Symbol is a new primitive type in ES6. Symbols are used mainly as property keys, and one of its main characteristics is that they are unique, even if they have the same description. This means they will never have a name clash with any other property key that is a symbol or string.

const MY_PROP_KEY = Symbol();
const obj = {};

obj[MY_PROP_KEY] = "ABC";
console.log(obj[MY_PROP_KEY]); 

In this example, the result of console.log would be ABC.

You can also have named Symbols like:

const APPLE    = Symbol('Apple');
const BANANA   = Symbol('Banana');
const GRAPE    = Symbol('Grape');

Each of these values are unique and cannot be overridden.

Providing an optional parameter (description) when creating primitive symbols can be used for debugging but not to access the symbol itself (but see the Symbol.for() example for a way to register/lookup global shared symbols).

Unlike most other JavaScript objects, symbols are not automatically converted into a string when performing concatenation.

let apple = Symbol('Apple') + ''; // throws TypeError!

Instead, they have to be explicitly converted into a string when necessary, (for example, to get a textual description of the symbol that can be used in a debug message) using the toString method or the String constructor.

const APPLE = Symbol('Apple');
let str1 = APPLE.toString(); // "Symbol(Apple)"
let str2 = String(APPLE);    // "Symbol(Apple)"

The Symbol.for method allows you to register and look up global symbols by name. The first time it is called with a given key, it creates a new symbol and adds it to the registry.

let a = Symbol.for('A');

The next time you call Symbol.for('A'), the same symbol will be returned instead of a new one (in contrast to Symbol('A') which would create a new, unique symbol that happens to have the same description).

a === Symbol.for('A') // true

but

a === Symbol('A') // false