Range.js

/*
 * A Range object represents a range of numbers {x: from <= x <= to}
 * Range defines a has() method for testing whether a given number is a member
 * of the range. Range is iterable and iterates all integers within the range.
 */
class Range {
    constructor (from, to) {
        this.from = from;
        this.to = to;
    }

    // Make a Range act like a Set of numbers
    has(x) { return typeof x === "number" && this.from <= x && x <= this.to; }

    // Return string representation of the range using set notation
    toString() { return `{ x | ${this.from} ≤ x ≤ ${this.to} }`; }

    // Make a Range iterable by returning an iterator object.
    // Note that the name of this method is a special symbol, not a string.
    [Symbol.iterator]() {
        // Each iterator instance must iterate the range independently of
        // others. So we need a state variable to track our location in the
        // iteration. We start at the first integer >= from.
        let next = Math.ceil(this.from);  // This is the next value we return
        let last = this.to;               // We won't return anything > this
        return {                          // This is the iterator object
            // This next() method is what makes this an iterator object.
            // It must return an iterator result object.
            next() {
                return (next <= last)   // If we haven't returned last value yet
                    ? { value: next++ } // return next value and increment it
                    : { done: true };   // otherwise indicate that we're done.
            },

            // As a convenience, we make the iterator itself iterable.
            [Symbol.iterator]() { return this; }
        };
    }
}

for(let x of new Range(1,10)) console.log(x); // Logs numbers 1 to 10
[...new Range(-2,2)]                          // => [-2, -1, 0, 1, 2]