⏫ metaproperties

Associate data with any object without modifying the object in any way. Implemented using WeakMaps. Works in browser and node.js.

const meta = require('metaproperties');

meta(someObject).someProperty = {
  created: 'yesterday',
  foo: 'bar'
};

someObject.someProperty // undefined

meta(someObject).someProperty.foo // 'bar'

meta(someObject) = 0;
meta(someObject)++;
// etc

motivation

There are many cases where one would like to store some information about or on an object, without affecting its state. For example, on objects created by an external library. Aside from defining a new property on the object, one would have to use a mapping implementation to associate the object with desired data.

Prior to the introduction of Maps/WeakMaps, objects couldn't be used as keys in hash-map-like implementations. Typically, and object would be assigned a unique string identifier as a property, which would be used as the key of a map object. However, the new data structures are designed solely to function as hash-maps, and they allow objects to be used as keys directly, which improves performance.

metaproperties is a thin abstraction over a WeakMap, offering some convenience and syntactic simplicity.

installation

npm install metaproperties --save

syntax

require('metaproperties')(object: Object [, key: Symbol]): Object
require('metaproperties').createKey(): Symbol

usage

const varsof = require('metaproperties');

let object = {
  weight: 100,
  shape: 'round',
  color: '#00ff00',
  child: {
    born: false
  }
};

varsof(object).secretProperty = 'foo';

console.log(object.secretProperty); // undefined, as expected

console.log(varsof(object).secretProperty); // foo, as expected

objects unaffected

let beforeProps = Object.getOwnPropertyNames(object), // properties on object
    beforeSyms = Object.getOwnPropertySymbols(object); // symbols on object
    
varsof(object).secretProperty = 'foo';

let afterProps = Object.getOwnPropertyNames(object),
    afterSyms = Object.getOwnPropertySymbols(object);
    
console.log(beforeProps.length === afterProps.length && beforeSyms.length ===
afterSyms.length); // true, as expected

let same = true;
beforeProps.forEach((prop, i) => {
  same = same && beforeProps[i] === afterProps[i]
});
beforeSyms.forEach((sym, i) => {
  same = same && beforeSyms[i] === afterSyms[i]
});

console.log(same); // true, as expected

performance

Without using the secret key functionality described below, performance is just slightly slower than native.

Run benchmark.js; this does not test secret key functionality. The first line represents native (setting properties directly on object) performance, and the second line represents metaproperties' performance. My typical results in node.js:

31.37ms average (native)
32.31ms average (meta)

Performance penalty should be minimal if varsof(...) is not used in critical performance sections.

Please let me know if your results differ!

Using the secret key functionality results in twice as slow as native performance; however, performance issues should generally not be noticeable.

secret keys

Specify a key to use as an access key. Must use varsof.createKey to create a key.

const varsof = require('metaproperties');

let key = varsof.createKey();

varsof(someObject, key).bigSecret = 'abc123';

varsof(someObject).bigSecret // undefined, as expected

varsof(someObject, key).bigSecret // 'abc123';

Dispose of access keys once no longer needed:

varsof.destroyKey(key: Symbol): void

etymology

"metaproperties" sounded cool. Also, metadata + properties = metaproperties.

license

MIT

feedback

Create issues here on github or email metapropertiesfeedback [at symbol] [google's email service].