Webpack plugin with loader that join sources by predefined method

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0.1.27 years ago7 years agoMinified + gzip package size for join-webpack-plugin in KB


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join plugin for webpack

Webpack plugin with loader that join sources by predefined method
This is webpack plugin produces single asset for set of files or multiple assets with grouping technique. The set of files may be splited to groups of set of files that produce group of assets.
The method of joining is defined by specified functions.
  • deep webpack integration
  • possibility to group files by simple criterion
  • autorebuild and reload on source file change (due to deep integration)
  • files may be loaded and joined by path pattern or by call function
require or import
This join plugin is enough flexible as it allows to predefine data join method. With that it is more general and requires to specify functions which produce joining.
Consider to use derivative plugins instead of this:
when it's necessary to merge json data (or data loaded from another formats with structured data like yaml for example).
build internationalization locale assets according to component internationalization approach.


npm install --save-dev join-webpack-plugin

Webpack configuration

This is minimal configuration to merge json into single asset:
var JoinPlugin = require("join-webpack-plugin");
const merge = require("merge");
module.exports = {
  module: {
    rules: [
        test: /\.(json)$/i,
        use: [
          // some preloaders
  plugins: [
    new JoinPlugin({
      search: './src/**/*.json',
      join: function(common, addition) {
        return merge.recursive(
          common ? common : {}, JSON.parse(addition)
      save: function(common) {
        return JSON.stringify(common);

The using this plugin directly in webpack configuration can lead to code duplication. A good solution in that case may be creation and sharing a new useful plugin. A good starting point in this case is to do fork of merge-webpack-plugin
and take it as a base.


var url1 = require("one-of-files.ext");
var url2 = require("another-file.ext");
// or describe files by pattern in plugin param

// url1 and url2 will be same name refers to same file
// which will also contain content of "third-file.ext"

Same in modern syntax:
import url1 from "one-of-files.ext"
import url2 from "another-file.ext"
import "third-file.ext"
// or describe files by pattern in plugin param

This returns public url of file with result of joining. This will be same url for each file joined together according to plugin configuration.
In order to involve files into join, files must be required by require function or configured by search param of plugin configuration.

Plugin configuration

JoinPlugin is created typically in webpack configuration file and waits hash of configuration options as its create param:
var JoinPlugin = require("join-webpack-plugin");

var join = new JoinPlugin({
  search: 'glob' || ['globs',...],
  skip: 'substr' || /regexp/ || [ 'substr', /regex/, ...],
  join: function(common, addition) { ... },
  save: function(common) { ... },
  group: '[name]',
  name: '[name].[hash].[ext]',

  • join - user defined joining function (mandatory required)
  • save - user defined asset saving function (mandatory required)
  • search - glob pattern or pattern array of files to find and prefetch
see glob module for reference
  • skip - substring or regular expression or array to skip from searched results
  • group - default group loader option (see below)
  • name - default name loader option (see below)

The search param works like multi-require with glob patterns. Only files that required by require function in code will be loaded in that case.
Any file that does not match to search or skip param and match to loader section in webpack config and is required in code by function require or import will be loaded and joined anyway.

Define joining

The joining process needs two function join and save (the pure functions is recommended).


The user defined joining function prototype:
join: function(common, addition)

  • common - common data structure collecting data to join. Here may be
any user defined data structure. At first call this param is null. With each next call common is result of previous call of this function.
  • addition - next peace information to join. This data is passed through
loaders chain from source file which currenly joined.


After all files are loaded and collected in common place. User defined function prototype producing the result:
save: function(common)

This function has the same param common as first param of join function. The common param contains information loaded form files collected on joining step. This user defined function must process common collected information and must produce result string and return it. The result of this function will be saved in asset.

Loader configuration

The loader() method includes join loader into loader chain.
var JoinPlugin = require("join-webpack-plugin");
var theJoin = new JoinPlugin({...})

  module: {
    rules: [
      { test: /\.(json)$/i,
        use: [
          // some more pre loaders
  plugins: [


Preliminary loaders must be applied before join loader. This means that
join loader must be final loader in loaders chain.

Loader function waits hash of configuration options as its param.
Default values of loader may be specified in plugin configuration
described above.

Loader options:

* `group` - devides files into separated assets by specifying
  groping pattern. May include template placeholders described
  below in groupping section. Grouping is not applied if
  value is not specified.
* `name` - specifies destination asset file name. String value
  may include template placeholders described below. Default
  value is `[hash]`.

Configuration values specified directly in `loader()` override
same values specified as default in plugin configuration.

The `loader()` function may be invoked as class function if only one plugin
instance is passed to config. Therefore it is better to use object form
instead of class form:

``` javascript
var theJoin = new JoinPlugin({...})

loaders: [
  // this form valid only for single plugin instance:
  // to avoid problems better to use always object form:


Files may be grouped by simple criterion. Grouping criterion is specified in group loader param. If group param is not specified than will be only one common group where will be all files joined togather.
Grouping criteria formed by template placeholders described in interpolateName() from loader-utils module. Some of that is:
  • [name] - to group files with same name set group param:
  • [ext] - to group files with same ext set group param:
  • [path] - to group files where each group contains files from same directory:

And any derivative combinations.