流类型错误。 对象类型与字符串不兼容

Question

I wrote a small Javascript class that has an argument which accepts either a string or an object. However, I get this error:

Error: utils/HttpError.js:8
  8:    message:string | ErrorObj
                         ^^^^^^^^ object type. This type is incompatible with
512:     message: string;
                  ^^^^^^ string. See lib: /private/tmp/flow/flowlib_33322c59/core.js:512

I've tried not using the type alias, and simplifying the ErrorObj to just {} , but I've had no luck. Is this a bug, or am I missing something? I'm using flow-bin ^0.51.0. My class is below.

//@flow
"use strict";

type ErrorObj = {[key:string]:Array<string>}

class HttpError extends Error {
    status:number
    message:string | ErrorObj
    constructor(status:number, message: string | ErrorObj){
        super();
        this.status = status;
        this.message = message;
    }

    toString():string{
        return `status: ${this.status} message: ${JSON.stringify(this.message)}`;
    }
}

module.exports = HttpError;

Answer 1

The internal definition of Error has the message member variable typed as simply a string. Even after subclassing, the base class ( Error ) still might interact with this.message in some way. If you were able to redefine it to be anything other than a string, and it attempted to call length or concat or some other member of the String class, it would fail. Therefore you cannot override member types of subclasses.

You could instead try something like:

type ErrorObj = {[key:string]:Array<string>}

class HttpError extends Error {
    status:number
    _customMessage:string | ErrorObj
    constructor(status:number, message: string | ErrorObj){
        super(typeof message === 'string' ? message : null);
        this.status = status;
        this._customMessage = message;
    }

    toString():string{
        return `status: ${this.status} message: ${JSON.stringify(this._customMessage)}`;
    }
}