JavaScript: Check Whether an Object Is an Array

#1 Using instanceof operator #
Example: var a = [0, 1, 1, 2], // array b = "string", // string c = 9, // number d = a instanceof Array, e = b instanceof Array, f = c instanceof Array; console.log(d); // Output: true console.log(e); // Output: false console.log(f); // Output: false

Additional

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Checking object #
This operator can also check if the input is an object, by using Object built-in constructor. As such: var the_object = {"something": 0, "something_else": 1000}, check = the_object instanceof Object; console.log(check); // Output: true

Or, we can use typeof method.
For example: var the_object = {"something": 0, "something_else": 1000}, check = typeof the_object === "object"; // standard form // previously: check = typeof(the_object) === "object"; console.log(check); // Output: true

typeof will return "object" (string) keyword for either object or array type input. Therefore, this operator can't be used for checking "array" input.

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Checking function #
We can check whether an object is a function: var the_function = function () { /*things*/ }, check = the_function instanceof Function; console.log(check); // Output: true

Or, using typeof operator.
For example: var the_function = function () { /*things*/ }, check = typeof the_function === "function"; console.log(check); // Output: true

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Checking string and number #
We can use typeof for those.
As such: var a = 156.23, b = "string", check_num = typeof a === "number", check_str = typeof b === "string"; console.log(check_num); // Output: true console.log(check_str); // Output: true

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Checking element #
We can use instanceof for that.
Example: var a = document.getElementById("some_elm"), b = document.querySelectorAll("div")[0], check_elm_id = a instanceof Element; check_elm_nl = b instanceof Element; console.log(check_elm_id); // Output: true (if there's such element on DOM) console.log(check_elm_nl); // Output: true (if there's such element on DOM)

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#2 Using Array.isArray(the_array) method #
var a = [0, 1, 1, 2], b = "string", // string c = 9, // number d = Array.isArray(a), e = Array.isArray(b), f = Array.isArray(c); console.log(d); // Output: true console.log(e); // Output: false console.log(f); // Output: false

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In conclusion #
There are 2 ways we can choose to check the "array"-ness.

1. the_array instanceof Array
2. Array.isArray(the_array) (supported on IE9+ and other current browsers)

For instanceof operator, we can create a custom constructor for a particular object we wanna test. More documentation at MDN