Relation of Garbage Collector and Closure in JavaScript

A Garbage Collector (GC) is a part of the JavaScript engine in the browser that automatically manages memory. Unlike languages like C or C++, where developers need to manually allocate and deallocate memory, JavaScript handles it for you. The garbage collector removes memory that is no longer being used, helping to avoid memory issues.

How Garbage Collection Works in JavaScript

• Automatic Memory Management: JavaScript automatically manages memory allocation and deallocation.
• Reachability: Variables and objects are checked for reachability to determine if they are still in use.
• Mark-and-Sweep: The garbage collector marks reachable objects and removes unreachable ones.
• Generational Collection: New objects are collected quickly, while older objects are collected less often.
• Memory Leaks: Closures can prevent garbage collection of unused variables, causing memory leaks.
• Smart Garbage Collection: Modern engines optimize collection for better performance.
• No Manual Management: Developers don’t need to manually allocate or deallocate memory.

Garbage Collection and Closures

A closure is when a function remembers the variables from its outer function even after the outer function has finished running. Closures are useful, but they can also cause problems with garbage collection.

Now let's understand this with the help of example:

<script>
    function a() {
        var x = 10;
    
        return function b() {
            console.log(x);
        }
    }
    
    var y = a();
    y();
</script>

Output:

10

In this example

• Function a creates the variable x and returns another function b that uses x.
• Normally, when a finishes, x would be removed from memory. But because b still uses x, it keeps x in memory.
• This is where the Garbage Collector doesn't remove x because it is still needed by the closure b.

Smart Garbage Collection

Modern JavaScript engines, like Chrome's V8, are smarter about garbage collection. Let's understand this with the help of example:

<script>
    function a() {
        var x = 10,
            z = 92;
    
        return function b() {
            console.log(x);
        }
    }
    
    var y = a();
    y();
    
    console.log(z);
</script>

Output:

10
Uncaught ReferenceError: z is not defined

In this example

• Both x and z are inside function a. However, only x is used in the closure b.
• Because z is not used, the Garbage Collector removes it from memory after a finishes.
• When you try to log z, it throws an error because z has already been removed from memory.

Conclusion

Garbage Collection in JavaScript helps manage memory by automatically freeing up unused variables. Closures can prevent the garbage collector from removing variables, leading to memory leaks. However, modern engines like V8 are smart enough to optimize memory usage by collecting unused variables more effectively. Understanding this process can help improve performance and avoid memory issues in your JavaScript programs.