This is due to the fact that isFunction is defined the following way

function isFunction(value) { // The use of Object#toStringavoids issues with thetypeof operator // in Safari 8-9 which returns 'object' for typed array and other constructors. var tag = isObject(value) ? objectToString.call(value) : ''; return tag == funcTag || tag == genTag; }

But on Safari 10, when you do Object.prototype.toString.call(window.Proxy)
you get "[object Proxy]", which is not equals to funcTag, nor genTag

Note that typeOf seems to work fine on Safari 10 BUT i have not tried on all type of objects