[Edit]: JavaScript: .indexOf() (#6981)

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* [Edit]: JavaScript: `.indexOf()`

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Author: mamtawardhani

content/javascript/concepts/arrays/terms/indexOf/indexOf.md

@@ -1,70 +1,187 @@
 ---
 Title: '.indexOf()'
-Description: 'Returns the first index at which an element can be found. Returns -1 if element is not found.'
+Description: 'Returns the first index at which a specified element can be found in an array, or -1 if not present.'
 Subjects:
-  - 'Web Development'
   - 'Computer Science'
+  - 'Web Development'
 Tags:
   - 'Arrays'
-  - 'Functions'
   - 'Methods'
+  - 'JavaScript'
+  - 'Search'
 CatalogContent:
   - 'introduction-to-javascript'
   - 'paths/front-end-engineer-career-path'
 ---
 
-The **`.indexOf()`** method returns the first index at which an element can be found. Returns `-1` if the element is not found.
+The **`.indexOf()`** method returns the first index at which a specified element can be found in an [array](https://www.codecademy.com/resources/docs/javascript/arrays), or -1 if the element is not present. This method searches the array from left to right and uses strict equality comparison to match elements.
+
+The `.indexOf()` method is commonly used for element detection, data validation, conditional logic, and implementing search functionality in web applications. It is a fundamental tool for determining whether specific values exist in arrays and their exact positions. It is essential for user input validation, menu systems, and data processing workflows.
 
 ## Syntax
 
 ```pseudo
-array.indexOf(element, startIndex);
+array.indexOf(searchElement, fromIndex)
 ```
 
-The following parameters are used in the `.indexOf()` method:
+**Parameters:**
+
+- `searchElement`: The element to search for in the array.
+- `fromIndex` (Optional): The index position to start the search from. The default value is 0. Negative values count back from the end of the array, but still search from left to right.
 
-- The `element` to be searched for in the `array`.
-- The optional `startIndex` position to begin searching from. If one is not given, the search starts from the beginning of the array. Negative indices will offset from the end of the `array`.
+**Return value:**
 
-## Examples
+The `.indexOf()` method returns a number representing the index position of the first occurrence of the specified element, or -1 if the element is not found.
 
-In the example below, the index for the number `12` is logged to the console:
+## Example 1: Basic Usage of the `.indexOf()` Method
+
+This example demonstrates the fundamental usage of the `indexOf()` method to locate elements in an array:
 
 ```js
-const numbers = [6, 12, 8, 10];
-const indexOf12 = numbers.indexOf(12);
+const fruits = ['apple', 'banana', 'orange', 'grape'];
+
+// Find the index of "banana"
+const bananaIndex = fruits.indexOf('banana');
+console.log(bananaIndex);
+
+// Search for an element that doesn't exist
+const kiwiIndex = fruits.indexOf('kiwi');
+console.log(kiwiIndex);
+
+// Check if an element exists in the array
+if (fruits.indexOf('orange') !== -1) {
+  console.log('Orange is in the array');
+} else {
+  console.log('Orange is not in the array');
+}
+```
+
+The output of this code will be:
 
-console.log(indexOf12);
-// Output: 1
+```shell
+1
+-1
+Orange is in the array
 ```
 
-If element is not found the result will be `-1`:
+This example shows how `.indexOf()` returns the zero-based index position of the first matching element. When "banana" is found at position 1, the method returns 1. When searching for "kiwi", which doesn't exist, it returns -1. The conditional check demonstrates a common pattern for testing the existence of an element.
+
+## Example 2: Product Inventory Management
+
+The following example shows how `.indexOf()` can be used in a real-world inventory management system to track product availability and prevent duplicates:
 
 ```js
-const pizzaToppings = ['pepperoni', 'olives', 'mushrooms'];
-const indexOfPineapple = pizzaToppings.indexOf('pineapple');
+const inventory = ['laptop', 'mouse', 'keyboard', 'monitor', 'speakers'];
+
+// Function to check if a product is in stock
+function checkStock(product) {
+  const index = inventory.indexOf(product);
+  if (index !== -1) {
+    return `${product} is in stock at position ${index}`;
+  } else {
+    return `${product} is out of stock`;
+  }
+}
+
+// Function to add new product if not already present
+function addProduct(product) {
+  if (inventory.indexOf(product) === -1) {
+    inventory.push(product);
+    console.log(`${product} added to inventory`);
+  } else {
+    console.log(`${product} already exists in inventory`);
+  }
+}
+
+// Test the functions
+console.log(checkStock('mouse'));
+console.log(checkStock('tablet'));
+
+addProduct('webcam');
+addProduct('laptop');
+
+console.log(inventory);
+```
 
-console.log(indexOfPineapple);
-// Output: -1
+The output of this code will be:
+
+```shell
+mouse is in stock at position 1
+tablet is out of stock
+webcam added to inventory
+laptop already exists in inventory
+[ 'laptop', 'mouse', 'keyboard', 'monitor', 'speakers', 'webcam' ]
 ```
 
