What is a Symbol used for in JavaScript? (ES6)

Question:

Reading the MDN documentation where it talks about the Symbol primitive data, no matter how much I read and reread, I don't understand what would be a use case for said element other than to iterate Symbol.iterator .

The documentation says that its use is for debugging purposes, but even with that I don't understand why use a Symbol to debug if console.log() or debugger exists.

I understand that it is a primitive data and therefore immutable. In the examples I have seen that they do something like the following:

var sym = Symbol('Llave'),
    obj = {
        [sym]: 'valor'
    };

console.log(obj[sym]); // valor
console.log(obj['Llave']); // undefined

Why use a Symbol ? if a traditional key (key: value) can be used.

Sources:

• Symbol-JavaScript | mdn
• lukehoban/es6features GitHub
• ECMAScript 6 — New Features: Overview & Comparison

Answer:

Symbols are unique data types that are immutable and can be used as object property identifiers. These are like the primitive Number , String , and Boolean types.

To create it, it is done as follows, without using the word new, since the Symbols have a Symbol function which is used to create them;

var sym1 = Symbol();
var sym2 = Symbol("foo");
var sym3 = Symbol("foo");

As I commented above, being unique types, it will create a new Symbol which will not be another:

Symbol("foo") === Symbol("foo"); // false

About debuging These are considered powerful, since they have a description, which is used only for debugging to make life easier when debugging by console:

console.log(Symbol('foo')); // prints "Symbol(foo)" to the console.  
assert(Symbol('foo').toString() === 'Symbol(foo)');  

What are they good for?:

1. To create and store values ​​like integer and string that will not change.

2. It can be used to store custom metadata for objects, which are children of the current object.

Conclusion:

They are small constants that have some extra properties that allow us to work better in debug and save unique values.

A good resource is this page where there is an example, and they explain much more extensively what symbols are.