Need to know whether an object belongs to a constructor’s inheritance chain? `typeof` only reports `"object"` for a constructed instance. `instanceof` gives a more useful test: `object instanceof Constructor`. section 7.2.3
The test is about prototypes, not a permanent record of who created the object. JavaScript checks whether the constructor’s current `prototype` object appears anywhere in the object’s prototype chain. A match returns `true`, whether the prototype is the object’s direct prototype or farther up the chain. section 7.3.2
Inheritance, not imitation
Suppose `Ninja.prototype = new Person()` before `const ninja = new Ninja()`. Both `ninja instanceof Ninja` and `ninja instanceof Person` are true. Merely copying a method from `Person.prototype` onto `Ninja.prototype` would give the object that method, but would not make `ninja instanceof Person` true. The chain matters; a matching method name does not. section 7.3 section 7.3.2
Watch for a moving target: replacing `Ninja.prototype` after creating `ninja` can make a previously true `ninja instanceof Ninja` test false. The operator compares against the constructor’s prototype *now*, while the existing object retains its earlier chain. section 7.3.2
Listing 7.12. Watch out for changes to constructor prototypes
function Ninja(){}
const ninja = new Ninja();
assert(ninja instanceof Ninja, "Our ninja is a Ninja!");
Ninja.prototype = {}; #A
assert(!(ninja instanceof Ninja), "The ninja is now not a Ninja!?"); #B
Run the before-and-after assertions to see a prototype replacement reverse an `instanceof` result for the same object. section 7.3.2
