How exactly does Javascript instanceof work? Is it slow style?

Question

How does the performance of instanceof fair for \"huge libraries\"?

Does it travel up the prototype chain one by one, similar to this?

Answer 1

According to what Felix Kling quoted, all that instanceof does (excluding the error checks) is to check whether the prototype property(which has to be an object) of the Function can be found somewhere down the prototype chain

person instanceof Object
// ROUGHTLY does
return (
     person.__proto__==Object.prototype
  || person.__proto__.__proto__==Object.prototype
  || ... );

Here's some pseudocode:

person instanceof Person
//ROUGHTLY equals
person.instanceOf(Person)

person.instanceOf = function(Person) {
    if(typeof Person!='object') throw new TypeError;
    if(!([[HasInstance]] in Person)) throw new TypeError;
    return Person.[[HasInstance]](this /* person */)
}


Person.[[HasInstance]] = function(V) {
    if(typeof V!='object') return false;
    var O = this.prototype;
    if(typeof O!='object') throw new TypeError;
    while(true) {
        V = V.__proto__; // [[prototype]] (hidden) property
        if(V==null) return false;
        if(V==O) return true;
    }
}