@pinemach/describe

This is a small, simple, permissively-licensed and zero-dependency library for getting a brief string description of arbitrary JavaScript values.

The default built-in description logic guarantees that strings will not be longer than 60 characters and will not contain newlines or control characters. This makes the package particularly suitable for generating human-readable descriptions of untrusted data, that is safe to output to a log.

Installation

You can add @pinemach/describe to your JavaScript project via NPM:

npm install --save @pinemach/describe

Usage

The simplest usage of this package is to import the default describe function and invoke it with the value that should be described.

Basic usage:

const {describe} = require("@pinemach/describe");

console.log(describe(null)); // Prints "null"
console.log(describe(1234.5)); // Prints "the number 1234.5"
console.log(describe({})); // Prints "an empty object"
console.log(describe([0, 1, 2, 3])); // Prints "an array with 4 elements"

The package also supports overriding description behavior via the addDescriptor function. Descriptor functions take precedence over default built-in logic, and descriptors added first take precedence over descriptors added last. When a descriptor function returns a non-empty string for an input value, that string will be produced by the describe function instead of its default output. When a descriptor function returns any falsey value, evaluation continues for remaining descriptor functions and then to the default description logic.

Intermediate usage, featuring custom value descriptors:

const {describe, addDescriptor} = require("@pinemach/describe");

// A custom class that we will want a custom description for
class MyClass {
    constructor(n) {
        this.n = n;
    }
}

// Prints "an object instance of \"MyClass\""
console.log(describe(new MyClass(50)));

// Add special logic for describing MyClass instances
addDescriptor((value) => {
    if(value instanceof MyClass) {
        return "MyClass #" + value.n;
    }
    else {
        return undefined;
    }
});

// Prints "MyClass #50"
console.log(describe(new MyClass(50)));

The package also provides a Describe class for instanced behavior. This means that custom descriptor functions may be added to a specific instance and applied only to uses of that instance's describe method, without changing the behavior of the global default describe function or any other Describe instances.

Advanced usage, featuring custom value descriptors and instanced Describe logic:

const {Describe, describe} = require("@pinemach/describe");

// A custom class that we will want a custom description for
class MyClass {
    constructor(n) {
        this.n = n;
    }
}

// Create a new Describe instance instead of using the global default
const myDescriber = new Describe();

// Add special logic for describing MyClass instances
myDescriber.addDescriptor((value) => {
    if(value instanceof MyClass) {
        return "MyClass #" + value.n;
    }
    else {
        return undefined;
    }
});

// Prints "MyClass #50"
console.log(myDescriber.describe(new MyClass(50)));

// Still prints "an object instance of \"MyClass\""
console.log(describe(new MyClass(50)));