dumpable: Make Object.toString useful

Because attaching a debugger isn't always an option, this library provides tools to easily implement a useful, consistent, and extensible override of Object.toString for your classes. After all, you're not using a dynamic language to see something as useless as [object Object] in your logs. It also provides methods to convert almost any value to a useful string or console dump.

This library and its tests and examples are written using Typescript, but it should work just as well with modern Javascript.

Installation

npm install dumpable

API

The entire API consists of one class (Dumpable) and two functions (dump and toDebugString). If you're using Typescript, you'll also need to import the PropertyDumper interface.

Dumpable base class

Just extend Dumpable and override dumpProperties to have a complete toString implementation:

import { Dumpable, PropertyDumper } from 'dumpable';

class MyClass extends Dumpable {
  constructor(private readonly parent: MyClass | null, private readonly config?: Map<string, any>) {
    super();
  }

  public dumpProperties(d: PropertyDumper): void {
    d.addRef('parent', this.parent);
    d.addIfTruthy('config', this.config);
  }
}

toString will return a string that looks like an object literal except for a special @ property that identifies the class and instance of the object:

const o1 = new MyClass(null, new Map(Object.entries({ a: 1, b: 'hi' })));
console.log(o1.toString()); // { @: [MyClass#1], parent: null, config: { "a": 1, "b": "hi" } }

const o2 = new MyClass(o1);
console.log(`${o2}`); // { @: [MyClass#2], parent: [MyClass#1] }

Subclasses can add properties by overriding dumpProperties and calling the super implementation:

class MySubclass extends MyClass {
  constructor(parent: MyClass | null, private readonly name: string, private readonly magic: number) {
    super(parent);
  }

  public dumpProperties(d: PropertyDumper): void {
    super.dumpProperties(d);
    d.add('magic', this.magic);
  }

  public toRefString(): string {
    return `[${this.constructor.name}:${this.name}]`;
  }
}

const o3 = new MySubclass(o2, 'morty', 42);
console.log(o3.toString()); // { @: [MySubclass:morty], parent: [MyClass#2], magic: 42 }

The previous example also demonstrates customizing the identity string of the class by overriding toRefString.

PropertyDumper interface

The PropertyDumper provided to dumpProperties has the following methods:

add

add(key: string, value: any): this

Adds a property with the given key and value to the object dump. The property is included even if null or undefined.

addRef

addRef(key: string, value: Dumpable | null | undefined): this

Adds a property with the given key and value to the object dump as a reference to another dumpable object. This is useful when only the identity, rather than the full description, of the object is desired. The property is included even if null or undefined.

addIfTruthy

addIfTruthy(key: string, value: any): this

Adds a property with the given key and value to the object dump if the value is truthy. Truthy values are any except null, undefined, false, 0, or ''.

addRefIfTruthy

addRefIfTruthy(key: string, value: Dumpable | null | undefined): this

Adds a property with the given key and value to the object dump as a reference to another dumpable object if the object reference is not null or undefined. This is useful when only the identity, rather than the full description, of the object is desired.

Utility methods

The library also includes two functions to make it easy to convert any value to a useful string or console dump.

toDebugString

import { toDebugString } from 'dumpable';

function toDebugString(value: any): string

Returns a string representation of the given value. In addition to the usual template literal and Object.toString conversions, this function has special handling for the following types:

• Date: calls toISOString (e.g. "1970-01-01T00:00:00.000Z")
• Map: formatted similar to an object literal (though the keys may be arbitrary types)
• Array/Iterable: formatted as an array literal
• Object: formatted as an object literal containing the object's own enumerable string-keyed properties

Structured objects are recursively formatted, and there are no depth constraints. Circular references are returned as the object identity (as returned by toRefString) for dumpable objects and the string [Circular] for others.

dump

import { dump } from 'dumpable';

function dump(...values: any[]): void

Dumps the given values to the console using console.log. Rather than converting each value to a string, this method passes through types that tend to receive special handling by the console, such as arrays, object literals, and certain built-in types. For instance, Node.js has special formatting for built-in types like BigInt, Date, Function, and Symbol. When using Node.js, there is also special formatting support for circular and dumpable object references. Note that unlike calling console.log directly, any strings passed directly to this function will be quoted.

License

dumpable is available under the ISC license.