {
+  var a = 7;
+
+  function y() {
+    console.log(a);
+  }
+
+  return y; // instead of y();
+}
+
+var z = x();
+
+console.log(z); // value of z is entire code of function y.
+```
+
+When functions are returned from another function, they still maintain their lexical
+scope.
+
+- When y is returned, not only is the function returned but the entire closure (function y + its lexical scope) is returned and put inside z. So when z is used somewhere else in program, it still remembers variable `a` inside x()
+- Closure is a very powerful concept of JS, just because this function remembers things even if they are not in their lexical scope
+
+### Some Gottchas:
+
+#### CODE - 01
+
+```javascript
+function x() {
+  var a = 7;
+
+  function y() {
+    console.log(a);
+  }
+
+  a = 100;
+
+  return y;
+}
+
+var z = x();
+z();
+```
+
+OUTPUT:
+
+    100
+
+#### CODE - 02
+
+```javascript
+function w() {
+  var b = 900;
+
+  function x() {
+    var a = 7;
+
+    function y() {
+      console.log(a, b);
+    }
+
+    y();
+  }
+
+  x();
+}
+
+var z = w();
+z();
+```
+
+OUTPUT:
+
+    7 900
+
+### Uses of Closure
+
+- Module Design Pattern,
+- Currying,
+- Functions like once(fxn that can be run only once),
+- memoize,
+- maintaining state in async world,
+- setTimeout,
+- iterators
+- ...