-Check if color `'blue'` is in the `colors` array starting with the second element:
+This example demonstrates practical usage where `.indexOf()` helps maintain inventory integrity by preventing duplicate entries and quickly checking product availability. The functions show how to combine `.indexOf()` with conditional logic for business operations.
 
-```js
-const colors = ['blue', 'orange', 'pink', 'yellow', 'teal'];
-const checkBlue = colors.indexOf('blue', 1);
+## Codebyte Example: User Permission System
+
+This example illustrates using `.indexOf()` with a starting position parameter to implement a user permission system with role hierarchy:
 
-console.log(checkBlue);
-// Output: -1
+```codebyte/javascript
+const userRoles = ["guest", "user", "moderator", "admin", "guest", "user"];
+const permissions = ["read", "write", "delete", "manage"];
+
+// Function to find user role starting from a specific position
+function findUserRole(role, startPosition = 0) {
+  const index = userRoles.indexOf(role, startPosition);
+  return index !== -1 ? index : "Role not found";
+}
+
+// Function to check permissions based on role hierarchy
+function checkPermission(userRole, action) {
+  const roleHierarchy = ["guest", "user", "moderator", "admin"];
+  const actionHierarchy = ["read", "write", "delete", "manage"];
+
+  const roleLevel = roleHierarchy.indexOf(userRole);
+  const actionLevel = actionHierarchy.indexOf(action);
+
+  if (roleLevel === -1 || actionLevel === -1) {
+    return "Invalid role or action";
+  }
+
+  return roleLevel >= actionLevel ? "Permission granted" : "Permission denied";
+}
+
+// Find all occurrences of "user" role
+let position = 0;
+const userPositions = [];
+while (position < userRoles.length) {
+  const foundIndex = userRoles.indexOf("user", position);
+  if (foundIndex === -1) break;
+  userPositions.push(foundIndex);
+  position = foundIndex + 1;
+}
+
+console.log("User role positions:", userPositions);
+console.log(checkPermission("moderator", "write"));
+console.log(checkPermission("guest", "delete"));
 ```
 
-## Codebyte Example
+The above example demonstrates advanced usage of the `fromIndex` parameter by finding multiple occurrences of a value and implementing a role-based permission system. The while loop demonstrates how to find all instances of an element by repeatedly calling `.indexOf()` with updated starting positions.
 
-Multiple matches will only return the first index where a match occurs:
+## Frequently Asked Questions
 
-```codebyte/javascript
-const repeatGreeting = ['hello world', 'hello world'];
-const firstGreeting = repeatGreeting.indexOf('hello world');
+### 1. What happens when `.indexOf()` searches for objects or arrays?
 
-console.log(firstGreeting);
-```
+The `.indexOf()` method uses strict equality comparison, which only matches the same object reference, not objects with identical content. For content-based matching of objects, use `findIndex()` with a custom comparison function.
+
+### 2. Can `.indexOf()` work with negative starting positions?
+
+Yes, negative values for the `fromIndex` parameter count backwards from the end of the array. However, the search still proceeds from left to right. If the calculated starting position is negative, the search begins from index 0.
+
+### 3. How does `.indexOf()` handle undefined and null values?
+
+The method can find `undefined` and `null` values in arrays using strict equality. It will return the index position of these values if they exist, or -1 if they don't.
+
+### 4. Is `.indexOf()` case-sensitive when searching strings?
+
+Yes, `.indexOf()` performs case-sensitive matching for string elements. "Apple" and "apple" are treated as different values.
+
+### 5. What is the difference between `.indexOf()` and `.lastIndexOf()`?
+
+Both `.indexOf()` and `.lastIndexOf()` search for a specified element in an array, but they differ in the direction of the search:
+
+- `.indexOf()` searches the array from left to right (starting from the first index).
+- `.lastIndexOf()` searches the array from right to left (starting from the last index).