Question

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What are the best practices to follow when declaring an array in Javascript?

Would be correct don’t use new Array after of defining a variable using []?

var v = [];
v = Array(5); // is new necessary?

Answer 1

You are creating two Array objects and throwing away one, so what you are doing is not necessary.

Declare the variable and create an array:

var v;
v = new Array(5);

Or as a single statement:

var v = new Array(5);

The preferred way of creating arrays is using new Array when you want to specify a size, and an array literal when you want to create an array from items:

var a1 = new Array(42); // creates an array with 42 undefined items
var a2 = [1, 2, 3]; // creates an array with the 3 items

For a zero size array you can use either:

var a1 = new Array(); // creates an empty array
var a2 = []; // creates an empty array

The Array constructor can also take a list of items, but that use should be avoided because it behaves differently if there is one item and that item is numerical:

var a1 = new Array(100, 101, 102); // creates an array with three items
var a2 = new Array(100, 101); // creates an array with two items
var a3 = new Array(100); // creates an array with 100 undefined items