vm.js is a pretty impressive fully
functional ECMAScript virtual machine that can be used from any
ECMAScript3-compatible environment. Eventually it will provide a
complete ECMAScript 6 environment (for now only some features are
supported).
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npm install vm.js |
It’s written in CoffeeScript and has plenty of tests. The most
impressive thing is that the tests themselves run inside vm.js.
Here are some possible use cases:
- Simple in-process javascript sandbox
- Async-to-sync API adapter using fibers (lightweight in-process threads, not node.js fibers)
- Use new ECMAScript features in very old browsers
The main API can be accessed through vm instances. Each vm is indirectly
associated with a global object (through a “realm”) and is isolated from
other vms.
Start by creating a new instance:
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> Vm = require('vm.js')> vm = new Vm() |
Evaluate simple expressions:
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> vm.eval('40 + 2')42> vm.eval('[a, b, c] = [1, 2, 3]') // Harmony destructuring assignment[1, 2, 3]> vm.realm.global.a1> vm.realm.global.b2> vm.realm.global.c3 |
Compile programs and run later
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// pass filename as second argument for stack traces/debugging> script = Vm.compile('2 + 2', 'sum.js')> vm.run(script)4 |
Compiled scripts can be serialized/deserialized to/from JSON-friendly
structures:
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> scriptObj = script.toJSON()> serializedScript = JSON.stringify(scriptObj)> deserializedScript = Vm.fromJSON(JSON.parse(serializedScript))> vm.run(deserializedScript) |
Expose objects to be used by code running inside the Vm
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> vm.realm.global.factorial = function factorial(n) { return n > 1 ? factorial(n - 1) * n : 1 }> vm.eval('factorial(5)')120 |
The inverse also works:
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> vm.eval('function factorial(n) { return n > 1 ? factorial(n - 1) * n : 1 }')> vm.realm.global.factorial(5)120 |
Return values asynchronously using fiber pause/resume:
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// created a paused fiber from compiled codefiber = vm.createFiber(Vm.compile('user = null; user = fetchAsync("/users/1");'))vm.realm.global.fetchAsync = function(url) { fiber.pause() // pause execution $.get(url, function(data) { // user === null fiber.setReturnValue(data) fiber.resume() // user === data });}// start fiberfiber.run() |
There are plenty of examples on the
github page.
