Overview

• “Symbol” value represents a unique identifier.
  • Symbols are guaranteed to be unique.
  • Even if we create many symbols with the same description, they are different values.
  • The description is just a label that doesn’t affect anything.
• A value of this type can be created using Symbol().

let id1 = Symbol();
let id2 = Symbol("id"); // id2 is a symbol with the description "id"
console.log(id1 == id2); // false

• Most values in JavaScript support implicit conversion to a string.
  • Symbols are special and they don’t auto-convert.
  • If we really want to show a symbol, we need to call toString() on it.
  • That’s a “language guard” against messing up, because strings and symbols are fundamentally different and should not occasionally convert one into another.
• Symbols allow us to create “hidden” properties of an object, that no other part of code can occasionally access or overwrite.
  • For instance, if we want to store an “identifier” for the object user, we can use a symbol as a key for it.
  • If we want to use a symbol in an object literal, we need square brackets.

let id = Symbol("id");

let user = { 
	name: "John" 
};
user[id] = "ID Value";

let user = {
  [id]: 1000,
  name: "John"
};

• Symbolic properties do not participate in for..in loop.
  • That’s a part of the general “hiding” concept.
  • In contrast, Object.assign copies both string and symbol properties.
  • The idea is that when we clone an object or merge objects, we usually want all properties to be copied.
• We can only use strings or symbols as keys in objects.
  • When other data types are used for keys, they are converted to strings.

let obj = {
  1: "test" // same as "1": "test"
};
console.log(obj["1"]);
console.log(obj[1]);

Global Symbol Registry

• Usually all symbols are different, even if they have the same names but sometimes we want same-named symbols to be same entities.
• With global symbol registry, we can create symbols in and access them later.
  • It guarantees that repeated accesses by the same name return exactly the same symbol.
  • In order to create or read a symbol in the registry, use Symbol.for(key).
  • That call checks the global registry, and if there’s a symbol described as key, then returns it.
    • Otherwise creates a new symbol Symbol(key) and stores it in the registry by the given key.
• Symbols inside the registry are called global symbols.

// Read from the global registry
let id = Symbol.for("id"); // if the symbol did not exist, it is created
let idAgain = Symbol.for("id"); // Read it again
console.log(id === idAgain); // true

Symbol.keyFor

• There’s a reverse call: Symbol.keyFor(sym), that returns a name by a global symbol.

let sym1 = Symbol.for("name");
let sym2 = Symbol.for("id");

console.log(Symbol.keyFor(sym1)); // name
console.log(Symbol.keyFor(sym2)); // id

• The Symbol.keyFor internally uses the global symbol registry to look up the key for the symbol.
  • So it doesn’t work for non-global symbols.
  • If the symbol is not global, it won’t be able to find it and return undefined.

console.log(Symbol.keyFor(Symbol.for("name"))); // name, global symbol
console.log(Symbol.keyFor(Symbol("name2"))); // undefined, the argument isn't a global symbol