1. val-loader

在构建时执行给定的模块以生成源代码。

1.1. 安装

npm install val-loader --save-dev

1.2. 用法

此loader加载的模块必须遵循以下接口

1.2.1. Module Interface

被加载的模块必须使用以下函数接口导出一个函数作为默认导出：

module.exports = function () {...};

被Babel 转换的模块也被支持

export default function (...) { ... }

1.2.2. Function Interface

该函数将使用loader选项来调用，并且必须返回一个对象或者Promise。

`Object`

Name	Type	Default	Description
`code`	`{String\	Buffer}`	`undefined`	(Required) The code that is passed to the next loader or to webpack
`sourceMap`	`{Object}`	`undefined`	(Optional) Will be passed to the next loader or to webpack
`ast`	`{Array<{Object}>}`	`undefined`	(Optional) An Abstract Syntax Tree that will be passed to the next loader. Useful to speed up the build time if the next loader uses the same AST
`dependencies`	`{Array<{String}>}`	`[]`	An array of absolute, native paths to file dependencies that need to be watched for changes
`contextDependencies`	`{Array<{String}>}`	`[]`	An array of absolute, native paths to directory dependencies that need to be watched for changes
`cacheable`	`{Boolean}`	`false`	Flag whether the code can be re-used in watch mode if none of the `dependencies` have changed

`Promise`

解析上面的对象接口后的对象

1.3. Options

val-loader itself has no options. The options are passed as they are (without cloning them) to the exported function

1.4. Examples

If you have a module like this

answer.js

function answer () {
  return {
    code: 'module.exports = 42;'
  }
};

module.exports = answer;

you can use the val-loader to generate source code on build time

webpack.config.js

module.exports = {
  ...
  module: {
    rules: [
      {
        test: require.resolve('path/to/answer.js'),
        use: [
          {
            loader: 'val-loader'
          }
        ]
      }
    ]
  }
};

1.4.1. Complete

A complete example of all available features looks like this

answer.js

const ask = require('./ask.js');
const generate = require('./generate.js');

function answer(options) {
  return ask(options.question)
    .then(generate)
    .then(result => ({
      ast: result.abstractSyntaxTree,
      code: result.code,
      // Mark dependencies of answer().
      // The function will be re-executed if one of these
      // dependencies has changed in watch mode.
      dependencies: [
        // Array of absolute native paths!
        require.resolve('./ask.js'),
        require.resolve('./generate.js')
      ],
      // Flag the generated code as cacheable.
      // If none of the dependencies have changed,
      // the function won't be executed again.
      cacheable: true
      sourceMap: result.sourceMap,
    })
  );
}

module.exports = answer;

webpack.config.js

module.exports = {
  ...
  module: {
    rules: [
      {
        test: require.resolve('path/to/answer.js'),
        use: [
          {
            loader: 'val-loader',
            options: {
              question: 'What is the meaning of life?'
            }
          }
        ]
      }
    ]
  }
};