Question

It is occasionally useful to know whether an object was derived from a specific constructor. For this, JavaScript provides a binary operator called instanceof .

console.log(new RTextCell("A") instanceof RTextCell);
// → true
console.log(new RTextCell("A") instanceof TextCell);
// → true
console.log(new TextCell("A") instanceof RTextCell);
// → false
console.log([1] instanceof Array);
// → true

The operator will see through inherited types. An RTextCell is an instance of TextCell because RTextCell.prototype derives from TextCell.prototype . The operator can be applied to standard constructors like Array . Almost every object is an instance of Object .

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.