selector-type.js

function invert(initial) {
    return Object.keys(initial)
        .reduce((obj, key) => {
            obj[initial[key]] = key;
            return obj;
        }, {});
}

const SELECTOR_FIRST_CHARS = {
    ID: '#',
    CLASS: '.',
    ATTRIBUTE: '['
};

const SELECTOR_TYPES = invert(SELECTOR_FIRST_CHARS);

const R_SELECTOR = /^[.#[]/;

class SelectorError extends Error {
    constructor(message) {
        super(message);
        this.name = 'SelectorError';
    }
}

class Selector extends String {
    constructor(string) {
        if (!string.match(R_SELECTOR)) {
            throw new SelectorError('Value must be a CSS selector');
        }
        super(string);
        this.type = SELECTOR_TYPES[string.charAt(0)];
    }
    toRaw() {
        let string = this.toString();
        if (this.type === 'ATTRIBUTE') {
            return string;
        }
        return string.slice(1);
    }
}

const idString = new Selector('#my-id');
const classString = new Selector('.my-class');
const attributeString = new Selector('[foo="bar"]');

console.log(idString.toString()); // #my-id
console.log(idString.toRaw()); // my-id
console.log(idString.type); // ID

console.log(classString.toString()); // .my-class
console.log(classString.toRaw()); // my-class
console.log(classString.type); // CLASS

console.log(attributeString.toString()); // [foo="bar"]
console.log(attributeString.toRaw()); // [foo="bar"]
console.log(attributeString.type); // ATTRIBUTE