Access logs from console.log() in Node.js vm module

Question

In node.js vm module, i can execute some javascript in another node.js process. What i want to achieve is being able to catch logs performed in the script runned by the vm.

For example i want to catch the "foobar" log:

var vm = require('vm')

vm.runInThisContext('console.log("foobar");', 'myfile.vm');

// how can i get the "foobar" log?

Thanks in advance

EDIT: The answer below works well, although there's a shorter version:

function captureStdout(callback) {
    var output = '', old_write = process.stdout.write

    // start capture
    process.stdout.write = function(str, encoding, fd) {
        output += str
    }

    callback()

    // end capture
    process.stdout.write = old_write

    return output
}

Answer 1

You can just wrap console.log directly:

function hook_consolelog(callback) {
    var old_log = console.log;

    console.log = (function(write) {
        return function() {
            write.apply(console, arguments)
            callback.apply(null, arguments);
        }
    })(console.log)

    return function() {
        console.log = old_log;
    }
}

var result;
var unhook = hook_consolelog(function(v) {
    result = v;
});

console.log('hello');
unhook();
console.log('goodbye');
console.log('the result is ', result);​

Since console.log simply calls process.stdout, another approach would be to capture the stdout events using a bit of wrapper magic like this:

var util = require('util')

function hook_stdout(callback) {
    var old_write = process.stdout.write

    process.stdout.write = (function(write) {
        return function(string, encoding, fd) {
            write.apply(process.stdout, arguments)
            callback(string, encoding, fd)
        }
    })(process.stdout.write)

    return function() {
        process.stdout.write = old_write
    }
}

var unhook = hook_stdout(function(string, encoding, fd) {
    util.debug('stdout: ' + util.inspect(string))
    if( string == 'foobar' ) { unhook(); }
});

Answer 2

Maybe it's better so just forward the current console to the newly created context object:

let contextObj = {
    console: console
};

Answer 3

I found another, IMHO simpler solution: we just need to overwrite the console.log() function inside the VM context to use the function from the outer context.

import { Script, createContext } from 'vm';

let contextObj = {
    console: {
        log: (...args) => {
            console.log(...args);
        }
    }
};

const vmContext = createContext(contextObj);
const script = new Script("console.log('hello from the other side');");
script.runInContext(vmContext);

Hope it helps anyone looking for a stupidly simple solution!