Using ES6 Symbols with typescript

Question

I am trying to run this simple line of code —

let FUNJECTOR_KEY = Symbol.for('funjector')

But I keep getting the error — Cannot find name 'Symbol'. I am new to typescript, so I am not sure if I need to include something?

In my case I don't want to use a poly fill as explained here — Using es-6 symbols in typescript

Answer 1

TypeScript compiler transpiles TS into JS. TSC cannot find the declaration for Symbol in es5 mode. So your error is purely in compile-time. You don't need polyfill for runtime.

To address this, you can either change your compiling target to es6 , so that Symbol is defined in standard library. Or you can manually add the definition ( 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;
}

Caveat: Some type checking does not work on symbol . For example, you cannot declare a property computed to symbol on interface.