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(); }
});