1. In the first line, you create a variable myArray and you add a value to it. Every variable in Javascript is an object, that has keys and properties (see more: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object)

Between [] you create an array type, that stores data and labels them with numbers. (see more: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array)

2. In the second line, you add a baz property to myArray with "baz" value

3. In the third line, you get the length property of the array

If you want to add a(n) "baz" element to the end of the array, you can use the .push() method. For example: myArray.push("baz");

You need to use Array.prototype.push() to add new elements to array. In your code myArray.baz = "baz" doesn't do anything.

var myArray = [ "foo", 42, "bar" ];

// your approch
myArray.baz = "baz"; 

console.log(myArray.length); // 3
console.log(myArray) // [ "foo", 42, "bar" ]


// correct 
myArray.push("baz")

console.log(myArray.length); // 4
console.log(myArray) // [ "foo", 42, "bar", "baz" ]