@babel/helper-compilation-targets

@babel/helper-compilation-targets 是一個輔助套件，可與編譯目標（瀏覽器或其他環境，例如節點）和相容性表格（知道哪個版本支援特定語法）搭配使用。@babel/preset-env 會使用它來根據 targets 選項來決定應該啟用哪個外掛程式。

JavaScript
import {
  filterItems,
  default as getTargets,
  isRequired,
} from "@babel/helper-compilation-targets";

filterItems

function filterItems(
  list: { [feature: string]: Targets },

  // A set of plugins that should always be included
  includes: Set<string>,

  // A set of plugins that should always be excluded
  excludes: Set<string>,
  targets: Targets,

  // A set of plugins that should always be included if `includes` is empty
  defaultIncludes: Array<string> | null,

  // A set of plugins that should always be excluded if `excludes` is empty
  defaultExcludes?: Array<string> | null,

  // A map from transform plugin to syntax plugin for backward compatibility with older `@babel/parser` versions
  pluginSyntaxMap?: Map<string, string | null>
): Set<string>; // A set of enabled plugins

提供相容性資料表格 list（例如 @babel/compat-data）和瀏覽器目標 targets，傳回所需的套件組。

範例

JavaScript
const compatData = {
  "transform-feature-1": {
    chrome: "1",
    firefox: "1",
  },
  "transform-feature-2": {
    chrome: "2",
    firefox: "2",
  },
  "transform-feature-3": {
    chrome: "3",
    firefox: "3",
  },
  "transform-feature-4": {
    chrome: "4",
    firefox: "4",
  },
};

// filter a set of plugins required when compiled to chrome 2
// returns new Set(["transform-feature-3", "transform-feature-4"])
filterItems(compatData, new Set(), new Set(), {
  chrome: 2,
});

// filter a set of plugins required when compiled to chrome 2 and firefox 1
// returns new Set(["transform-feature-2", "transform-feature-3", "transform-feature-4"])
filterItems(compatData, new Set(), new Set(), {
  chrome: 2,
  firefox: 1,
});

// always include "transform-feature-2" and exclude "transform-feature-4"
// returns new Set(["transform-feature-2", "transform-feature-3"])
filterItems(
  compatData,
  new Set(["transform-feature-2"]),
  new Set(["transform-feature-4"]),
  {
    chrome: 2,
  }
);

// syntax-feature-2 is required to allow older @babel/parser to parse
// the feature-2 syntax supported in chrome 2

// returns new Set(["syntax-feature-2", "transform-feature-3", "transform-feature-4"])
filterItems(
  compatData,
  new Set(),
  new Set(),
  {
    chrome: 2,
  },
  null,
  null,
  new Map([["transform-feature-2", "syntax-feature-2"]])
);

註解

當新的 ES 功能達到第 4 階段時，它將在 @babel/parser 中成熟，這表示它將始終被解析，而不管外掛程式為何。然而，我們需要語法外掛程式才能使用較舊的 @babel/parser。

getTargets

type GetTargetsOption = {
  // This is not the path of the config file, but the path where start searching it from
  configPath?: string;

  // The path of the config file
  configFile?: string;

  // The env to pass to browserslist
  browserslistEnv?: string;

  // true to disable config loading
  ignoreBrowserslistConfig?: boolean;
};

type InputTargets = {
  ...Targets,

  browsers?: Browsers,

  // When `true`, this completely replaces the `browsers` option.
  // When `intersect`, this is intersected with the `browsers`
  // option (giving the higher browsers as the result).
  esmodules?: boolean | "intersect",
};

function getTargets(
  inputTargets: InputTargets = {},
  options: GetTargetsOption = {}
): Targets;

將使用者指定的 targets 標準化為支援的目標清單。另請參閱 (@babel/preset-env)[preset-env.md#options] 以取得 GetTargetsOption

範例

JavaScript
// Return the default compilation targets
// returns {}
getTargets();

空的編譯目標等同於強制所有轉換。預設編譯目標將變更為瀏覽器清單查詢 defaults, not IE 11 在 Babel 8 中。

也可以使用 ES 模組支援查詢編譯目標，例如 @vue/babel-preset-app 所做的，以提供一組現代目標。

JavaScript
/* returns {
  "android": "61.0.0",
  "chrome": "61.0.0",
  "edge": "16.0.0",
  "firefox": "60.0.0",
  "ios": "10.3.0",
  "node": "13.2.0",
  "opera": "48.0.0",
  "safari": "10.1.0",
  "samsung": "8.2.0",
} */
getTargets({
  esmodules: true,
});

注意：ES 模組相容性資料是由 MDN 所產生。

isRequired

function isRequired(
  name: string,
  targets: Targets,
  {
    compatData = pluginsCompatData,
    includes,
    excludes,
  }: {
    compatData?: { [feature: string]: Targets };
    includes?: Set<string>;
    excludes?: Set<string>;
  } = {}
): boolean;

給定瀏覽器目標 targets，查詢 compatData 是否需要外掛程式 name 來進行編譯。當未指定 compatData 時，預設資料來源為 @babel/compat-data

範例

babel.config.js
module.exports = api => {
  const targets = api.targets();
  // The targets have native optional chaining support
  // if `transform-optional-chaining` is _not_ required
  const optionalChainingSupported = !isRequired(
    "transform-optional-chaining",
    targets
  );
};

外掛作者可以使用 isRequired 來最佳化外掛輸出，以符合不同的 targets

example-babel-plugin.js
// a naive plugin replace `a.b` to `a != null && a.b`
module.exports = api => {
  const targets = api.targets();
  // The targets have native optional chaining support
  // if `transform-optional-chaining` is _not_ required
  const optionalChainingSupported = !isRequired(
    "transform-optional-chaining",
    targets
  );
  const visited = new WeakSet();
  return {
    visitor: {
      MemberExpression(path) {
        if (path.matchesPattern("a.b")) {
          if (visited.has(path.node)) return;
          visited.add(path.node);
          if (optionalChainingSupported) {
            // When optional chaining is supported,
            // output `a?.b` instead of `a != null && a.b`
            path.replaceWith(api.templates`a?.b`);
          } else {
            path.replaceWith(api.templates`a != null && ${path.node}`);
          }
        }
      },
    },
  };
};

@babel/plugin-transform-object-rest-spread 使用 isRequired 來判斷目標是否已經有原生 Object.assign 支援。

filterItems​

getTargets​

isRequired​

filterItems

getTargets

isRequired