Add params and arguments

Author: geekysrm

13-functions-param-arguments.md

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+## Functions - arguments and parameters
+
+- Parameters are like _placeholders_ for data that will be passed to a function.
+
+- Arguments are the actual values passed to a function while calling it
+
+  ```javascript
+  function calculateBill(billAmount, taxRate) { // here billAmount, taxRate are parameters const total = billAmount + billAmount * taxRate return total; }
+
+  calculateBill(100, 0.13); // here 100, 0.13 are arguments
+  ```
+
+![https://res.cloudinary.com/geekysrm/image/upload/v1576699296/parameters-arguments.jpg](https://res.cloudinary.com/geekysrm/image/upload/v1576699296/parameters-arguments.jpg)
+
+- Parameters are variables local to the function; available only inside the function.
+
+- You can also pass variables as arguments during a function call.
+
+- We can also pass _expressions_ as arguments to a function.
+
+  ```
+    myTotal3 = calculateBill(20+20+30, 0.3);
+
+  ```
+
+- So, we can either pass direct value or variables holding value or expressions resulting in a value to a function as arguments.
+
+- **Passing functions as arguments:**
+
+  ```javascript
+  function doctorize(name) {
+    return `Dr. ${name}`;
+  }
+
+  function yell(name) {
+    return `HEY ${name.toUpperCase()}`;
+  }
+
+  // We can pass a function inside another
+  yell(doctorize("Soumya")); // HEY DR. SOUMYA
+  // Above, returned value of doctorize function is passed to yell function
+  ```
+
+  - Default values:
+
+    ```javascript
+    function yell(name = "Silly Goose") {
+      return `HEY ${name.toUpperCase()}`;
+    }
+
+    yell("Soumya"); // HEY SOUMYA
+    yell(); // HEY SILLY GOOSE
+
+    // Above, if we don't pass any argument to yell function, then it takes the default value in function definition,
+    // here Silly Goose, else it takes whatever we pass as argument.
+    ```
+
+  - **Important gotcha:**
+
+    ```javascript
+    function calculateBill(billAmount, taxRate = 0.13, tipRate = 0.15) {
+      console.log("Running Calculate Bill!!");
+      const total = billAmount + billAmount * taxRate + billAmount * tipRate;
+      return total;
+    }
+
+    // Suppose above, we want to pass the tipRate but not the taxRate and want taxRate to be default,
+    // then the only thing we can do is:
+
+    calculateBill(100, undefined, 0.66); // here the taxRate will default to 0.13 as
+    // we have passed undefined to it and the tipRate will be 0.66 as passed
+    ```