add custom functions

Author: geekysrm

12-functions-custom.md

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+
+## Functions - Custom
+
+-   Functions are **created/ defined** then they are **called**.
+    
+-   Defining a function:
+    
+    ```javascript
+      // Function definition
+      
+      function calculateBill() {
+      	// this is the function body
+      	console.log('running calculateBill');
+      }
+    
+    ```
+    
+-   Calling a function:
+    
+    ```javascript
+      // Function call or run
+      
+      calculateBill(); // running calculateBill (returns undefined)
+    
+    ```
+    
+-   Variables created inside a function are not available outside the function. e.g. `total` above.
+    
+    It is a **temporary variable.** After running of the function is complete, the variable is cleaned up or garbage-collected.
+    
+-   **Returning value from function:**
+    
+    ```javascript
+      function calculateBill() {
+      	const total = 100 * 1.13;
+      	return total; // total is returned
+      }
+      
+      calculateBill(); // returns 112.999999999
+    
+    ```
+    
+-   Capturing returned value from a function into a variable:
+    
+    `const myTotal = calculateBill();` (myTotal will have value 112.999999999)