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问题:

Babel's 6th version changes the functioning of export default and in particular its relation with commonjs require.

To summarise, while until babel5, require('module') where giving the default export of the module, it now always returns the module object containing all of the exports of the module. If one only wants the default, he/she must use require('module').default. As explained here, there is very good reasons behind this and the aim of this question is not to break or hack this behaviour.

However, if one is building a library, he/she usually does not want to distribute a module but the export value of his library (e.g. a function, whatever module system is used internally). This is well dealt with by webpack and the output.library configuration when using commonjs or AMD. Because prior babel's versions allowed the default export to be required with commonjs, babel was also compatible with this mechanism. However it is not the case anymore: the library now always provides an es6 module object.

Here is an example.

src/main.js

export default "my lib content";

webpack.config.js

var path = require("path");
var webpack = require("webpack");

module.exports = {
  entry: {
    lib: [ path.resolve(__dirname, "src/main.js") ],
  },
  output: {
    path: path.join(__dirname, "dist"),
    filename: "mylib-build.js",
    library: 'myLib'
  },
  module: {
    loaders: [
      {
        test: /\.js$/,
        loader: "babel",
        include: path.join(__dirname, "src"),
        query: { presets: ['es2015'] }
      }
    ]
  }
};

test.html

<html>
<head></head>
<body>
<script src="dist/mylib-build.js"></script>
<!-- `myLib` will be attached to `window` -->
<script>
  console.log(JSON.stringify(myLib)); // { default: "my lib content" }
</script>
</body>
</html>

This is a very simple example but I obviously want the export of mylib to be the string "my lib content" instead of { default: "my lib content" }.

One solution could be to create an export source file in commonjs to perform the transformation:

module.exports = require('./main').default;

However I find this solution quite poor. One should be able to solve it at the compilation level, without changing the source code. Any idea?

回答1:

Was just going at this my self. Whether one like to call it a workaround or solution, there seem to be a Babel plugin that "solve it".

Using the plugin babel-plugin-add-module-exports as referenced in https://stackoverflow.com/a/34778391/1592572

Example config

var webpackOptions = {
    entry: {
        Lib1: './src/Lib1.js',
        Lib2: './src/Lib2.js'
    },
    output: {
        filename: "Master.[name].js",
        library: ["Master","[name]"],
        libraryTarget: "var"
    },
    module: {
        loaders: [
            {
                loader: 'babel',
                query: {
                    presets: ['es2015'],
                    plugins: ["add-module-exports"]
                }
            }
        ]
    }
};

This yields Master.Lib1 to be lib1 instead of Master.Lib1.default.

回答2:

Webpack 2 now supports es6 modules which partially solves this issue. Migrating from webpack 1 to webpack 2 is relatively painless. One just need to remember to disable babel's es6 module to commonjs conversion to make this work:

.babelrc

{
  "presets": [
    ["es2015", {"modules": false}]
  ]
}

However, unfortunately, it does not work properly with export default (but an issue is opened, hopefully a solution will be released eventually).

EDIT

Good news! Webpack 3 supports the output.libraryExport option that can be used to directly expose the default export:

var path = require("path");
var webpack = require("webpack");

module.exports = {
  entry: {
    lib: [ path.resolve(__dirname, "src/main.js") ],
  },
  output: {
    path: path.resolve(__dirname, "dist"),
    filename: "mylib-build.js",
    library: "myLib",
    // Expose the default export.
    libraryExport: "default"
  },
  module: {
    loaders: [
      {
        test: /\.js$/,
        loader: "babel",
        include: path.resolve(__dirname, "src")
      }
    ]
  }
};

回答3:

You can use this solution (this is more like workaround, but it allow you to keep your sources from change):

There is a loader called callback-loader. It allow you to change your sources in a build time by calling a callback and put a result instead of it. In other words you can turn all you require('module') into a require('module').default automatically in a build time.

Here is your config for it:

var webpackConfig = {
    module: {
        loaders: [
            { test: /\.js$/, exclude: /node_modules/, loader: 'callback' },
            ...
        ]
    },
    ...
    callbackLoader: {
        require: function() {
            return 'require("' + Array.prototype.join.call(arguments, ',') + '").default';
        }
    }
};