Allow appending objects in `mergeWithRules`

Consider the following example:

const a = {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [{ loader: "style-loader", options: { someOption: true } }, { loader: "sass-loader" }],
      },
    ],
  },
};
const b = {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [
          {
            loader: "style-loader",
            options: {
              modules: true,
            },
          },
        ],
      },
    ],
  },
};
const result = {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [
          {
            loader: "style-loader",
            options: {
              someOptions: true,
              modules: true,
            },
          },
          { loader: "sass-loader" },
        ],
      },
    ],
  },
};

assert.deepStrictEqual(
  mergeWithRules({
    module: {
      rules: {
        test: "match",
        use: {
          loader: "match",
          options: "append",
        },
      },
    },
  })(a, b),
  result
);

Currently this will fail, since append only supports arrays. However, merging loaders options is a perfectly valid scenario and should be pretty easy to support.
Any chance we can get that?

[bebraw]

Also would be nice to get proper exception for certain option not being allowed for objects/arrays. Today if I use append on object I’d get “concat is not a function".

Yeah, I can add a type check and throw for sure.

[bebraw]

I understand. Instead of type checking Webpack types though you could check for object/array. Regardless of the application (whether you merge webpack config or something else) you can limit the append/prepend to arrays only and merge to objects only. This way you’ll have proper warnings/errors while avoiding things like “concat is not a function”. And it also will be pretty easy to document. WDYT?

Yeah, I see what you mean now. I added simple type checks to guard against append/prepend/merge. I think that covers it well and now it’s easier to notice if you are trying to merge types that the system doesn’t expect.

If you notice missing checks, open separate issues with small test cases for them and I’ll add logic based on that.