Checking for existance

If all these things are True Run the function

  if ((obj instanceof Object) &&
      (Array.isArray(obj[key])) &&
      (obj.hasOwnProperty(key)) &&
      (obj[key].length !== 0)) {

instanceof Object

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/instanceof

typeof

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof

if (obj instanceof Object && obj[key] instanceof Array && obj[key] != []) {};

Array.isArray()

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray

Array.isArray([1, 2, 3])
var animals = ['tapir','giraffe', 'ostrich']
Array.isArray(animals)

var a;
a = 123;
console.log(typeof a); // 'number'
a = "text string";
console.log(typeof a); // 'string'
a = [];
console.log(typeof a); // 'object'
a = {};
console.log(typeof a); // 'object'
a = true;
console.log(typeof a); // 'boolean'
a = null;
console.log(typeof a); // 'object'
a = undefined;
console.log(typeof a); // 'undefined'
a = function(){};
console.log(typeof a); // 'function'
a = new Date();
console.log(typeof a); // 'object'

https://stackoverflow.com/questions/5515310/is-there-a-standard-function-to-check-for-null-undefined-or-blank-variables-in

You can just check if the variable has a truthy value or not. That means

if( value ) {
}
will evaluate to true if value is not:

null
undefined
NaN
empty string ("")
0
false

Other stuff

var obj = {
    first: "John", 
    last: "Doe"
};

//  Visit non-inherited enumerable keys

Object.keys(obj).forEach(function(key) {

    console.log(key, obj[key]);

});

JavaScript Properties

Properties are the values associated with a JavaScript object. A JavaScript object is a collection of unordered properties. Properties can usually be changed, added, and deleted, but some are read only.