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Symbol Back

Symbol is a primitive data type arisen from ECMAScript 2015(ES6), which purpose is to be an identifier for object properties. It means that it is immutable and unique:

console.log(Symbol() === Symbol()); /** => false */

const obj = {
    id: Symbol(),
};

Just like strings, symbols can also be used as keys for object properties:

const str = 'key';
const symbol = Symbol('key');

const obj = {
    [str]: 'value',
    [symbol]: 'symbol value',
};

Certainly, the member have to be accessed with specific declared symbol:

console.log(obj[symbol]); /** => symbol value */
console.log(obj[Symbol('key')]); /** undefined */

Can we generate an unreachable property in this way?

const obj = {
    [(() => Symbol('key'))()]: 'unreachable value',
};

We cannot access including using for...in operators, except Object.getOwnPropertySymbols().

console.log(obj[Object.getOwnPropertySymbols(obj)[0]]); /** => 'unreachable value' */

For more details around Symbol including its built-in methods, we can check the document from TypeScript or MDN directly.