vime, a faster vm module

Synopsis

Execute JS code in a new VM context.

Motivation

The built-in vm module is flexible but slow. Code sometimes runs 200x slower inside a vm context than outside. Vime sacrifices some flexibility in exchange for fast and reliable performance.

Usage

The module exports a single function.

const vime = require('vime');
const result = vime('1 + 1');
console.log(result);  // Prints "2".

To execute more code in the same vime context, first grab a reference to the global object (or any other object from the context):

const vime = require('vime');
const context = vime('this');
vime('var x = 1', context);
console.log(vime('x + x', context));  // Prints "2".

Exporting functions from the main context is done in similar fashion:

const vime = require('vime');
const context = Object.assign(vime('this'), { console, require });
vime('console.log(require("./package.json"))');

It follows that vime() is not restricted to running JS code in a new context, you can also execute it in the current context:

const vime = require('vime');
const context = global;  // Or any object from this context.
vime('console.log("ok")', context);  // Prints "ok".

Caveats

instanceof checks don't work across contexts because each context gets its own copy of Object, Array, and other built-ins.

const vime = require('vime');
const array = vime('[]');
console.log(Array.isArray(array));  // Prints "true".
console.log(array instanceof Array);  // Prints "false".

References to objects from a context keep the context alive until the last reference goes away. Because contexts are heavy-weight objects (a context is a complete JS runtime), keeping many contexts around has a notable impact on memory usage:

const vime = require('vime');
console.log(process.memoryUsage().rss >>> 20);  // 25 MB on my machine.
const leak = [];
for (let i = 0; i < 100; ++i) leak.push(vime('this'));
console.log(process.memoryUsage().rss >>> 20);  // 100 MB on my machine.

Use node-heapdump or the built-in inspector to debug such issues.

Important note: this module is currently not designed for executing untrusted code ("sandboxing.") If that is an important use case for you, please file a bug report.

Reporting bugs

Report bugs at https://github.com/bnoordhuis/vime/issues. Please check existing issues first. If possible, include a test case demonstrating the bug.

UNIX users, please include the output of node -v and uname -a.

Windows users, please include the output of node -v and winver.

When reporting build errors, include the full terminal output from npm install vime on down and the output of g++ -v or clang++ -v (if you are a UNIX user) or the version of Visual Studio or Build Tools (if you are a Windows user.)

License

ISC. See the LICENSE file in the top-level directory.