Determining if a Variable is an Object in JavaScript – JavaScript type checking
In JavaScript, checking the type of a variable to determine if it's an object is a common task but it comes with certain pitfalls. Using typeof can sometimes lead to unexpected results.
Common Mistake with typeof
The typeof operator can misleadingly suggest that null is an object:
var bar = null;
console.log(typeof bar === "object"); // logs true!
This happens because, historically, null was considered a type of object due to a bug in JavaScript's initial implementation. Even today, this behavior persists for compatibility reasons.
Correctly Checking for Objects
To avoid the above pitfall, it's crucial to check not only that typeof returns "object" but also that the variable is not null:
var bar = null;
console.log(bar !== null && typeof bar === "object"); // logs false
Considering Functions and Arrays
Functions in JavaScript are objects but not in the typical sense. If you also want to consider functions as objects:
var bar = function() ;
console.log(bar !== null && (typeof bar === "object" || typeof bar === "function")); // logs true
Arrays are also technically objects. If you need to specifically exclude arrays when checking for objects:
var bar = [];
console.log(bar !== null && typeof bar === "object" && !Array.isArray(bar)); // logs false
More Precise Checks with Constructors
For a more robust check, especially to ensure the variable is exactly an object (not any subclass or other types), you can check the constructor:
var bar = ;
console.log(bar !== null && bar.constructor === Object); // logs true
This method will return true only if bar is explicitly an instance of Object.
Using jQuery for Checking
If you're using jQuery, it provides utilities to simplify these checks, like $.isArray() for array detection:
var bar = [];
console.log(bar !== null && typeof bar === "object" && !$.isArray(bar)); // logs false
Simplified Array Checking in ES5
Lastly, ES5 introduced Array.isArray(), which simplifies array checking and includes its own null checks:
var bar = [];
console.log(Array.isArray(bar)); // logs true
Conclusion
By understanding the nuances of JavaScript's type system and utilizing appropriate checks, you can avoid common mistakes and ensure your type checks are accurate and meaningful. This approach helps in writing more robust and maintainable JavaScript code.
