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).

Standard Merging

merge(...configuration | [...configuration])

merge is the core, and the most important idea, of the API. Often this is all you need unless you want further customization.

// 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]);

merge({ customizeArray, customizeObject })(...configuration | [...configuration])

merge behavior can be customized per field through a curried customization API.

// Customizing array/object behavior
var output = merge(
  {
    customizeArray(a, b, key) {
      if (key === 'extensions') {
        return _.uniq([...a, ...b]);
      }

      // Fall back to default merging
      return undefined;
    },
    customizeObject(a, b, key) {
      if (key === 'module') {
        // Custom merging
        return _.merge({}, a, b);
      }

      // Fall back to default merging
      return undefined;
    }
  }
)(object1, object2, object3, ...);

For example, if the previous code was invoked with only object1 and object2 with object1 as:

{
    foo1: ['object1'],
    foo2: ['object1'],
    bar1: { object1: {} },
    bar2: { object1: {} },
}

and object2 as:

{
    foo1: ['object2'],
    foo2: ['object2'],
    bar1: { object2: {} },
    bar2: { object2: {} },
}

then customizeArray will be invoked for each property of Array type, i.e:

customizeArray(['object1'], ['object2'], 'foo1');
customizeArray(['object1'], ['object2'], 'foo2');

and customizeObject will be invoked for each property of Object type, i.e:

customizeObject({ object1: {} }, { object2: {} }, bar1);
customizeObject({ object1: {} }, { object2: {} }, bar2);

merge.unique(<field>, <fields>, field => field)

const output = merge({
  customizeArray: merge.unique(
    'plugins',
    ['HotModuleReplacementPlugin'],
    plugin => plugin.constructor && plugin.constructor.name
  )
})({
  plugins: [
    new webpack.HotModuleReplacementPlugin()
  ]
}, {
  plugins: [
    new webpack.HotModuleReplacementPlugin()
  ]
});

// Output contains only single HotModuleReplacementPlugin now.

Merging with Strategies

merge.strategy({ <field>: '<prepend|append|replace>''})(...configuration | [...configuration])

Given you may want to configure merging behavior per field, there's a strategy variant:

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

merge.smartStrategy({ <key>: '<prepend|append|replace>''})(...configuration | [...configuration])

The same idea works with smart merging too (described below in greater detail).

var output = merge.smartStrategy(
  {
    entry: 'prepend', // or 'replace'
    'module.rules': 'prepend'
  }
)(object1, object2, object3, ...);

Smart Merging

merge.smart(...configuration | [...configuration])

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.

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, {
    ...
  });
}

...

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 array values loaders: ['babel'] can be reordered by including original loaders.

merge.smart({
  loaders: [{
    test: /\.js$/,
    loaders: ['babel']
  }]
}, {
  loaders: [{
    test: /\.js$/,
    loaders: ['react-hot', 'babel']
  }]
});
// will become
{
  loaders: [{
    test: /\.js$/,
    // order of second argument is respected
    loaders: ['react-hot', 'babel']
  }]
}

This also works in reverse - the existing order will be maintained if possible:

merge.smart({
  loaders: [{
    test: /\.css$/,
    use: [
      { loader: 'css-loader', options: { myOptions: true } },
      { loader: 'style-loader' }
    ]
  }]
}, {
  loaders: [{
    test: /\.css$/,
    use: [
      { loader: 'style-loader', options: { someSetting: true } }
    ]
  }]
});
// will become
{
  loaders: [{
    test: /\.css$/,
    use: [
      { loader: 'css-loader', options: { myOptions: true } },
      { loader: 'style-loader', options: { someSetting: true } }
    ]
  }]
}

In the case of an order conflict, the second order wins:

merge.smart({
  loaders: [{
    test: /\.css$/,
    use: [
      { loader: 'css-loader' },
      { loader: 'style-loader' }
    ]
  }]
}, {
  loaders: [{
    test: /\.css$/,
    use: [
      { loader: 'style-loader' },
      { loader: 'css-loader' }
    ]
  }]
});
// will become
{
  loaders: [{
    test: /\.css$/,
    use: [
      { loader: 'style-loader' }
      { loader: 'css-loader' },
    ]
  }]
}

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'
  }]
}

Multiple Merging

merge.multiple(...configuration | [...configuration])

Sometimes you may need to support multiple targets, webpack-merge will accept an object where each key represents the target configuration. The output becomes an array of configurations where matching keys are merged and non-matching keys are added.

var path = require('path');
var baseConfig = {
    server: {
      target: 'node',
      output: {
        path: path.resolve(__dirname, 'dist'),
        filename: 'lib.node.js'
      }
    },
    client: {
      output: {
        path: path.resolve(__dirname, 'dist'),
        filename: 'lib.js'
      }
    }
  };

// specialized configuration
var production = {
    client: {
      output: {
        path: path.resolve(__dirname, 'dist'),
        filename: '[name].[hash].js'
      }
    }
  }

module.exports = merge.multiple(baseConfig, production)

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

Development

1. npm i
2. npm run build
3. npm run watch

Before contributing, please open an issue where to discuss.

License

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