webpack-merge - Merge designed for Webpack

webpack-merge provides a merge function that concatenates arrays and merges objects creating a new object. If functions are encountered, it will execute them, run the results through the algorithm, and then wrap the returned values within a function again.

This behavior is particularly useful in configuring webpack although it has uses beyond it. Whenever you need to merge configuration objects, webpack-merge can come in handy.

There's also a webpack specific merge variant known as merge.smart that's able to take webpack specifics into account (i.e., it can flatten loader definitions).

API

// Default API
var output = merge(object1, object2, object3, ...);

// You can pass an array of objects directly.
// This works with all available functions.
var output = merge([object1, object2, object3]);

// Customizing array/object behavior
var output = merge(
  {
    customizeArray(a, b, key) { return [...a, ...b]; },
    customizeObject(a, b, key) { return mergeWith(a, b); }
  }
)(object1, object2, object3, ...);

// Smarter merging for loaders, see below
var output = merge.smart(object1, object2, object3, ...);

// Merging with a specific merge strategy
var output = merge.strategy(
  {
    entry: 'prepend', // or 'replace'
    'module.loaders': 'prepend'
  }
)(object1, object2, object3, ...);

// The same idea works with smart merging too
var output = merge.smartStrategy(
  {
    entry: 'prepend', // or 'replace'
    'module.loaders': 'prepend'
  }
)(object1, object2, object3, ...);

Check out SurviveJS - Webpack and React to dig deeper into the topic.

Example

package.json

{
  "scripts": {
    "start": "webpack-dev-server",
    "build": "webpack"
  },
  ...
}

webpack.config.js

var path = require('path');
var merge = require('webpack-merge');

var TARGET = process.env.npm_lifecycle_event;

var common = {
  entry: path.join(__dirname, 'app'),
  ...
  module: {
    loaders: [
      {
        test: /\.css$/,
        loaders: ['style', 'css'],
      },
    ],
  },
};

if(TARGET === 'start') {
  module.exports = merge(common, {
    module: {
      // loaders will get concatenated!
      loaders: [
        {
          test: /\.jsx?$/,
          loader: 'babel?stage=1',
          include: path.join(ROOT_PATH, 'app'),
        },
      ],
    },
    ...
  });
}

if(TARGET === 'build') {
  module.exports = merge(common, {
    ...
  });
}

...

Smart Merging of Loaders

Webpack-merge tries to be smart about merging loaders when merge.smart is used. Loaders with matching tests will be merged into a single loader value.

Note that the logic picks up webpack 2 rules kind of syntax as well. The examples below have been written in webpack 1 syntax.

Loader string values loader: 'babel' override each other.

merge.smart({
  loaders: [{
    test: /\.js$/,
    loader: 'babel'
  }]
}, {
  loaders: [{
    test: /\.js$/,
    loader: 'coffee'
  }]
});
// will become
{
  loaders: [{
    test: /\.js$/,
    loader: 'coffee'
  }]
}

Loader array values loaders: ['babel'] will be merged, without duplication.

merge.smart({
  loaders: [{
    test: /\.js$/,
    loaders: ['babel']
  }]
}, {
  loaders: [{
    test: /\.js$/,
    loaders: ['coffee']
  }]
});
// will become
{
  loaders: [{
    test: /\.js$/,
    // appended because Webpack evaluated these from right to left
    // this way you can specialize behavior and build the loader chain
    loaders: ['babel', 'coffee']
  }]
}

Loader query strings loaders: ['babel?plugins[]=object-assign'] will be overridden

merge.smart({
  loaders: [{
    test: /\.js$/,
    loaders: ['babel?plugins[]=object-assign']
  }]
}, {
  loaders: [{
    test: /\.js$/,
    loaders: ['babel', 'coffee']
  }]
});
// will become
{
  loaders: [{
    test: /\.js$/,
    loaders: ['babel', 'coffee']
  }]
}

Loader arrays in source values will have loader strings merged into them.

merge.smart({
  loaders: [{
    test: /\.js$/,
    loader: 'babel'
  }]
}, {
  loaders: [{
    test: /\.js$/,
    loaders: ['coffee']
  }]
});
// will become
{
  loaders: [{
    test: /\.js$/,
    // appended because Webpack evaluated these from right to left!
    loaders: ['babel', 'coffee']
  }]
}

Loader strings in source values will always override.

merge.smart({
  loaders: [{
    test: /\.js$/,
    loaders: ['babel']
  }]
}, {
  loaders: [{
    test: /\.js$/,
    loader: 'coffee'
  }]
});
// will become
{
  loaders: [{
    test: /\.js$/,
    loader: 'coffee'
  }]
}

Development

1. npm i
2. npm run watch

Before contributing, please open an issue where to discuss.

License

webpack-merge is available under MIT. See LICENSE for more details.