javascript - Check if "instanceof" is going to fail -

function a(){} a.prototype = "foo bar";  new a() instanceof a; // typeerror: function has non-object prototype 'foo bar' in instanceof check 

as can see, if prototype of constructor not object, fail , throw error. there way make sure instanceof not fail?

typeof new a().constructor.prototype === "object" 

and

typeof object.getprototypeof(new a()) === "object" 

apparently not work.

the error says a.prototype needs object, should check that:

function isobject(x) {     return x != null && (typeof x == "object" || typeof x == "function"); } 

but isobject(a.prototype) not can assert instanceof call won't throw. going spec, should test

function allowsinstancecheck(c) {     try {         if (!isobject(c)) return false;         var m = c[symbol.hasinstance];         if (m != null) return typeof m == "function";         if (typeof c != "function") return false;         return isobject(c.prototype);     } catch (e) {         // of property accesses threw         return false;     } }