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This module enables code hotswapping in node.js
Branch: master


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hotswap.js disable "persistent" flag in so node can exit naturally
package.json 1.1.0

Basic usage

Just write this in your main file (or in the repl):


And then add this line to modules you want to be reloaded (it tells node-hotswap to watch this file):

module.change_code = 1;

Now if you change your modules, they will be reloaded automatically.

Demo (you have to wait more than 1 second between writings - it's file system limitations):

var fs = require('fs');
var hotswap = require('hotswap');

// writing the initial version of test.js
fs.writeFileSync("hotswap-test.js", "module.exports.version = 0; module.change_code = true;");

// requiring it
var test = require('./hotswap-test')

hotswap.on('swap', function() {
        // we are going to console.log(test) whenever it's changed

setTimeout(function() {
        fs.writeFile("hotswap-test.js", "module.exports.version = 1; module.change_code = true;")
}, 1000);

setTimeout(function() {
        fs.writeFile("hotswap-test.js", "module.exports.hi_there = function(){}; module.change_code = true;")
}, 2000);

setTimeout(function() {
        fs.writeFile("hotswap-test.js", "module.exports = {wow: 'thats working'}; module.change_code = true;")
}, 3000);

/* outputs:
 * { version: 0 }
 * { version: 1 }
 * { hi_there: [Function] }
 * { wow: 'thats working' }

What does it do?

This module overrides default functions in require.extension and do some magic there. It remembers all references to exports objects of modules with function module.change_code defined. When module is changed, contents of it's old exports object is replaced with contents of the new one.

// So, this will work fine, m will be changed:
var m = require('hot-swapping-module');

// but "hotswap" have to way to replace m.func with new value
// so dont do this unless you really want to use old code, 
var m.func = require('hot-swapping-module').func;


require('hotswap') returns an EventEmitter that emits the following events:

  • change - when one of the watched modules has changed
  • swap - after successful replacing an old module with a new version
  • error - if there was a filesystem error or something like that

Local variables

When old module is replaced by the new one, local variables of the old module will be lost. If you want to save them, you can use module.change_code.

If module.change_code is defined as a function it will be called before module is reloaded. So you can write up something like that:

module.cache = {} // it's important data you want to save

module.change_code = function(oldmod, newmod, exports_object) {
  newmod.cache = oldmod.cache;


You can configure node-hotswap using configure function.

    // a list of extensions registered by hotswap
    // you can define your own extension for such files if you are afraid 
    // that this module would mess up with some other modules (shouldn't 
    // happen though)
    // default: {'.js': 'js', '.coffee': 'coffee'}
    extensions: {'.js': ['js', 'jsx'], '.coffee': 'coffee'},

    // enable or disable on hotswapping files
    // default: true
    watch: true,

    // automatically reload files on change
    // default: true
    autoreload: true,


I should really write up some good documentation and examples here -_-

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