How to create an array if an array does not exist yet?

Question

How do I create an array if it does not exist yet? In other words how to default a variable to an empty array?

Answer 1

If you want to check whether an array x exists and create it if it doesn't, you can do

x = ( typeof x != 'undefined' && x instanceof Array ) ? x : []

Answer 2

var arr = arr || [];

Answer 3

const list = Array.isArray(x) ? x : [x];

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray

Or if x could be an array and you want to make sure it is one:

const list = [].concat(x);

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat

Answer 4

You can use the typeof operator to test for undefined and the instanceof operator to test if it’s an instance of Array:

if (typeof arr == "undefined" || !(arr instanceof Array)) {
    var arr = [];
}