7.3 Operations on Objects https://tc39.github.io/ecma262/#sec-operations-on-objects 7.3.9 GetMethod(V, P) https://tc39.github.io/ecma262/#sec-getmethod
function GetMethod(V, P) {
            var func = V[P];
            if (func === undefined || func === null)
                return undefined;
            if (!IsCallable(func))
                throw new TypeError();
            return func;
        }