SOURCE

function _instanceOf(target, origin) {
    if (typeof target !== 'object') return false;
    while(target) {
        if (target.__proto__ === null) return false;
        if (target.__proto__ === origin.prototype) {
            return true;
        }
        target = target.__proto__;
    }
}

var a = _instanceOf('1', String)
console.log(a)