@@ -185,3 +185,44 @@ function runWhileLoopForNSeconds(sec) {
185185 now = Date . now ( ) ;
186186 }
187187}
188+ 189+ // Write a function where suppose you want to use a variable for counting something, and you want this counter to be available to all functions.
190+ 191+ // Initiate counter
192+ let counter = 0 ;
193+ 194+ // Function to increment counter
195+ function add ( ) {
196+ counter += 1 ;
197+ }
198+ 199+ // Call add() 3 times
200+ add ( ) ;
201+ add ( ) ;
202+ add ( ) ;
203+ 204+ // The counter should now be 3
205+ 206+ // Solve
207+ const add = ( function ( ) {
208+ let counter = 0 ;
209+ return function ( ) {
210+ counter += 1 ;
211+ return counter ;
212+ } ;
213+ } ) ( ) ;
214+ 215+ add ( ) ;
216+ add ( ) ;
217+ add ( ) ; // the counter is now 3
218+ 219+ // Explained
220+ // The variable add is assigned to the return value of a self-invoking function.
221+ 222+ // The self-invoking function only runs once. It sets the counter to zero (0), and returns a function expression.
223+ 224+ // This way add becomes a function. The "wonderful" part is that it can access the counter in the parent scope.
225+ 226+ // This is called a JavaScript closure. It makes it possible for a function to have "private" variables.
227+ 228+ // The counter is protected by the scope of the anonymous function, and can only be changed using the add function.
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