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JavaScript provides a variety of data structures to be used ranging from simple objects over arrays, sets, maps, the weak variants as well as ArrayBuffers.

Over the half past year I found myself in the spot to recreate some of the more common structures like Dequeues, count maps and mostly different variants of trees.

While looking at the Ecma specification I could not find a description on how arrays implemented on a memory level, supposedly this is up to the underlying engine?

Contrary to languages I am used to, arrays in JavaScript have a variable length, similar to list. Does that mean that elements are not necessarily aligned next to each other in memory? Does a splice push and pop actually result in new allocation if a certain threshold is reached, similar to for example ArrayLists in Java? I am wondering if arrays are the way to go for queues and stacks or if actual list implementations with references to the next element might be suited in JavaScript in some cases (e.g. regarding overhead opposed to the native implementation of arrays?).

If someone has some more in-depth literature, please feel encouraged to link them here.

Peter Mortensen
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Kilian
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  • Also, depending on your app requirements, you may be digging deep into a rabbit hole when you worry about performance and your own data structure implementations w/o actual profile testing. The most common usages of Javascript are not low level operation handling, so w/o profile testing, you wouldn't know if you're getting any actual benefit from it – Christopher Francisco Jul 17 '20 at 18:07

1 Answers1

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While looking at the Ecma specification I could not find a description on how arrays implemented on a memory level, supposedly this is up to the underlying engine?

The ECMAScript specification does not specify or require a specific implementation. That is up to the engine that implements the array to decide how best to store the data.

Arrays in the V8 engine have multiple forms based on how the array is being used. A sequential array with no holes that contains only one data type is highly optimized into something similar to an array in C++. But, if it contains mixed types or if it contains holes (blocks of the array with no value - often called a sparse array), it would have an entirely different implementation structure. And, as you can imagine it may be dynamically changed from one implementation type to another if the data in the array changes to make it incompatible with its current optimized form.

Since arrays have indexed, random access, they are not implemented as linked lists internally which don't have an efficient way to do random, indexed access.

Growing an array may require reallocating a larger block of memory and copying the existing array into it. Calling something like .splice() to remove items will have to copy portions of the array down to the lower position.

Whether or not it makes more sense to use your own linked list implementation for a queue instead of an array depends upon a bunch of things. If the queue gets very large, then it may be faster to deal with the individual allocations of a list so avoid having to copy large portions of the queue around in order to manipulate it. If the queue never gets very large, then the overhead of a moving data in an array is small and the extra complication of a linked list and the extra allocations involved in it may not be worth it.

As an extreme example, if you had a very large FIFO queue, it would not be particularly optimal as an array because you'd be adding items at one end and removing items from the other end which would require copying the entire array down to insert or remove an item from the bottom end and if the length changed regularly, the engine would probably regularly have to reallocate too. Whether or not that copying overhead was relevant in your app or not would need to be tested with an actual performance test to see if it was worth doing something about.

But, if your queue was always entirely the same data type and never had any holes in it, then V8 can optimize it to a C++ style block of memory and when calling .splice() on that to remove an item can be highly optimized (using CPU block move instructions) which can be very, very fast. So, you'd really have to test to decide if it was worth trying to further optimize beyond an array.

Here's a very good talk on how V8 stores and optimizes arrays:

Elements Kinds in V8

Here are some other reference articles on the topic:

How do JavaScript arrays work under the hood

V8 array source code

Performance tips in V8

How does V8 optimize large arrays

Peter Mortensen
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jfriend00
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