npm i ud


Utilities for updating code live with hot module replacement

by Chris Cowan

3.3.1 (see all)License:MITTypeScript:Built-In
npm i ud


Node.js CI GitHub license npm version

Ud is a small set of utilities for updating code live with hot module replacement, as supported by Browserify-HMR and Webpack.

These functions let you accomplish common tasks easily without needing to use the Hot Module Replacement API directly.

When the module.hot API is not available, all of the functions act as simple pass-throughs.


All of ud's functions require a reference to your local module object to be passed in, and take an optional key. Each of the functions can only be used once per module with a given key.

ud.defonce(module, function, key?)

On the first run, the function will be called, and its return value will be returned. On future reloads, the function will not be called again, and instead its first return value will be returned again. You can use this to define values once that must be persisted across reloads.

ud.defobj(module, object, key?)

On the first run, the object will be returned. On a reload, the original object will be updated to have all of the values of the newest object and then will be returned.

ud.defn(module, function, key?)

A wrapper around the function will be returned which calls the given function. On a reload, the wrapper will be updated so that it calls the most recent version of the function.

The prototype of the function will be updated too, so you can pass a class constructor to defn and have its methods be kept up to date.


var _ = require('lodash');
var ud = require('ud');

var shared = ud.defonce(module, _.constant({counter: 0}));

var inc = ud.defn(module, function() {
  shared.counter += 1;
  console.log('counter', shared.counter);

// Function still can be updated even if you export it.
module.exports = inc;

The inc function may be updated and will work as expected. If ud.defonce were not used to define the counter object, then each new reload would create a brand new counter. If ud.defn were not used to define the inc function, then the previously exported function that other modules may have local copies of would not be updated.

No-op / Production Builds

For non-HMR builds such as typical production builds, all of ud's functions will work correctly as they normally do on the first run. However, ud's code and its dependencies (mostly babel-included polyfills; it adds up to ~50kb, though if your bundle is already using these same dependencies, such as if you or your other dependencies are already also using babel-runtime or babel-polyfill in the bundle, then ud isn't necessarily bringing as much into the bundle!) may be dead weight that can be safely removed. You can swap out ud for a simpler no-op implementation by configuring your build process to use the "ud/noop" module in place of "ud". Here's an example of doing this with Browserify via the CLI:

browserify -r ud/noop:ud main.js > bundle.js


Both TypeScript and Flow type definitions for this module are included! The type definitions won't require any configuration to use.

See Also

  • ud-kefir, a companion library which integrates with Kefir and emits events when reloads happen.
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