Symbol

A symbol is a unique and immutable data type and may be used as an identifier for object properties

Why use Symbol

See Inversify page Why use symbol over string for concrete example.

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Symbol.keyFor()

The Symbol.keyFor() static method retrieves a shared symbol key from the global symbol registry for the given symbol

// Global symbol
const globalSym = Symbol.for('foo');
console.log(Symbol.keyFor(globalSym)); // "foo"

// Local symbol
const localSym = Symbol();
console.log(Symbol.keyFor(localSym)); // undefined