Monotonic sequence that is also its current value (in ES6)

index.js

'use strict'; 

class SequencedNumber extends Number {
    advance() { this._next = this._current + 1; }
    
    constructor (initial) {
        super(initial);
        this._current = initial;
        this._next = initial;
    }
    
    valueOf() { return this._current; }
    
    next() {
        this._current = this._next;
        this.advance();
        return { done: false, value: this._current };
    }
    
    [Symbol.iterator]() { return this; }
}

let square = (x) => x * x;

module.exports = Array.from(function* () {
    let i = 0, sn = new SequencedNumber(3);
    for (let n of sn) {
        if (i++ >= 5) break;
        yield [square(n), square(sn)];
    }
}());