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+```markdown
+# Functions and Higher-Order Functions (HOF)
+
+In JavaScript, functions are blocks of code that can be executed multiple times. They can accept parameters and return values. Here's a basic function example:
+
+```javascript
+function sum(a, b) {
+  if (a > b) {
+    return a + b;
+  } else {
+    return a - b;
+  }
+}
+
+console.log(sum(15, 10)); // Output: 25
+console.log(sum(4, 7));   // Output: -3
+```
+
+## Callback Functions
+
+Callback functions are functions passed as arguments to another function to be executed later. For instance:
+
+```javascript
+function abc() {
+  setInterval(() => {
+    console.log("Hello");
+  }, 1000);
+}
+
+abc(); // Calls abc function, which prints "Hello" every second
+```
+
+## Higher-Order Functions (HOF)
+
+Higher-order functions are functions that can accept other functions as arguments or return functions as output. Some common examples are `forEach`, `map`, `filter`, `reduce`, and `sort`.
+
+### forEach
+
+```javascript
+let arr = [9, 8, 7, 6, 5, 4, 3, 2, 1];
+let sum = 0;
+
+arr.forEach((ele) => {
+  sum += ele;
+});
+
+console.log(sum); // Output: 45
+```
+
+### Map
+
+```javascript
+let arr = [9, 8, 7, 6, 5, 4, 3, 2, 1];
+let res = arr.map((ele) => {
+  return ele * 3;
+});
+
+console.log(res); // Output: [27, 24, 21, 18, 15, 12, 9, 6, 3]
+```
+
+### Filter
+
+```javascript
+let arr = [9, 8, 7, 6, 5, 4, 3, 2, 1];
+let res = arr.filter((ele) => {
+  return ele % 2 === 0;
+});
+
+console.log(res); // Output: [8, 6, 4, 2]
+```
+
+### Reduce
+
+```javascript
+let arr = [9, 8, 7, 6, 5, 4, 3, 2, 1];
+let sum = arr.reduce((acc, ele) => {
+  return acc + ele;
+}, 0);
+
+console.log(sum); // Output: 45
+```
+
+### Chaining
+
+```javascript
+let arr = [9, 8, 7, 6, 5, 4, 3, 2, 1];
+let res = arr.map((ele) => {
+  return ele * 3;
+}).filter((ele) => {
+  return ele % 2 === 0;
+}).reduce((acc, ele) => {
+  return acc + ele;
+}, 0);
+
+console.log(res); // Output: 72
+```
+
+### Sort
+
+```javascript
+let arr = [9, 8, 7, 6, 5, 4, 3, 29, 10];
+arr.sort((a, b) => {
+  return a - b;
+});
+
+console.log(arr); // Output: [3, 4, 5, 6, 7, 8, 9, 10, 29]
+```
+
+These higher-order functions are powerful tools in JavaScript for working with arrays in a functional and declarative manner.