Throws error on invalid usage of delete. The delete operator (used to remove properties from objects) cannot be used on non-configurable properties of the object. Non-strict code will fail silently when an attempt is made to delete a non-configurable property, whereas strict mode will throw an error in such a case.
in which case, it's useful?
Disallows duplicate property names or parameter values.
Prevents accidental globals. Without strict mode, assigning a value to an undeclared variable automatically creates a global variable with that name. This is one of the most common errors in JavaScript. In strict mode, attempting to do so throws an error.
this is most important one.
A client-side Javascript SDK for authenticating with OAuth2 (and OAuth1 with a oauth proxy) web services and querying their REST API's. HelloJS standardizes paths and responses to common API's like Google Data Services, Facebook Graph and Windows Live Connect. It's modular, so that list is growing. No more spaghetti code!
javascript facebook client
javascript web applications and elasticsearch
Good article on using ElasticSearch with JS
Developers these days are spoiled with choice when it comes to selecting an MV* framework for structuring and organizing their JavaScript web apps.
Cool project to help choose a framework, by building an app that uses each framework.
Rich JavaScript Applications – the Seven Frameworks (Throne of JS, 2012)
Good summary of JS frameworks