使用带有打字稿的 ES6 符号

Question

我正在尝试运行这行简单的代码——

let FUNJECTOR_KEY = Symbol.for('funjector')

但我不断收到错误消息 — Cannot find name 'Symbol'. 我是打字稿的新手，所以我不确定是否需要包含一些内容？

就我而言，我不想使用此处解释的多边形填充 — Using es-6 symbols in typescript

Answer 1

TypeScript 编译器将 TS 转换为 JS。 TSC 在 es5 模式下找不到 Symbol 的声明。所以你的错误纯粹是在编译时。运行时不需要 polyfill。

要解决这个问题，您可以将编译目标更改为es6，以便在标准库中定义Symbol。或者您可以手动添加定义 (source)。

declare class Symbol {
    /** Returns a string representation of an object. */
    toString(): string;

    /** Returns the primitive value of the specified object. */
    valueOf(): Object;

    /**
      * Returns a new unique Symbol value.
      * @param  description Description of the new Symbol object.
      */
    constructor(description?: string);

    /**
    * Returns a Symbol object from the global symbol registry matching the given key if found.
    * Otherwise, returns a new symbol with this key.
    * @param key key to search for.
    */
    static for(key: string): Symbol;

    /**
      * Returns a key from the global symbol registry matching the given Symbol if found. 
      * Otherwise, returns a undefined.
      * @param sym Symbol to find the key for.
      */
    static keyFor(sym: Symbol): string;
}

// Well-known Symbols
declare module Symbol {
    /** 
      * A method that determines if a constructor object recognizes an object as one of the 
      * constructor’s instances.Called by the semantics of the instanceof operator. 
      */
    const hasInstance: Symbol;

    /** 
      * A Boolean value that if true indicates that an object should be flatten to its array 
      * elements by Array.prototype.concat.
      */
    const isConcatSpreadable: Symbol;

    /** 
      * A Boolean value that if true indicates that an object may be used as a regular expression. 
      */
    const isRegExp: Symbol;

    /** 
      * A method that returns the default iterator for an object.Called by the semantics of the 
      * for-of statement. 
      */
    const iterator: Symbol;

    /** 
      * A method that converts an object to a corresponding primitive value.Called by the 
      * ToPrimitive abstract operation. 
      */
    const toPrimitive: Symbol;

    /** 
      * A String value that is used in the creation of the default string description of an object.
      * Called by the built- in method Object.prototype.toString. 
      */
    const toStringTag: Symbol;

    /** 
      * An Object whose own property names are property names that are excluded from the with 
      * environment bindings of the associated objects.
      */
    const unscopables: Symbol;
}

警告： 某些类型检查不适用于symbol。例如，您不能在接口上声明计算为符号的属性。