symbols2.js

let obj = {}

const symbol1 = Symbol("a")
const symbol2 = Symbol("a")

const sharedSymbol = Symbol.for('b')

obj[symbol1] = 'a'
obj[sharedSymbol] = 'b'
obj['c'] = 'c'
obj.d = 'd'

obj[symbol1] // 'a'
obj[symbol2] // undefined, symbol1 and symbol2 are two different symbols, "a" it's just a description and not an identifier 

obj[sharedSymbol] // 'b'
obj[Symbol.for('b')] // 'b', Symbol.for access the symbol in the symbol registry, so the symbol it's always the same
                     // in this case 'b' it's used as unique key and not just as description

for (let i in obj) {
   console.log(i)  // logs "c" and "d", symbols are not enumerable in for...in so they are not gonna be printed 
}

for(let i of Object.getOwnPropertySymbols(obj) ) {
   console.log(obj[i]); // instead you can use getOwnPropertySymbols that returns an array of Symbols
}

JSON.stringify(obj) // {c: 'c', d: 'd'}, Symbol-keyed properties will be completely ignored when using JSON.stringify()
                    // you could use this a trick to save only same values in a DB and leave the others locally