Symbol Predicates Proposal

Status

Stage 2

Champions: Ashley Claymore, Jordan Harband

Authors: Robin Ricard, Ashley Claymore, Jordan Harband

Motivation

A proposal to introduce ways to differentiate symbols.

This proposal adds the following predicates:

• Symbol.isRegistered(symbol)
• Symbol.isWellKnown(symbol)

Not all symbols are the same and knowing more about what they are can be useful, notably for libraries.

Knowing if a symbol is truly unique, is forgeable (registered) or is shared across realms (well-known), can be important depending on the use case.

For instance Symbols as WeakMap keys require symbols to not be registered.

Use Cases

You can detect in a library if a symbol can be used as a WeakMap key:

function isWeakMapKey(key) {
  switch (typeof key): {
    case "object":
      return key !== null;
    case "function":
      return true;
    case "symbol":
      return !Symbol.isRegistered(sym);
  }
  return false;
}

isWeakMapKey({}); // true
isWeakMapKey(Symbol()); // true
isWeakMapKey("foo"); // false
isWeakMapKey(Symbol.for("foo")); // false
isWeakMapKey(Symbol.asyncIterator); // true

You can also find out if you are being given a truly unique symbol:

const isUniqueSymbol = sym => typeof sym === "symbol" && !(Symbol.isRegistered(sym) || Symbol.isWellKnown(sym));

isUniqueSymbol(Symbol()); // true
isUniqueSymbol(Symbol.for("foo")); // false
isUniqueSymbol(Symbol.asyncIterator); // false
isUniqueSymbol({}); // false

Description

Symbol.isRegistered(value)

Takes an unknown value as the only argument, returns a boolean: true if the symbol is registered, false otherwise.

Symbol.isWellKnown(value)

Takes an unknown value as the only argument, returns a boolean: true if the symbol is one of the well known symbols as defined by ECMA262 & ECMA402, false otherwise.

Implementations

Note that the two predicate packages belo are not polyfills; but they precisely match the current spec text, so they represent valid implementations of the proposal.

• Symbol.isRegistered(symbol): https://github.com/inspect-js/is-registered-symbol
• Symbol.isWellKnown(symbol): https://github.com/inspect-js/is-well-known-symbol

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Maintain the proposal repo

1. Make your changes to spec.emu (ecmarkup uses HTML syntax, but is not HTML, so I strongly suggest not naming it ".html")
2. Any commit that makes meaningful changes to the spec, should run npm run build and commit the resulting output.
3. Whenever you update ecmarkup, run npm run build and commit any changes that come from that dependency.