Assorted

typeof bar === "object" to determine if bar is an object

typeof bar === "object" is a reliable way of checking if bar is an object

But:null is also considered an object!

Therefore:

var bar = null;
console.log(typeof bar === "object");  // logs true!

Solution: also check if bar is null:

console.log((bar !== null) && (typeof bar === "object")); 
// logs false

There are two other things worth noting:

First, the above solution will return false if bar is a function. In most cases, this is the desired behavior, but in situations where you want to also return true for functions, you could amend the above solution to be:

console.log((bar !== null) && ((typeof bar === "object") || (typeof bar === "function")));

Second, the above solution will return true if bar is an array (e.g., if var bar = [];). In most cases, this is the desired behavior, since arrays are indeed objects, but in situations where you want to also false for arrays, you could amend the above solution to be:

console.log((bar !== null) && (typeof bar === "object") && (toString.call(bar) !== "[object Array]"));

However, there’s one other alternative that returns false for nulls, arrays, and functions, but true for objects:

console.log((bar !== null) && (bar.constructor === Object));

Or, if you’re using jQuery:

console.log((bar !== null) && (typeof bar === "object") && (! $.isArray(bar)));

ES6 makes the array case quite simple, including its own null check with Array.isArray():

console.log(Array.isArray(bar));

NaN

Definition

Represents a value that is not a number.

Results from an operation that could not be performed.

Gotchas

1. Although NaN means “not a number”, its type is, believe it or not, Number:

console.log(typeof NaN === "number");  // logs "true"

2. NaN compared to anything – even itself! – is false:

console.log(NaN === NaN);  // logs "false"

Testing whether a number is equal to NaN

built-in function isNaN() (imperfect solution)

value !== value only true if the value is NaN

More reliable solution: ES6 Number.isNaN() function