Produce boolean value from other types
All objects in JavaScript can be converted to boolean implicitly, look at these examples:

0 == false; // true
1 == true; // true
'' == false // true
null == false // true

but these values are not the type of boolean.
Therefore if we compare empty string with false, the result is false:

0 === false; // false
1 === true; // false
'' === false // false
null === false // false

Now, the problem is how to generate boolean values from other types:

!!0 === false; // true
!!1 === true; // true
!!'' === false // true
!!null === false // true

pretty cool!

Initialize default value
JavaScript doesn’t have the overload feature of other object-oriented languages.
But the arguments of JavaScript function are optional, missed arguments are passed in with the value of undefined:

function plus(base, added) {
    return base + added;
}
plus(2); // NaN

In this example, plus(2) is the same as plus(2, undefined), the result of 2 + undefined is NaN.

Now the question is how to set the default value of the argument added:

function plus(base, added) {
    added = added || 1;
    return base + added;
}
plus(2); // 3
plus(2, 2); // 4

Prevent your page from loading in frames
If your site become famous, other people or feed aggregator may embed your page in there site with their interface. This most likely to steal data.

Now it’s time to prevent this action:

if(top !== window) {
	top.location.href = window.location.href;
}

This piece of JavaScript code should be put in the head section of all your pages.

Replace string
The String.prototype.replace method always suprise somebody who are familar with C# or Java.
Consider this code for example:

'Hello world, hello world'.replace('world', 'JavaScript');
// The result is "Hello JavaScript, hello world"

The first parameter of replace is a Regular Expression.
If it’s presented as a string, only the first found string will be replaced.

To solve this problem, we must use Regular Expression in JavaScript like this:

'Hello world, hello world'.replace(/world/g, 'JavaScript');
// The result is "Hello JavaScript, hello JavaScript"

Also, we can specify to ignore case when replace string:

'Hello world, hello world'.replace(/hello/gi, 'Hi');
// The result is "Hi world, Hi world"

Transform the arguments object into an array
Arguments variable that exist in function is a built-in, array-like object.
It’s weird that the arguments has length properties, but doen’t support slice, push, sort, etc.

Now it’s the show time to convert this fake array to a real array:

function args() {return [].slice.call(arguments, 0);}args(2, 5, 8); 
// [2, 5, 8]