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@babel/plugin-transform-new-target

info

这个插件包含在 @babel/preset-env 中

示例​

🌐 Example

JavaScript
function Foo() {
  console.log(new.target);
}

Foo(); // => undefined
new Foo(); // => Foo
JavaScript
class Foo {
  constructor() {
    console.log(new.target);
  }
}

class Bar extends Foo {}

new Foo(); // => Foo
new Bar(); // => Bar

注意事项​

🌐 Caveats

这个插件依赖于 this.constructor，这意味着在使用未转换的类时，super 必须已经被调用。

🌐 This plugin relies on this.constructor, which means super must already have been called when using untransformed classes.

JavaScript
class Foo {}

class Bar extends Foo {
  constructor() {
    // This will be a problem if classes aren't transformed to ES5
    new.target;
    super();
  }
}

此外，当使用带有 ES5 函数类（转换后的 ES6 类）的 newTarget 时，这个插件不能转换所有的 Reflect.construct 情况。

🌐 Additionally, this plugin cannot transform all Reflect.construct cases when using newTarget with ES5 function classes (transformed ES6 classes).

JavaScript
function Foo() {
  console.log(new.target);
}

// Bar extends Foo in ES5
function Bar() {
  Foo.call(this);
}
Bar.prototype = Object.create(Foo.prototype);
Bar.prototype.constructor = Bar;

// Baz does not extend Foo
function Baz() {}

Reflect.construct(Foo, []); // => Foo (correct)
Reflect.construct(Foo, [], Bar); // => Bar (correct)

Reflect.construct(Bar, []); // => Bar (incorrect, though this is how ES5
// inheritance is commonly implemented.)
Reflect.construct(Foo, [], Baz); // => undefined (incorrect)

安装​

🌐 Installation

npm install --save-dev @babel/plugin-transform-new-target

用法​

🌐 Usage

🌐 With a configuration file (Recommended)

babel.config.json
{
  "plugins": ["@babel/plugin-transform-new-target"]
}

通过 CLI​

🌐 Via CLI

Shell
babel --plugins @babel/plugin-transform-new-target script.js

通过 Node API​

🌐 Via Node API

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