callback instanceof Function

typeof callback == "function"

alert(typeof undefinedVariable); // alerts the string "undefined"
alert(undefinedVariable instanceof Object); // throws an exception

var myNullVar = null;
alert(typeof myNullVar ); // alerts the string "object"
alert(myNullVar  instanceof Object); // alerts "false"

function isFunction(obj) {
  return typeof(obj) == "function";
}
function isArray(obj) {
  return typeof(obj) == "object"
      && typeof(obj.length) == "number"
      && isFunction(obj.push);
}

var str = 'hello word';

str instanceof String   // false

typeof str === 'string' // true

var ClassFirst = function() { };
var ClassSecond = function() { };
var instance = new ClassFirst();
console.log(typeof instance); //returns object
console.log(typeof instance == 'ClassFirst'); //obviously this one is false
console.log(instance instanceof Object); //true
console.log(instance instanceof ClassFirst); //true
console.log(instance instanceof ClassSecond); //false

callback instanceof Function