Javascript Application Isolation

From Google's v8 documentation...

In V8, a context is an execution environment that allows separate, unrelated, JavaScript applications to run in a single instance of V8. You must explicitly specify the context in which you want any JavaScript code to be run.

Why is this necessary? Because JavaScript provides a set of built-in utility functions and objects that can be changed by JavaScript code. For example, if two entirely unrelated JavaScript functions both changed the global object in the same way then unexpected results are fairly likely to happen.

In terms of CPU time and memory, it might seem an expensive operation to create a new execution context given the number of built-in objects that must be built. However, V8's extensive caching ensures that, while the first context you create is somewhat expensive, subsequent contexts are much cheaper....

With the V8 snapshot feature (activated with build option snapshot=yes, which is the default) the time spent creating the first context will be highly optimized as a snapshot includes a serialized heap which contains already compiled code for the built-in JavaScript code. Along with garbage collection, V8's extensive caching is also key to V8's performance, for more information see V8 Design Elements.

When you have created a context you can enter and exit it any number of times. While you are in context A you can also enter a different context, B, which means that you replace A as the current context with B. When you exit B then A is restored as the current context. This is illustrated below:

Note that the built-in utility functions and objects of each context are kept separate. You can optionally set a security token when you create a context. See the Security Model section for more information.

The motivation for using contexts in V8 was so that each window and iframe in a browser can have its own fresh JavaScript environment.

Thank you.