How to create an array if an array does not exist yet?
JavascriptJavascript Problem Overview
How do I create an array if it does not exist yet? In other words how to default a variable to an empty array?
Javascript Solutions
Solution 1 - Javascript
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 : []
Solution 2 - Javascript
var arr = arr || [];
Solution 3 - Javascript
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
Solution 4 - Javascript
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 = [];
}
Solution 5 - Javascript
If you want to check if the object is already an Array, to avoid the well known issues of the instanceof operator when working in multi-framed DOM environments, you could use the Object.prototype.toString method:
arr = Object.prototype.toString.call(arr) == "[object Array]" ? arr : [];
Solution 6 - Javascript
<script type="text/javascript">
array1 = new Array('apple','mango','banana');
var storearray1 =array1;
if (window[storearray1] && window[storearray1] instanceof Array) {
alert("exist!");
} else {
alert('not find: storearray1 = ' + typeof storearray1)
}
</script>
Solution 7 - Javascript
If you are talking about a browser environment then all global variables are members of the window object. So to check:
if (window.somearray !== undefined) {
somearray = [];
}