Closures in JavaScript

1// Closure: Function has access to outer function's variables
2
3// Example 1: Basic closure
4function outerFunction(x) {
5    // Outer function's variable
6    let outerVar = x;
7    
8    // Inner function (closure)
9    function innerFunction(y) {
10        console.log("Outer:", outerVar, "Inner:", y);
11        return outerVar + y;
12    }
13    
14    return innerFunction;
15}
16
17let closure = outerFunction(10);
18console.log("Result:", closure(5));
19
20// Example 2: Counter using closure
21function createCounter() {
22    let count = 0;
23    
24    return function() {
25        count++;
26        return count;
27    };
28}
29
30let counter1 = createCounter();
31let counter2 = createCounter();
32
33console.log("\nCounter 1:", counter1());
34console.log("Counter 1:", counter1());
35console.log("Counter 2:", counter2());
36console.log("Counter 1:", counter1());
37
38// Example 3: Private variables
39function createBankAccount(initialBalance) {
40    let balance = initialBalance; // Private variable
41    
42    return {
43        deposit: function(amount) {
44            balance += amount;
45            return balance;
46        },
47        withdraw: function(amount) {
48            if (amount <= balance) {
49                balance -= amount;
50                return balance;
51            } else {
52                return "Insufficient funds";
53            }
54        },
55        getBalance: function() {
56            return balance;
57        }
58    };
59}
60
61let account = createBankAccount(100);
62console.log("\nInitial balance:", account.getBalance());
63console.log("After deposit 50:", account.deposit(50));
64console.log("After withdraw 30:", account.withdraw(30));
65console.log("Final balance:", account.getBalance());
66
67// Example 4: Function factory
68function createMultiplier(multiplier) {
69    return function(number) {
70        return number * multiplier;
71    };
72}
73
74let double = createMultiplier(2);
75let triple = createMultiplier(3);
76
77console.log("\nDouble 5:", double(5));
78console.log("Triple 5:", triple(5));
79
80// Example 5: Closure with loop (common mistake)
81console.log("\nClosure in loop:");
82for (var i = 0; i < 3; i++) {
83    setTimeout(function() {
84        console.log("var i:", i); // Prints 3, 3, 3
85    }, 100);
86}
87
88// Solution: Use let
89for (let j = 0; j < 3; j++) {
90    setTimeout(function() {
91        console.log("let j:", j); // Prints 0, 1, 2
92    }, 200);
93}