eval.js

A JavaScript interpreter written in JavaScript.

Why?

You might be working in a JavaScript environment where eval() isn't allowed (and you have a genuinely good reason why you want to use it). Maybe this'll slip under the radar. You could also extend this to make it execute ES6 code in an ES5 environment. PRs welcome!

How?

Most of the heavy lifting is done by acorn, a JavaScript parser written in JavaScript. eval.js converts the AST it generates into JavaScript function closures, which when run execute the whole program.

It's also possible to use eval.js with esprima.

Command line interface

This npm package comes with a REPL which allows you to experiment with it. It's easy to install and use:

marten@procyon:~/git/evaljs$ npm install -g evaljs
marten@procyon:~/git/evaljs$ evaljs
> 1 + 1
2
> new Error('Hello World!')
[Error: Hello World!]
> throw new Error('Hello World!')
Error: Hello World!
    at newWithArgs (/home/marten/git/evaljs/index.js:255:10)
    at /home/marten/git/evaljs/index.js:249:12
    at Array.0 (/home/marten/git/evaljs/index.js:581:11)
    at /home/marten/git/evaljs/index.js:466:31
    at REPLServer.repl.start.eval (/home/marten/git/evaljs/bin/evaljs:12:34)
    at repl.js:249:20
    at REPLServer.repl.start.eval (/home/marten/git/evaljs/bin/evaljs:14:7)
    at Interface.<anonymous> (repl.js:239:12)
    at Interface.EventEmitter.emit (events.js:95:17)
    at Interface._onLine (readline.js:202:10)
> marten@procyon:~/git/evaljs$

API

• evaljs.evaluate(code) A drop in alternative for window.eval().

• new evaljs.Environment([scopesOrGlobalObject]) Generates a new JS Environment to 'run' code in. The argument can be one of the following:

  • a global object
  • nothing (in this case, '{}' is used as the global object)
  • a list of objects. The first will be the global object, others will be other scopes loaded into the interpreter. Kind of like wrapping the code in a with statement for each further object in the array. This is handy for emulating Node.js (for passing in require(), exports, and module.)

  A JS Environment has the following properties:

  • env.gen(node): Takes either the result of acorn's parse() method (an AST), or a JS string containing source code. This AST/code will be converted into a function that, when run, executes the AST/code passed in and returns the result.
  • env.DEBUG: When set to true, evaljs will write debug information to stdout.

Size?

16.3kB min+gzip

License?

ISC

Is it complete?

No labeled statements; no nice error handling (although there is a DEBUG option). There are probably bugs. That said, it can run itself and acorn, so its supported subset of JS is usable. PRs containing improvements welcome!

How slow is it?

Not sure. I only tested with small snippets so far in Node.js, for which the speed difference isn't notable. But it's probably slow.

Who?

eval.js is written by Marten de Vries. Maintained by Jason Huggins. Credits for the original idea go to closure-interpreter.