symbol is a primitive data type in JavaScript. The function Symbol() returns a value of type symbol which is always unique. symbol is used for creating identifiers (because of their unique nature) and mostly as object properties.

Creating a symbol

let symbol1 = Symbol();
let symbol2 = Symbol("text");

🚨 Symbol() does not have a constructor and cannot support new Symbol(). Doing so will result in an error.

let symbol1 = new Symbol(); // Symbol is not a constructor

When we create a symbol by passing a string, it does not make the string as a symbol. It still generates a new symbol every time. The description is just a label that doesn’t affect anything.

console.log(Symbol("text") === Symbol("text")); // false

Using symbol as an object property

We use square bracket notation to use a symbol as an object property.

let userId = Symbol("id");

let employee = {
    name: "Parwinder",
    [userId]: 727
}

console.log(employee[userId]); // 727

// above log is not similar to using the string literal "userId"
console.log(employee["userId"]); // undefined

Creating hidden properties with symbol

We can create hidden properties on objects that no one can modify or overwrite.

let car = {
    name: "BMW"
};

let hiddenField = Symbol("price");

car[hiddenField] = 70000;

console.log(car); // { name: 'BMW', [Symbol(price)]: 70000 }
console.log(car[hiddenField]); // 70000

Creating hidden property on an object using symbol could have significant advantages in the right situations.

1. The hidden property will not impact any other module, library or user using the car object as they won't be able to see it.
2. If someone else wants a hidden property on the object, it will not conflict with yours due to uniqueness.

Even though symbol might seem hidden in an object but we have Object.getOwnPropertySymbols() that allows us to get all symbols in an object.

let car = {
    name: "BMW"
};

let hiddenField = Symbol("price");
let anotherHiddenField = Symbol("release");

car[hiddenField] = 70000;
car[anotherHiddenField] = "07/18/2020";

console.log(Object.getOwnPropertySymbols(car)); // [ Symbol(price), Symbol(release) ]

Gotcha: Symbols do not show up in for..in.

let car = {
    name: "BMW",
    model: "Cooper",
    color: "Pearl White"
};

let hiddenField = Symbol("price");
let anotherHiddenField = Symbol("release");

car[hiddenField] = 70000;
car[anotherHiddenField] = "07/18/2020";

console.log(car[hiddenField]); // 70000
console.log(car[anotherHiddenField]); // 07/18/2020

for (let properties in car) {
    console.log(properties); // name, model, color
}