dynamic-error

TL/DR: A dynamic error class with support for TypeScript, ES6, and old-style JavaScript supporting even very old browsers.

This package provides the abstract class DynamicError. You can use it define error classes, that compute their error message dynamically from the internal state of the error object. For example, you can define an InvalidValueError<T> that takes a value argument in its constructor and computes the error message as you wish. It is derived from ExtendableError from my ts-error package, which enables the proper inheritance hierarchy (subclassing Error normally does not work properly in JS).

The package is well-tested, very small and only has one production dependency (ts-error), which itself has no production dependencies.

Install

You can install the dynamic-error package from NPM with the command:

npm install dynamic-error

If you're using TypeScript, you will also need the @types/node dev dependency (unless you have type checking disabled).

npm install --save-dev @types/node

Usage

You subclass DynamicError, call super() in the constructor, define the method errorMessage() and then you can use your new error class in your code. If you want to pass state parameters to the constructor, you must call super with a function argument, in which you can set the state of your error object (this is due to the JS quirks with Error objects).

In TypeScript:

import { DynamicError } from "dynamic-error";
 
// A subclass of DynamicError, here also using generics.
class ValueError<T> extends DynamicError {
    protected value: T;
 
    constructor(value: T) {
        // You need to use function(){} here and not arrow notation, because of `this`, which is not supported by arrow functions.
        super(function () {
          this.value = value;
        });
    }
 
    public errorMessage() {
        return `The value ${this.value} is invalid.`;
    }
 
    // This method is not required, but I use it to demonstrate dynamic recalculation.
    public updateValue(value: T) {
        this.value = value;
        // this.update() recomputes the error message and stack trace.
        // The stack trace remains the same it was at error creation, only the message in the stack trace is updated.
        this.update();
    }
}
 
// Another subclass
class NumberValueError extends ValueError<number> {
    public errorMessage() {
        return `The number ${this.value} is invalid.`;
    }
}
 
try {
    throw new NumberValueError(-1);
    throw new ValueError("abc");
} catch (e) {
    if (e instanceof NumberValueError) {
        // ...
    } else {
        // ...
    }
}

In ES6 / esnext:

import { DynamicError } from "dynamic-error";
 
// A subclass of DynamicError
class ValueError extends DynamicError {
    constructor(value) {
        // You need to use function(){} here and not arrow notation, because of `this`, which is not supported by arrow functions.
        super(function () {
          this.value = value;
        });
    }
 
    errorMessage() {
        return `The value ${this.value} is invalid.`;
    }
 
    // This method is not required, but I use it to demonstrate dynamic recalculation.
    updateValue(value) {
        this.value = value;
        // this.update() recomputes the error message and stack trace.
        // The stack trace remains the same it was at error creation, only the message in the stack trace is updated.
        this.update();
    }
}
 
// Another subclass
class NumberValueError extends ValueError {
    public errorMessage() {
        return `The number ${this.value} is invalid.`;
    }
}
 
try {
    throw new NumberValueError(-1);
    throw new ValueError("abc");
} catch (e) {
    if (e instanceof NumberValueError) {
        // ...
    } else {
        // ...
    }
}

In ES5:

var DynamicError = require("dynamic-error").DynamicError;
 
// This is taken from TypeScript compiler output, because it works quite reliably.
// There are various other methods though, so use whatever you like, if you have to use ES5.
var __extends = (this && this.__extends) || (function () {
    var extendStatics = Object.setPrototypeOf ||
        ({ __proto__: [] } instanceof Array && function (d, b) { d.__proto__ = b; }) ||
        function (d, b) { for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p]; };
    return function (d, b) {
        extendStatics(d, b);
        function __() { this.constructor = d; }
        d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
    };
})();
 
var ValueError = /** @class */ (function (_super) {
    __extends(ValueError, _super);
    function ValueError(value) {
        return _super !== null && _super.call(this, function () {
            this.value = value;
        }) || this;
    }
    ValueError.prototype.errorMessage = function () {
        return "The value " + this.value + " is invalid.";
    };
    ValueError.prototype.updateValue = function (value) {
        this.value = value;
        this.update();
    };
    return ValueError;
}(DynamicError));
 
var NumberValueError =  /** @class */ (function (_super) {
    __extends(NumberValueError, _super);
    function NumberValueError() {
        return _super !== null && _super.apply(this, arguments) || this;
    }
    NumberValueError.prototype.errorMessage = function () {
        return "The number " + this.value + " is invalid.";
    };
    return NumberValueError;
}(ValueError));
 
try {
    throw new NumberValueError(-1);
    throw new ValueError("abc");
} catch (e) {
    if (e instanceof NumberValueError) {
        // ...
    } else {
        // ...
    }
}

Tests

Run npm run test to run the mocha/chai test suite. The ExtendableError package is well-tested in node and all browsers, including very old versions, so this Error class will be compatible even with very old and mobile browsers.

If you encounter any issues, please file an issue and I will investigate and fix it.

License

MIT (see ./LICENSE).