syms

1.1.0 • Public • Published

syms - quick/easy/canonical Symbol.for() access

This library is a "magic" proxy object, which generates Symbol.for() type symbols on property access after performing a few key normalization steps, resulting in the following:

  • syms.foo becomes Symbol.for("foo")
  • syms.TWO_WORDS becomes Symbol.for("two.words")
  • syms._two_words becomes Symbol.for("two.words")
  • syms.twoWords becomes Symbol.for("two.words")
  • syms.TwoWords becomes Symbol.for("two.words")
  • syms.kTwoWords becomes Symbol.for("two.words")

In short, the following:

const syms = require("syms");
const { kSomething, ANOTHER_THING, yetAnotherThing, ["A Fourth thing"]THING4 } = syms;
const THING5 = syms["thing #5"]
 
// with a namespace:
const { example, kAnotherExample, EXAMPLE_THREE } = syms[syms.ns]('my.app.name')

Is equivalent to:

const kSomething = Symbol.for("something");
const ANOTHER_THING = Symbol.for("another.thing");
const yetAnotherThing = Symbol.for("yet.another.thing");
const THING4 = Symbol.for("a.fourth.thing");
const THING5 = Symbol.for("thing.5");
 
const example = Symbol.for("my.app.name.example")
const kAnotherExample = Symbol.for("my.app.name.another.example")
const EXAMPLE_THREE = Symbol.for("my.app.name.example.three")

Motivation

JavaScript Symbol objects are an incredibly handy generic construct in modern JavaScript which can be utilized for many purposes, including:

  • marking/documenting "internal" methods -- for example using this[Symbol.for('perform-internal-action')]() rather than something like this._performInternalAction()
  • faking classical-OO style "private" methods / properties -- While these aren't technically "private", using regular string-keyed methods/props for "public" api and symbol-keyed methods/props for "private" api is one way to separate "intended public API" vs "implementation-specific" API is an entirely valid use of Symbols in javascript
  • attaching your own metadata to an object you don't control -- while this is not often considered good practice, if you ARE going to do it, it's probably safer to use a Symbol-keyed object
  • "hiding" data from JSON.stringify -- JSON.stringify will skip symbol-keyed entries when serializing objects

There are two ways of constructing a symbol: Symbol("some-name") or Symbol.for("some-name"), the difference being that Symbol.for() will always return the same symbol object when given the same key, while Symbol() always returns a unique symbol object (in other words, Symbol("foo") !== Symbol("foo") while Symbol.for("foo") === Symbol.for("foo")). You can think of this library as Symbol.fuzzyFor().

Namespaces

In order to be clean with Symbol.for(), it is helpful to namespace your keys with a unique prefix. For example, if you have a program named 'crayon box' and you want to define red, green, and blue symbols, you may want to define these as Symbol.for('crayon.box.red'), rather than Symbol.for('red'), to keep your symbols from unintentionally colliding with another color management program.

A helper is provided for creating multiple symbols using the same prefix, accessed by syms[Symbol.for('ns')]("my.namespace") or syms[syms.ns]("my.namespace"):

const syms = require("syms");
const test = Symbol.for("one.two.three.four.five");
const {twoThreeFourFive: check1} = syms[Symbol.for('ns')]('one');
const {THREE_FOUR_FIVE: check2} = syms[Symbol.for('ns')]('one_two');
const {four_five: check3} = syms[syms.ns]('one :: two');
const {kFive: check4} = syms[syms.ns]('one/two/three');
 
assert.equal(test, check1);
assert.equal(test, check2);
assert.equal(test, check3);
assert.equal(test, check4);

Why "." ?

The reason that . is used as a word separator is because modern versions of node use Symbol.for("nodejs.foo.bar") style symbols, allowing things like this:

const syms = require("syms");
 
const sym1 = require("util").inspect.custom;
const sym2 = Symbol.for("nodejs.util.inspect.custom");
const { NODEJS_UTIL_INSPECT_CUSTOM } = syms;
const { nodejsUtilInspectCustom } = syms;
 
// with the `syms[syms.ns](namespaceName)` helper, you can do stuff like this:
const { inspectCustom } = syms[syms.ns]('nodejs.util');
 
// sym1, NODEJS_UTIL_INSPECT_CUSTOM, and nodejsUtilInspectCustom are all the same symbol, which is also the symbol returned by ``
const {strict:assert} = require("assert")
assert.equal(sym1, sym2)
assert.equal(sym1, NODEJS_UTIL_INSPECT_CUSTOM);
assert.equal(sym1, nodejsUtilInspectCustom);
assert.equal(sym1, inspectCustom);
 
class MyThing() {
  [inspectCustom]() {
    return "[[ my-thing instance ]]";
  }
}
 
console.log(require("util").inspect(new MyThing()));

"Internal" Methods

One handy usage is to "clean up" your public/official api, without actually restricting access to the actual underlying methods/properties, which can feel slightly nicer than naming like this._internalThing.

const { SEND, OUT_STREAM } = require("syms");
class Whatever {
  constructor({ out = process.stdout }) {
    this[OUT_STREAM] = out;
  }
  async doSomething() {
    await someOperation();
    this[SEND]();
  }
  [SEND]() {
    this[OUT_STREAM].write("i'm sending you a message, friend, and i hope you receive it swiftly!\n");
  }
}

In the above, you could could change these vars to kSend+kOutStream, or send+outStream, or whatever other format pleases you. You could also use const $ = require("syms") and use this[$.outStream]. Consumers of a Whatever instance (const whatevs = new Whatever({out});) can easily call whatevs[Symbol.for("send")]() and whatevs[Symbol.for("out stream")], without even having to know that you're using this library, and it would hopefully be pretty obvious that they're reaching into your internal / non-official / non-stable APIs and should do so at their own risk:

const out = fs.createWriteStream("/tmp/useless-file.txt");
const _send = Symbol.for('send');
const _outs = Symbol.for('out.stream');
const whatever = new Whatever({out})
whatever[_send]();
whatever[_outs].write("hello");

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