错误类型错误：构造函数 Map 在 ES5 中需要“新”？

Question

Code is:

export class ExtendedMap<T, U> extends Map {
    constructor() {
        super();
    }

    toggle(key: T, value: U) {
        if (this.has(key)) {
            super.delete(key);
        } else {
            super.set(key, value);
        }
    }

    has(key: T): boolean {
        return super.has(key);
    }
}

I get this error for ES5 compilation:

ERROR TypeError: Constructor Map requires 'new'

I'm using it like this:

public registryLayers = new ExtendedMap<number, any>();

Snippet (without TypeScript parts commented out):

 /*export*/ class ExtendedMap/*<T, U>*/ extends Map { constructor() { super(); } toggle(key/*: T*/, value/*: U*/) { if (this.has(key)) { super.delete(key); } else { super.set(key, value); } } has(key/*: T*/)/*: boolean*/ { return super.has(key); } } /*public*/ registryLayers = new ExtendedMap/*<number, any>*/(); console.log("Worked without error");

Answer 1

TypeScript should be complaining about Map if you're targeting ES5 (it does in this example ).

If you ignore that error from TypeScript and then run the code that TypeScript generates in a modern(ish) JavaScript engine, the problem is that the code TypeScript created is trying to call Map like an old-style constructor function. When you configure TypeScript to emit ES5 code, it doesn't emit class constructs because ES5 didn't have them. Instead, it emits constructor function syntax, and in your ExtendedMap constructor, it tries to call Map like this:

return _super.call(this) || this; // Where `_super` is set to `Map`

But in a modern environment, Map is defined as a class constructor and cannot be called that way, resulting in the error you're getting.

If you need your code to run in an ES5 environment, you need to include a Map polyfill (since Map didn't exist in ES5) that is written to be compatible with an ES5 environment so that Map can be called via .call as above. And if your code is running in a modern(ish) environment that already has Map , you'll still need to include the polyfill because of the way it gets called.