Callback Functions in JavaScript

1// Callback: Function passed as argument to another function
2
3// Example 1: Simple callback
4function greet(name, callback) {
5    console.log("Hello, " + name);
6    callback();
7}
8
9greet("John", function() {
10    console.log("Callback executed!");
11});
12
13// Example 2: Callback with parameters
14function processArray(arr, callback) {
15    let result = [];
16    for (let item of arr) {
17        result.push(callback(item));
18    }
19    return result;
20}
21
22let numbers = [1, 2, 3, 4, 5];
23let doubled = processArray(numbers, function(num) {
24    return num * 2;
25});
26
27console.log("\nDoubled:", doubled);
28
29// Example 3: Using arrow functions
30let squared = processArray(numbers, num => num ** 2);
31console.log("Squared:", squared);
32
33// Example 4: setTimeout callback
34console.log("\nStart");
35setTimeout(function() {
36    console.log("This runs after 1 second");
37}, 1000);
38console.log("End");
39
40// Example 5: Array methods with callbacks
41let arr = [1, 2, 3, 4, 5];
42
43// forEach
44arr.forEach(function(num) {
45    console.log("Number:", num);
46});
47
48// map
49let doubled2 = arr.map(num => num * 2);
50console.log("\nMapped:", doubled2);
51
52// filter
53let evens = arr.filter(num => num % 2 === 0);
54console.log("Filtered:", evens);
55
56// Example 6: Custom callback with error handling
57function fetchData(successCallback, errorCallback) {
58    let success = true; // Simulate API call
59    
60    if (success) {
61        successCallback({ data: "Success!" });
62    } else {
63        errorCallback("Error occurred");
64    }
65}
66
67fetchData(
68    function(data) {
69        console.log("\nSuccess:", data);
70    },
71    function(error) {
72        console.log("Error:", error);
73    }
74);