Variant of merge that's useful for webpack configuration


Keywords
webpack, merge
License
MIT
Install
npm install webpack-merge@4.1.1

Contributors

Juho Vepsäläinen Jonathan Felchlin Kyle Herock Dan Kirkham Artem Zakharchenko David Gómez Matt Shwery siready Fernando Montoya Lucretiel Aaron Jensen Ken Powers Artem Sapegin C.J. Winslow Christian Hoffmeister


See all contributors


Documentation

build status bitHound Score codecov

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.loaders': '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.loaders': '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']
  }]
}

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 watch

Before contributing, please open an issue where to discuss.

License

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