Numbas are kinda ranges. Similar to interval arithmetics. https://en.wikipedia.org/wiki/Interval_arithmetic
Here is an example:
var myNumba1 = new Numba(1, 6); // [1..6]
var myNumba2 = new Numba(-4, 3); // [-4..3]
console.log(Numbas.add(myNumba1, myNumba2).toString());
// [-3..9]
console.log(myNumba1.multiply(myNumba2).toString());
// [-4..18]
if (Numbas.gte(myNumba1, myNumba2)) console.log(myNumba1 + ' is greater than or equal to ' + myNumba2);
// [-4..18] is greater than or equal to [-4..3]
if (Numbas.in(myNumba2, myNumba1)) console.log(myNumba2 + ' is in ' + myNumba1);
// [-4..3] is in [-4..18]
console.log(myNumba1.add(1).toString());
// [-3..19]
console.log(myNumba1.diff());
// 22
myNumba2.min = Number.NEGATIVE_INFINITY;
myNumba2.max = Number.POSITIVE_INFINITY;
console.log(myNumba2.toString());
// [-Infinity..Infinity]