How do I create an array if it does not exist yet? In other words how to default a variable to an empty array?
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8 Answers
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 : []
6 Comments
James
This works in most cases, but it won't cover you if you plan on checking an array instantiated from a different global
Array constructor - this might occur when scripting across frames.
Rich
That is true. How would one handle that case?
Christian C. Salvadó
@Rich: See my answer here: stackoverflow.com/questions/1961528/…, it will behave correctly on multi-framed DOM environments.
mynameistechno
The correct way to do it nowadays is to use
Array.isArray(x): stackoverflow.com/a/38670091/271442
Claudiu
It doesn't work when it is null. So a complete check would be
if (Array.isArray(array) && array.length) You can find more here stackoverflow.com/a/24403771/364951 |
var arr = arr || [];
5 Comments
Gumbo
But this doesn’t check if arr is an array.
Brian Campbell
You're right. It wasn't clear from the question if checking that it was an array in advance is necessary, or just checking to see if the variable is already defined. This is a common idiom, and is shorter and simpler than the other ones posted, so I figured I'd post it and let the questioner decide if it's sufficient.
James
If
arr is already defined in the relevant scope, then why bother with var ...? You can just do arr = arr || [];Brian Campbell
The question was to check whether the array already exists, and create it if not. Thus, you don't know if it's already defined. This idiom is useful if you have several files to be linked together, and don't know which one of them will be loaded first to define some global variable that they will all share.
whitfin
@BrianCampbell I might be wrong but unless that's being run in global scope, wouldn't
var be hoisted and therefore it'd always just become []? Edit: nvm, I am wrong :Dconst 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
2 Comments
huysentruitw
Throws
x is not defined in both casesmynameistechno
x is the variable that could be an array or not. You define that. This is just example code ...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 = [];
}
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 : [];
answered Dec 25, 2009 at 18:24
Christian C. Salvadó
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2 Comments
Justin Johnson
@kangax Declaration isn't necessary in this context.
Christian C. Salvadó
The declaration is necessary, but I thought it was pretty obvious that if
arr is undeclared a ReferenceError will be thrown in the right-hand side of the assignment...<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>
Comments
The nullish coalescing assignment operator could be a more modern approach to consider.
let a = [1, 2, 3]
let b
a ??= []
b ??= []
console.log(a) // [1, 2, 3]
console.log(b) // []
Comments
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 = [];
}
