Understanding JavaScript Memory Management

Memory Management in JavaScript

In most cases there is no need for you to have the thorough knowledge about how memory management works if you code in JavaScript.  Since after all, the JavaScript engine is wired to take care of that for you.

Nevertheless, sooner or later you might face certain issues – for instance, memory leaks. To be able to fix it quickly you should understand exactly how memory allocation operates.

This article describes the basics of memory management, explains how to prevent the most common types of memory leaks and presents the garbage collector solution.

The Lifecycle of Memory in JavaScript

In JavaScript, when we script functions, variables, and others the engine makes up some memory space for them and frees it once that memory is no longer necessary.

Thus, memory allocation could be defined as the process of “stocking” an area of memory, while freeing it is the return of some space to the system, so that previously occupied space can be reused for other purpose.

When you declare a variable or create a function, the required memory unit goes through the following steps:

Memory assignment. JavaScript handles this task: it assigns the memory needed for the object we built.

Memory application. This is what we have to script in the code: here, reading and writing to memory is came as reading and writing to a variable.

Releasing memory. This step is also completed by the JavaScript engine: right after letting go of particular memory space, you can re-apply it for other objects.

When we mention “objects”, it does not only address JavaScript objects, but also functions with their scopes.

Stack and Heap Allocation Options

Now that you know that the JavaScript engine assigns memory for everything that we include in the code, and frees it when this memory is no longer required. You may ask a reasonable question: where is everything kept exactly?

In JavaScript, there are two options for stockpiling data: on the stack and on the heap. Both are names of data structures that are applied by the engine for different means.


A stack is a data structure that is set to reserve static data, or the data which size is known at compile time. In JavaScript, this data contains primitive values such as string, number, boolean, undefined, and null; as well as references to functions and objects.

Since the size of the data does not change, the engine allocates only a fixed amount of memory for each value.

Such process of assigning memory right before execution is called static.

Because the engine shares a fixed amount of memory for such values, it is possible to assume that there is a limit on the size of primitive values. And depending on the browser used, these limits, as well as the maximum allowed size of the entire stack, may differ.


A memory heap is set to stock objects and functions data .

Unlike the above stack, the engine does not know how much space will be enough for certain objects, and therefore assigns memory as needed.

This particular allocation pattern is called dynamic.

Memory in JavaScript

Utilizing allocated memory in JavaScript typically means reading and scripting it.

This can be achieved by reading or scripting the value of a variable or an object body, or possibly even by forwarding an argument to a function.

Releasing Memory Space

Most issues with memory management take place at the stage of releasing memory units. The difficult part here is concluding when the assigned memory is no longer needed by the program. Generally, this takes a developer who can seek where in the program a certain piece of memory is no longer necessary and get rid of it.

High-level languages by default have a subsystem called the garbage collector. The main aim of this subsystem is to track all the memory operations and find out when a piece of allocated memory is no longer required. Once it finds it, the garbage collector automatically releases this component.

However, this procedure is not all perfect, since the typical issue of searching whether a certain memory item is needed or not cannot simply be solved through an algorithm.

Majority of garbage collectors operate by collecting memory that cannot be accessed, that is, one that all variables pointing to are out of reach. At the same time, there is always a possibility of freeing memory, since at any time a certain memory area may have variables pointing to it in a certain scope, even though that memory area have never been used in the program.


This article briefly discusses the key points of memory management in JavaScript, particularly focusing on the process of releasing it, the so-called garbage collection. It is at this phase only that potential issues might occur, which are communicated in the fact that memory taken by unnecessary components cannot be cleared. Therefore, despite the automated work of the garbage collector, the developer should understand that he also holds a certain responsibility for the efficient use of memory.


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