How do you check if a variable is an array in JavaScript?
This question already has an answer here:
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:
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!
Array.isArray(variable)
This last one is, in my opinion the ugliest, and it is one of the slowest. Running about 1/5 the speed as the first example. 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
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/33 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.
You could also use:
if (value instanceof Array) {
alert('value is Array!');
} else {
alert('Not an array');
}
This seems to me a pretty elegant solution, but to each his own.
Edit:
As of ES5 there is now also:
Array.isArray(value);
But this will break on older browsers, unless you are using polyfills (basically... IE8 or similar).
I noticed someone mentioned jQuery, but I didn't know there was an isArray() function. It turns out it was added in version 1.3.
jQuery implements it as Peter suggests:
isArray: function( obj ) {
return toString.call(obj) === "[object Array]";
},
Having put a lot of faith in jQuery already (especially their techniques for cross-browser compatibility) I will either upgrade to version 1.3 and use their function (providing that upgrading doesn't cause too many problems) or use this suggested method directly in my code.
Many thanks for the suggestions.
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