[ES6] Symbol

A symbol is a unique and immutable data type that is often used to identify object properties.

To create a symbol, you write Symbol() with an optional string as its description.

const sym1 = Symbol('apple');
console.log(sym1); // Symbol(apple)

This will create a unique symbol and store it in sym1. The description "apple" is just a way to describe the symbol, but it can’t be used to access the symbol itself.

And just to show you how this works, if you compare two symbols with the same description…

const sym2 = Symbol('banana');
const sym3 = Symbol('banana');
console.log(sym2 === sym3); // false

Example that Symbol can be useful:

const bowl = {
  'apple': { color: 'red', weight: 136.078 },
  'banana': { color: 'yellow', weight: 183.151 },
  'orange': { color: 'orange', weight: 170.097 },
  'banana': { color: 'yellow', weight: 176.845 }
};
console.log(bowl); // Object {apple: Object, banana: Object, orange: Object}

Instead of adding another banana to the bowl, our previous banana is overwritten by the new banana being added to the bowl. To fix this problem, we can use symbols.

const bowl = {
  [Symbol('apple')]: { color: 'red', weight: 136.078 },
  [Symbol('banana')]: { color: 'yellow', weight: 183.15 },
  [Symbol('orange')]: { color: 'orange', weight: 170.097 },
  [Symbol('banana')]: { color: 'yellow', weight: 176.845 }
};
console.log(bowl); // Object {Symbol(apple): Object, Symbol(banana): Object, Symbol(orange): Object, Symbol(banana): Object}

By changing the bowl’s properties to use symbols, each property is a unique Symbol and the first banana doesn’t get overwritten by the second banana.