CODEWITHSUNDEEP
javascriptarrays
user3164317
user3164317

Reputation: 195

Why set arrayLength in "new Array(arrayLength)"?

For example:

array1 = new Array(5); array2 = new Array(10);

Both console.log(array1) and console.log(array2) would return []. then, what is the role of arrayLength here?

Upvotes: 3

Views: 134

Answers (2)

Thriggle
Thriggle

Reputation: 7059

Passing a number to the Array constructor sets the length property of the array without setting the indices of the items (which is why your console.log isn't showing anything).

To quote JavaScript Garden:

Being able to set the length of the array in advance is only useful in a few cases, like repeating a string, in which it avoids the use of a loop.

Here's an example of doing just that:

new Array(count + 1).join(stringToRepeat);

Upvotes: 1

StriplingWarrior
StriplingWarrior

Reputation: 156504

JavaScript hides a lot of the details that you'd typically have to deal with when working with arrays in many other languages. For example, an array in JavaScript can grow automatically as you push values to it.

However, under the covers, the runtime is still dealing with the same sort of memory allocation issues that languages like C or Java make visible to the developer. The runtime may set aside a little extra memory for the array to grow into, and then once it runs out of contiguous memory space, it'll reallocate a larger piece of memory somewhere else and copy all of the values from the first set of memory to another location.

(This is vastly oversimplifying things, but hopefully you get the general idea.)

If you know ahead of time exactly how many items you can expect to put into the array, using new Array(number) will give the runtime a hint that it can begin by allocating that much memory, and avoid the need for the memory to be reallocated and copied around as you grow it.

It's in this light, for example, that this page suggests the following practices to achieve maximum performance in the V8 javascript engine:

  • Don't pre-allocate large Arrays (e.g. > 64K elements) to their maximum size, instead grow as you go
  • Initialize using array literals for small fixed-sized arrays
  • Preallocate small arrays (<64k) to correct size before using them

Upvotes: 4

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