Span.js

// This is the constructor function for our subclass
function Span(start, span) {
    if (span >= 0) {
        this.from = start;
        this.to = start + span;
    } else {
        this.to = start;
        this.from = start + span;
    }
}

// Ensure that the Span prototype inherits from the Range prototype
Span.prototype = Object.create(Range.prototype);

// We don't want to inherit Range.prototype.constructor, so we
// define our own constructor property.
Span.prototype.constructor = Span;

// By defining its own toString() method, Span overrides the
// toString() method that it would otherwise inherit from Range.
Span.prototype.toString = function() {
    return `(${this.from}... +${this.to - this.from})`;
};