Memory Management

Memory life cycle

Regardless of the programming language, the memory life cycle is pretty much always the same:

1. Allocate the memory you need

2. Use the allocated memory (read, write)

3. Release the allocated memory when it is not needed anymore

The second part is explicit in all languages. The first and last parts are explicit in low-level languages but are mostly implicit in high-level languages like JavaScript.

Allocation in JavaScript

Value initialization

In order to not bother the programmer with allocations, JavaScript will automatically allocate memory when values are initially declared.

var n = 123; // allocates memory for a number
var s = 'azerty'; // allocates memory for a string

var o = {
  a: 1,
  b: null
}; // allocates memory for an object and contained values

// (like object) allocates memory for the array and
// contained values
var a = [1, null, 'abra'];

function f(a) {
  return a + 2;
} // allocates a function (which is a callable object)

// function expressions also allocate an object
someElement.addEventListener('click', function() {
  someElement.style.backgroundColor = 'blue';
}, false);

Release when the memory is not needed anymore (What is garbage collector)

Some high-level languages, such as JavaScript, utilize a form of automatic memory management known as garbage collection (GC). The purpose of a garbage collector is to monitor memory allocation and determine when a block of allocated memory is no longer needed and reclaim it. This automatic process is an approximation since the general problem of determining whether or not a specific piece of memory is still needed is undecidable.

The garbage collector, or just collector, attempts to reclaim garbage, or memory occupied by objects that are no longer in use by the program. Garbage

Garbage collection

As stated above, the general problem of automatically finding whether some memory "is not needed anymore" is undecidable. As a consequence, garbage collectors implement a restriction of a solution to the general problem. This section will explain the concepts that are necessary for understanding the main garbage collection algorithms and their respective limitations.

Limitation: Circular references

There is a limitation when it comes to circular references. In the following example, two objects are created with properties that reference one another, thus creating a cycle. They will go out of scope after the function call has completed. At that point they become unneeded and their allocated memory should be reclaimed. However, the reference-counting algorithm will not consider them reclaimable since each of the two objects has at least one reference pointing to them, resulting in neither of them being marked for garbage collection. Circular references are a common cause of memory leaks.

function f() {
  var x = {};
  var y = {};
  x.a = y;        // x references y
  y.a = x;        // y references x

  return 'azerty';
}

f();

Real-life example

Internet Explorer 6 and 7 are known to have reference-counting garbage collectors for DOM objects. Cycles are a common mistake that can generate memory leaks:

var div;
window.onload = function() {
  div = document.getElementById('myDivElement');
  div.circularReference = div;
  div.lotsOfData = new Array(10000).join('*');
};

In the above example, the DOM element "myDivElement" has a circular reference to itself in the "circularReference" property. If the property is not explicitly removed or nulled, a reference-counting garbage collector will always have at least one reference intact and will keep the DOM element in memory even if it was removed from the DOM tree. If the DOM element holds a large amount of data (illustrated in the above example with the "lotsOfData" property), the memory consumed by this data will never be released and can lead to memory related issues such as the browser becoming increasingly slower.