Babel 也有配置文件！許多其他工具都有類似的配置文件：ESLint (.eslintrc)、Prettier (.prettierrc)。

所有 Babel API 參數(shù) 都可以被配置。然而，如果該參數(shù)需要用到 JavaScript 代碼，你可能需要使用 JavaScript 代碼版的 配置文件。

你的使用場(chǎng)景是什么？?

• 你是否采用的是單一倉(cāng)庫(kù)（monorepo）模式？
• 你是否需要編譯 ?node_modules?？

那么 ?babel.config.json ?文件可以滿足你的的需求！

• 你的配置文件是否僅適用于項(xiàng)目的某個(gè)部分？

那么 ?.babelrc.json? 文件適合你！

• Guy Fieri 是你崇拜的對(duì)象？

我們建議使用? babel.config.json ?格式的配置文件。  Babel 自身使用的就是這種格式。

babel.config.json?

在項(xiàng)目的根目錄（package.json 文件所在的目錄）下創(chuàng)建一個(gè)名為 babel.config.json 的文件，并輸入如下內(nèi)容。

babel.config.json

{
  "presets": [...],
  "plugins": [...]
}

請(qǐng)參閱 babel.config.json 文檔 以了解更多關(guān)于配置參數(shù)的信息。

.babelrc.json?

在你的項(xiàng)目中創(chuàng)建名為 .babelrc.json 的文件，并輸入以下內(nèi)容。

.babelrc.json

{
  "presets": [...],
  "plugins": [...]
}

請(qǐng)參閱 .babelrc 文檔 以了解更多關(guān)于配置參數(shù)的信息。

package.json?

或者，還可以選擇將 .babelrc.json 中的配置信息作為 babel 鍵（key）的值添加到 package.json 文件中，如下所示：

package.json

{
  "name": "my-package",
  "version": "1.0.0",
  "babel": {
    "presets": [ ... ],
    "plugins": [ ... ],
  }
}

用 JavaScript 編寫配置文件?

你還可以用 JavaScript 來(lái)編寫 babel.config.js 和 .babelrc.js 文件：

babel.config.js

module.exports = function (api) {
  api.cache(true);

  const presets = [ ... ];
  const plugins = [ ... ];

  return {
    presets,
    plugins
  };
}

你還可以調(diào)用 Node.js 的任何 API，例如基于進(jìn)程環(huán)境進(jìn)行動(dòng)態(tài)配置：

babel.config.js

module.exports = function (api) {
  api.cache(true);

  const presets = [ ... ];
  const plugins = [ ... ];

  if (process.env["ENV"] === "prod") {
    plugins.push(...);
  }

  return {
    presets,
    plugins
  };
}

你可以在此 文檔 中閱讀更多關(guān)于 JavaScript 配置文件的信息。

使用 CLI (@babel/cli)

Shell

babel --plugins @babel/plugin-transform-arrow-functions script.js

請(qǐng)參閱 babel-cli 文檔 以了解更多關(guān)于配置參數(shù)的信息。

使用 API (@babel/core)?

JavaScript

require("@babel/core").transformSync("code", {
  plugins: ["@babel/plugin-transform-arrow-functions"],
});

請(qǐng)參閱 babel-core 文檔 以了解更多關(guān)于配置參數(shù)的信息。

Print effective configs?

You can tell Babel to print effective configs on a given input path

• Shell
• powershell

# *nix or WSL
BABEL_SHOW_CONFIG_FOR=./src/myComponent.jsx npm start

BABEL_SHOW_CONFIG_FOR accepts both absolute and relative file paths. If it is a relative path, it will be resolved from cwd.

Once Babel processes the input file specified by BABEL_SHOW_CONFIG_FOR, Babel will print effective configs to the console. Here is an example output:

Babel configs on "/path/to/cwd/src/index.js" (ascending priority):
config /path/to/cwd/babel.config.json
{
  "sourceType": "script",
  "plugins": [
    "@foo/babel-plugin-1"
  ],
  "extends": "./my-extended.js"
}

config /path/to/cwd/babel.config.json .env["test"]
{
  "plugins": [
    [
      "@foo/babel-plugin-3",
      {
        "noDocumentAll": true
      },
    ]
  ]
}

config /path/to/cwd/babel.config.json .overrides[0]
{
  "test": "src/index.js",
  "sourceMaps": true
}

config /path/to/cwd/.babelrc
{}

programmatic options from @babel/cli
{
  "sourceFileName": "./src/index.js",
  "presets": [
    "@babel/preset-env"
  ],
  "configFile": "./my-config.js",
  "caller": {
    "name": "@babel/cli"
  },
  "filename": "./src/index.js"
}

Babel will print effective config sources ordered by ascending priority. Using the example above, the priority is:

babel.config.json < .babelrc < programmatic options from @babel/cli

In other words, babel.config.json is overwritten by .babelrc, and .babelrc is overwritten by programmatic options.

For each config source, Babel prints applicable config items (e.g. overrides and env) in the order of ascending priority. Generally each config sources has at least one config item -- the root content of configs. If you have configured overrides or env, Babel will not print them in the root, but will instead output a separate config item titled as .overrides[index], where index is the position of the item. This helps determine whether the item is effective on the input and which configs it will override.

If your input is ignored by ignore or only, Babel will print that this file is ignored.

How Babel merges config items?

Babel's configuration merging is relatively straightforward. Options will overwrite existing options when they are present and their value is not undefined. There are, however, a few special cases:

• For assumptions, parserOpts and generatorOpts, objects are merged, rather than replaced.
• For plugins and presets, they are replaced based on the identity of the plugin/preset object/function itself combined with the name of the entry.

Option (except plugin/preset) merging?

As an example, consider a config with:

JavaScript

{
  sourceType: "script",
  assumptions: {
    setClassFields: true,
    iterableIsArray: false
  },
  env: {
    test: {
      sourceType: "module",
      assumptions: {
        iterableIsArray: true,
      },
    }
  }
};

When NODE_ENV is test, the sourceType option will be replaced and the assumptions option will be merged. The effective config is:

JavaScript

{
  sourceType: "module", // sourceType: "script" is overwritten
  assumptions: {
    setClassFields: true,
    iterableIsArray: true, // assumptions are merged by Object.assign
  },
}

Plugin/Preset merging?

As an example, consider a config with:

JavaScript

plugins: [
  './other',
  ['./plug', { thing: true, field1: true }]
],
overrides: [{
  plugins: [
    ['./plug', { thing: false, field2: true }],
  ]
}]

The overrides item will be merged on top of the top-level options. Importantly, the plugins array as a whole doesn't just replace the top-level one. The merging logic will see that "./plug" is the same plugin in both cases, and { thing: false, field2: true } will replace the original options, resulting in a config as

JavaScript

plugins: [
  './other',
  ['./plug', { thing: false, field2: true }],
],

Since merging is based on identity + name, it is considered an error to use the same plugin with the same name twice in the same plugins/presets array. For example

JavaScript

plugins: ["./plug", "./plug"];

is considered an error, because it's identical to plugins: ['./plug']. Additionally, even

JavaScript

plugins: [["./plug", { one: true }], ["./plug", { two: true }]];

is considered an error, because the second one would just always replace the first one.

If you actually do want to instantiate two separate instances of a plugin, you must assign each one a name to disambiguate them. For example:

JavaScript

plugins: [
  ["./plug", { one: true }, "first-instance-name"],
  ["./plug", { two: true }, "second-instance-name"],
];

because each instance has been given a unique name and thus a unique identity.