defining array in javascript

Question

I want to create an array in javascript and remember two ways of doing it so I just want to know what the fundamental differences are and if there is a performance difference in these two "styles"

var array_1 = new Array("fee","fie","foo","fum");
var array_2 = ['a','b','c'];

for (let i=0; i<array_1.length; i++){
  console.log(array_1[i])
}

for (let i=0; i<array_2.length; i++){
  console.log(array_2[i])
}

Answer 1

They do the same thing. Advantages to the [] notation are:

• It's shorter.
• If someone does something silly like redefine the Array symbol, it still works.
• There's no ambiguity when you only define a single entry, whereas when you write new Array(3), if you're used to seeing entries listed in the constructor, you could easily misread that to mean [3], when in fact it creates a new array with a length of 3 and no entries.
• It may be a tiny little bit faster (depending on JavaScript implementation), because when you say new Array, the interpreter has to go look up the Array symbol, which means traversing all entries in the scope chain until it gets to the global object and finds it, whereas with [] it doesn't need to do that. The odds of that having any tangible real-world impact in normal use cases are low. Still, though...

So there are several good reasons to use [].

Advantages to new Array:

• You can set the initial length of the array, e.g., var a = new Array(3);

I haven't had any reason to do that in several years (not since learning that arrays aren't really arrays and there's no point trying to pre-allocate them). And if you really want to, you can always do this:

var a = [];
a.length = 3;