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Understanding Garbage Collection in JavaScript and Beyond

Susan Sarandon
Release: 2025-01-27 22:32:17
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Understanding Garbage Collection in JavaScript and Beyond

Recently, I was asked in a technical interview how different programming languages ​​handle garbage collection. This was a surprising yet refreshing question, and it really piqued my interest - I'd never encountered such an in-depth discussion of memory management in an interview before. I like this question and would like to explore this topic further in a blog post.


Efficient memory management is critical for high-performance applications. Garbage Collection (GC) Ensures automatic recycling of unused memory to prevent memory leaks and crashes. In this article, we'll focus on how garbage collection works in JavaScript, explore other methods used in programming languages, and provide examples to illustrate these concepts.


What is garbage collection?

Garbage collection is the process of reclaiming memory occupied by objects that are no longer used. Languages ​​with automatic garbage collection abstract this process so developers don't have to manually manage memory. For example, JavaScript uses a tracking garbage collector, while other languages ​​use different techniques.


Garbage collection in JavaScript

JavaScript relies on the tracking garbage collection method, specifically the mark-sweep algorithm. Let’s break it down:

1. Mark-clear algorithm

This algorithm determines which objects in memory are "reachable" and frees those that are unreachable:

  1. Mark stage:
    • Start from a "root" object (for example, window in a browser or the global object in Node.js).
    • Iterate through all objects reachable from these root objects and mark them as "alive".
  2. Clear Phase:
    • Scan the heap and release objects that are not marked as reachable.

Example:

function example() {
  let obj = { key: "value" }; // obj 可达
  let anotherObj = obj; // anotherObj 引用 obj

  anotherObj = null; // 引用计数减少
  obj = null; // 引用计数减少到 0
  // obj 现在不可达,将被垃圾回收
}
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2. Generational garbage collection

Modern JavaScript engines (such as V8 in Chrome/Node.js) use Generational GC to optimize garbage collection. Memory is divided into:

  • New Generation: Short-lived objects (such as function scope variables) are stored here and collected frequently.
  • Old generation: Long-lived objects (such as global variables) are stored here and collected less frequently.

Why is generational GC more efficient?

  • Most objects in JavaScript are ephemeral and can be collected quickly.
  • Long-lived objects are moved to the old generation, reducing the need for frequent scans.

Other garbage collection strategies

Let’s explore how other languages ​​handle garbage collection:

<.> 1. Reference counting

How many references to the reference counting are directed to an object. When the reference count is reduced to 0, the object will be released.

<点> Advantages:

Simple and immediately recycled memory.
  • Before predicting.
  • <缺> Disadvantages:

<循> Circular reference

: If the two objects are referenced with each other, their counts will never reach 0.
  • <示> Example: (python reference count)

<.> 2. Manual memory management

<<> C
function example() {
  let obj = { key: "value" }; // obj 可达
  let anotherObj = obj; // anotherObj 引用 obj

  anotherObj = null; // 引用计数减少
  obj = null; // 引用计数减少到 0
  // obj 现在不可达,将被垃圾回收
}
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and

C Language requires developers to express and release memory.

<示> Example:

(C memory management) <点> Advantages:

Completely control the use of memory.

a = []
b = []
a.append(b)
b.append(a)
# 这些对象相互引用,但不可达;现代 Python 的循环收集器可以处理这种情况。
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<缺> Disadvantages:

Memory leaks (forgot to release memory) and suspended pointers (prematurely release memory).
  • <.> 3. Tracking garbage recycling with a circular collector

Some languages ​​(such as Python) combine the reference count

and
    cycle detection
  • to treat the cycle reference.

The cycle collector scan the object regularly to detect the cycle (the mutual reference object group that cannot be accessed from the root object). Once the cycle is found, the collector will destroy it and recycle memory. The cycle collector solves the biggest disadvantage (cycle reference) of pure reference counting. They increase additional expenses, but make sure they do not cause memory leaks due to cycle.

<.> 4. Rust's borrowing inspection device (no GC) Rust uses a different method,

completely avoiding garbage recycling
    . On the contrary, RUST uses <> borrowing the inspection device
  • to enforce strict ownership rules:
  • <权> 🎜
: There is only one owner at a time at a time.

<借> Borrow : You can borrow reference (uncharacteristic or variable), but only allow only one variable reference to prevent

data competition . <命> The life cycle

: When the compiler's inference value exceeds the scope of the scope and automatically releases the memory.
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