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Memory Lifecycle

Memory Lifecycle

Introduction

Every piece of memory in JavaScript goes through three stages: allocate, use, and release.

The Three Stages

flowchart LR
    A["1. Allocate Memory\nVariables, objects, arrays"]
    B["2. Use Memory\nRead/write operations"]
    C["3. Release Memory\nGarbage collection"]

    A --> B
    B --> C
    C --> A

Stage 1: Allocation

JavaScript automatically allocates memory when you create values:

// Allocating primitives on the stack
const name = 'Alice';    // string
const age = 30;          // number
const active = true;     // boolean

// Allocating objects on the heap
const user = {           // object
  name: 'Alice',
  skills: ['JS', 'React'] // array (also on heap)
};

// Allocating functions (on the heap)
function greet() {
  return 'Hello!';
}

Stage 2: Usage

When you read or write variables, you’re using allocated memory:

const user = { name: 'Alice', age: 30 };

// Reading (using the memory)
console.log(user.name); // 'Alice'

// Writing (modifying the memory)
user.age = 31;
user.skills = ['JS'];

Stage 3: Release

Memory is released when it’s no longer reachable. The garbage collector handles this automatically:

function process() {
  const temp = { data: 'temporary' };
  // temp is allocated and used here
  return 42;
  // After the function returns, temp is unreachable
  // Its memory can be released
}

// Global references persist
const persistent = { data: 'stays in memory' };

Reachability

An object is “reachable” if it can be accessed from the roots (global object, current function’s local variables, call stack):

let user = { name: 'Alice' };      // ✅ reachable (via 'user')
user = null;                         // ❌ unreachable → eligible for GC

// Longer chain
let data = { value: 42 };           // ✅ reachable
let reference = data;               // ✅ reachable (via 'reference' too)
reference = null;                    // ✅ still reachable (via 'data')
data = null;                         // ❌ unreachable → eligible for GC

Best Practices

Release references when done: obj = null
Avoid unnecessary global variables
Clear arrays/lists when no longer needed
Remove event listeners when unmounting

Further Reading