contextify

Contextify

Turn an object into a V8 execution context. A contextified object acts as the global 'this' when executing scripts in its context. Contextify adds 3 methods to the contextified object: run(code, filename), getGlobal(), and dispose(). The main difference between Contextify and Node's vm methods is that Contextify allows asynchronous functions to continue executing in the Contextified object's context. See vm vs. Contextify below for more discussion.

Examples

var Contextify = require('contextify');
var sandbox = { console : console, prop1 : 'prop1'};
Contextify(sandbox);
sandbox.run('console.log(prop1);');
sandbox.dispose(); // free the resources allocated for the context.

var sandbox = Contextify(); // returns an empty contextified object.
sandbox.run('var x = 3;');
console.log(sandbox.x); // prints 3
sandbox.dispose();

var sandbox = Contextify({setTimeout : setTimeout});
sandbox.run("setTimeout(function () { x = 3; }, 5);");
console.log(sandbox.x); // prints undefined
setTimeout(function () {
    console.log(sandbox.x); // prints 3
    sandbox.dispose();
}, 10);

Details

Contextify([sandbox])

sandbox - The object to contextify, which will be modified as described below
          If no sandbox is specified, an empty object will be allocated and used instead.

Returns the contextified object.  It doesn't make a copy, so if you already have a reference
to the sandbox, you don't need to catch the return value.

A Contextified object has 2 methods added to it:

run(code, [filename])

code - string containing JavaScript to execute
filename  - an optional filename for debugging.

Runs the code in the Contextified object's context.

getGlobal()

Returns the actual global object for the V8 context. The global object is initialized with interceptors (discussed below) which forward accesses on it to the contextified object. This means the contextified object acts like the global object in most cases. Sometimes, though, you need to make a reference to the actual global object.

For example:

var window = Contextify({console : console});
window.window = window;
window.run("console.log(window === this);");
// prints false.

var window = Contextify({console : console});
window.window = window.getGlobal();
window.run("console.log(window === this);");
// prints true

The global object returned by getGlobal() can be treated like the contextified sandbox object, except that defining getters/setters will not work on it. Define getters and setters on the actual sandbox object instead.

dispose()

Frees the memory allocated for the underlying V8 context. If you don't call this when you're done, the V8 context memory will leak, as will the sandbox memory, since the context's global stores a strong reference to the sandbox object. You can still use your sandbox object after calling dispose(), but it's unsafe to use a global previously returned from getGlobal(). run, getGlobal, and dispose will be removed from the sandbox object.

Install

npm install contextify

require('vm') vs. Contextify

Node's vm functions (runInContext etc) work by copying the values from the sandbox object onto a context's global object, executing the passed in script, then copying the results back. This means that scripts that create asynchronous functions (using mechanisms like setTimeout) won't have see the results of executing those functions, since the copying in/out only occurs during an explicit call to runInContext and friends.

Contextify creates a V8 context, and uses interceptors (see: http://code.google.com/apis/v8/embed.html#interceptors) to forward global object accesses to the sandbox object. This means there is no copying in or out, so asynchronous functions have the expected effect on the sandbox object.

Tests

Testing is done with nodeunit. Run the tests with

nodeunit test/

Output:

OK: 92 assertions (27ms)

Building

node-waf configure build

Acknowledgments

Inspiration taken from Assaf's Zombie.js context solution: https://github.com/assaf/zombie