JavaScript doesn’t have block scope
Block doesn’t have scope in javascript, only function has scope.

for(var i = 0; i < 2; i ++) { } i;  // 2

If you want to create scope, use anonymous function:

(function (){ for(var i = 0; i < 2; i ++) { } })(); typeof(i) === 'undefined';  // true

NaN is weird
NaN indentifies a value which is not a number.
It’s weird because comparing NaN with anything (even NaN itself) is always false.

NaN === NaN; // false

Therefore, the following code may confuse somebody:

parseInt('hello', 10); // NaN parseInt('hello', 10) == NaN; // false parseInt('hello', 10) === NaN; // false

Then, how to check whether a number is NaN or not?
A built-in function can help you, check it:

isNaN(parseInt('hello', 10)); // true

Truthy and falsy values
All values in javascript can be converted into boolean implied.
Note that the following values can be converted into false ones automatically, which called falsy values:
null, undefined, NaN, 0, ‘’, false
Therefore you don’t need to do like this:

if(obj === undefined || obj === null) { }

Simply do as follows:

if(!obj) { }

How to modify arguments
In this post , I give a method to convert arguments into a real array.
Then, how to modify arguments?
For example, add a new value to the end of the arguments:

function add() { arguments.push('new value'); } add(); // error - arguments.push is not a function

Of course there would be an error, because arguments is not a real array.
Following is the solution:

function add() { Array.prototype.push.call(arguments, 'new value'); return arguments; } add()[0]; // "new value"

Boolean and new Boolean
You’d better consider Boolean as a constructor function. It can be used to produce a Boolean literal value:

Boolean(false) === false; // true Boolean('') === false; // true

Boolean(0) is equal to !!0, which was mentioned in this post.
Also, we can use new keyword to produce a boolean object:

new Boolean(false) === false; // false new Boolean(false) == false; // true typeof(new Boolean(false)); // "object" typeof(Boolean(false)); // "boolean"

This is interesting.