Singleton Pattern - sgml/signature GitHub Wiki
The language parser is a singleton controller, the symbol table is a singleton model, the language runtime is a singleton view. So overloading or overriding the global or local symbols with delegation, decoration, or memoization is a temporary extension to the language itself.
ES6 has a global resource for creating symbols: the symbol registry. The symbol registry provides us with a one-to-one relationship between strings and symbols. The registry returns symbols using Symbol.for( key ).
Symbol.for( key1 ) === Symbol.for( key2 ) whenever key1 === key2. This correspondance works even across service workers and iframes.
Symbols can be given a description, which is really just used for debugging to make life a little easier when logging them to a console: var symbol = Symbol.for('foo')
Symbol.toPrimitive is used when the JavaScript engine needs to convert your Object into a primitive value. There is one really bad feature of all this, which is that ToPrimitive does not enforce any type-checking on the return values, other than that they are primitive
Given a symbol symbol, Symbol.keyFor(symbol) returns the key that was associated with symbol when the symbol was added to the global registry.
console.log(Symbol.keyFor(symbol))
Symbol.iterator also lets developers redefine navigation patterns without cloning the object or modifying its order in-place.
These patterns can then be written once in a function, class, or module and used anywhere. That means you can loop through an Array forwards, backwards, randomly, or infinitely in a modular fashion.
function addPointOrVector(a, b) {
if (a instanceof Point) a = new Vector([a])
if (b instanceof Point) b = new Vector([b])
return new Vector(...a.points, ...b.points])
}
class Vector() {
constructor(points) { ... }
get [Symbol.operator('+')]() { return addPointOrVector }
}
class Point() {
constructor(x, y) { this.x = x, this.y = y }
get [Symbol.operator('+')]() { return addPointOrVector }
}
Point(1, 0) + Point(1, 1) == Vector([[1,1], [1,0]])
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