How to Detect If a Variable Is an Array

How do I check if a variable is an array in JavaScript?

There are several ways of checking if an variable is an array or not. The best solution is the one you have chosen.

variable.constructor === Array

This is the fastest method on Chrome, and most likely all other browsers. All arrays are objects, so checking the constructor property is a fast process for JavaScript engines.

If you are having issues with finding out if an objects property is an array, you must first check if the property is there.

variable.prop && variable.prop.constructor === Array

Some other ways are:

Array.isArray(variable)

Update May 23, 2019 using Chrome 75, shout out to @AnduAndrici for having me revisit this with his question
This last one is, in my opinion the ugliest, and it is one of the slowest fastest. Running about 1/5 the speed as the first example. This guy is about 2-5% slower, but it's pretty hard to tell. Solid to use! Quite impressed by the outcome. Array.prototype, is actually an array. you can read more about it here https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray

variable instanceof Array

This method runs about 1/3 the speed as the first example. Still pretty solid, looks cleaner, if you're all about pretty code and not so much on performance. Note that checking for numbers does not work as variable instanceof Number always returns false. Update: instanceof now goes 2/3 the speed!

So yet another update

Object.prototype.toString.call(variable) === '[object Array]';

This guy is the slowest for trying to check for an Array. However, this is a one stop shop for any type you're looking for. However, since you're looking for an array, just use the fastest method above.

Also, I ran some test: http://jsperf.com/instanceof-array-vs-array-isarray/35 So have some fun and check it out.

Note: @EscapeNetscape has created another test as jsperf.com is down. http://jsben.ch/#/QgYAV I wanted to make sure the original link stay for whenever jsperf comes back online.

How to detect if a variable is an array

Type checking of objects in JS is done via instanceof, ie

obj instanceof Array

This won't work if the object is passed across frame boundaries as each frame has its own Array object. You can work around this by checking the internal [[Class]] property of the object. To get it, use Object.prototype.toString() (this is guaranteed to work by ECMA-262):

Object.prototype.toString.call(obj) === '[object Array]'

Both methods will only work for actual arrays and not array-like objects like the arguments object or node lists. As all array-like objects must have a numeric length property, I'd check for these like this:

typeof obj !== 'undefined' && obj !== null && typeof obj.length === 'number'

Please note that strings will pass this check, which might lead to problems as IE doesn't allow access to a string's characters by index. Therefore, you might want to change typeof obj !== 'undefined' to typeof obj === 'object' to exclude primitives and host objects with types distinct from 'object' alltogether. This will still let string objects pass, which would have to be excluded manually.

In most cases, what you actually want to know is whether you can iterate over the object via numeric indices. Therefore, it might be a good idea to check if the object has a property named 0 instead, which can be done via one of these checks:

typeof obj[0] !== 'undefined' // false negative for `obj[0] = undefined`
obj.hasOwnProperty('0') // exclude array-likes with inherited entries
'0' in Object(obj) // include array-likes with inherited entries

The cast to object is necessary to work correctly for array-like primitives (ie strings).

Here's the code for robust checks for JS arrays:

function isArray(obj) {
    return Object.prototype.toString.call(obj) === '[object Array]';
}

and iterable (ie non-empty) array-like objects:

function isNonEmptyArrayLike(obj) {
    try { // don't bother with `typeof` - just access `length` and `catch`
        return obj.length > 0 && '0' in Object(obj);
    }
    catch(e) {
        return false;
    }
}

How to check if a variable is an array?

In GNU Awk, there's an answer, but the recommended approach depends on what version you are running.

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From GNU Awk 4.2, released in October 2017, there is a new function typeof() to check this, as indicated in the release notes from the beta release:

15. The new typeof() function can be used to indicate if a variable or array element is an array, regexp, string or number. The isarray() function is deprecated in favor of typeof().

So now you can say:

$ awk 'BEGIN { a[1] = "a"; print typeof(a) }'
array

And perform the check as follows:

$ awk 'BEGIN { a = "a"; if (typeof(a) == "array") print "yes" }'    
$ awk 'BEGIN { a[1] = "a"; if (typeof(a) == "array") print "yes" }'
yes

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In older versions, you can use isarray():

$ awk 'BEGIN { a = "a"; if (isarray(a)) print "yes" }'
$ awk 'BEGIN { a[1] = "a"; if (isarray(a)) print "yes" }'
yes

From the man page:

isarray(x)
    Return true if x is an array, false otherwise.

How to check if variable is an object or an array?

This would help.

var a = [], b = {};
console.log(Object.prototype.toString.call(a).indexOf("Array")>-1);console.log(Object.prototype.toString.call(b).indexOf("Object")>-1);
console.log(a.constructor.name == "Array");console.log(b.constructor.name == "Object");

How can I check if an object is an array?

In modern browsers you can do:

Array.isArray(obj)

(Supported by Chrome 5, Firefox 4.0, Internet Explorer 9, Opera 10.5 and Safari 5)

For backward compatibility you can add the following:

// Only implement if no native implementation is available
if (typeof Array.isArray === 'undefined') {
  Array.isArray = function(obj) {
    return Object.prototype.toString.call(obj) === '[object Array]';
  }
};

If you use jQuery you can use jQuery.isArray(obj) or $.isArray(obj). If you use Underscore.js you can use _.isArray(obj).

If you don't need to detect arrays created in different frames you can also just use instanceof:

obj instanceof Array

python: how to identify if a variable is an array or a scalar

>>> import collections.abc
>>> isinstance([0, 10, 20, 30], collections.abc.Sequence)
True
>>> isinstance(50, collections.abc.Sequence)
False

note: isinstance also supports a tuple of classes, check type(x) in (..., ...) should be avoided and is unnecessary.

You may also wanna check not isinstance(x, (str, unicode))

As noted by @2080 and also here this won't work for numpy arrays. eg.

>>> import collections.abc
>>> import numpy as np
>>> isinstance((1, 2, 3), collections.abc.Sequence)
True
>>> isinstance(np.array([1, 2, 3]), collections.abc.Sequence)
False

In which case you may try the answer from @jpaddison3:

>>> hasattr(np.array([1, 2, 3]), "__len__")
True
>>> hasattr([1, 2, 3], "__len__")
True
>>> hasattr((1, 2, 3), "__len__")
True

However as noted here, this is not perfect either, and will incorrectly (at least according to me) classify dictionaries as sequences whereas isinstance with collections.abc.Sequence classifies correctly:

>>> hasattr({"a": 1}, "__len__")
True
>>> from numpy.distutils.misc_util import is_sequence
>>> is_sequence({"a": 1})
True
>>> isinstance({"a": 1}, collections.abc.Sequence)
False

You could customise your solution to something like this, add more types to isinstance depending on your needs:

>>> isinstance(np.array([1, 2, 3]), (collections.abc.Sequence, np.ndarray))
True
>>> isinstance([1, 2, 3], (collections.abc.Sequence, np.ndarray))
True

How do I check if variable is an array?

According to this wiki page, you can use this command:

declare -p variable-name 2> /dev/null | grep -q '^declare \-a'

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