What’s the difference between “Array()” and “[]” while declaring a JavaScript array?

Question

What\'s the real difference between declaring an array like this:

var myArray = new Array();

and

var myArray = [];
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Answer 1

Well, var x = new Array() is different than var x = [] is different in some features I'll just explain the most useful two (in my opinion) of them.

Before I get into expalining the differences, I will set a base first; when we use x = [] defines a new variable with data type of Array, and it inherits all the methods that belong to the array prototype, something pretty similar (but not exactly) to extending a class. However, when we use x = new Array() it initilizes a clone of the array prototype assigned to the variable x.

Now let's see what are the difference

The First Difference is that using new Array(x) where x is an integer, initilizes an array of x undefined values, for example new Array(16) will initialize an array with 16 items all of them are undefined. This is very useful when you asynchronously fill an array of a predefined length. For example (again :) ) let's say you are getting the results of 100 competitiors, and you're receiving them asynchronously from a remote system or db, then you'll need to allocate them in the array according to the rank once you receive each result. In this very rare case you will do something like myArray[result.rank - 1] = result.name, so the rank 1 will be set to the index 0 and so on.

The second difference is that using new Array() as you already know, instanciates a whole new clone of the array prototype and assigns it to your variable, that allows you to do some magic (not recommended btw). This magic is that you can overwrite a specific method of the legacy array methods. So, for example you can set the Array.push method to push the new value to the beginning of the array instead of the end, and you can also add new methods (this is better) to this specific clone of the Array Prototype. That will allow you to define more complex types of arrays throughout your project with your own added methods and use it as a class.

Last thing, if you're from the very few people (that I truly love) that care about processing overhead and memory consumption of your app, you'd never tough new Array() without being desperate to use it :).

I hope that has explained enough about the beast new Array() :)