JavaScript "undefined" vs "null"

1// "undefined" means this value does not exist. typeof returns "undefined"
2// "null" means this value exists with an empty value
3
4typeOf(null) // => object
5typeOf(randomKey) // => undefined
6
7// All JS falsy values
8x = false;
9x = 0;
10x = “";
11x = null;
12x = undefined;
13x = NaN (Not A Number)
14
15// All JS primitive values
16typeof true; // => Boolean
17typeof x=null; // => Null
18typeof b; // => Undefined
19typeof 3; // => Number
20typeof 'sample'; // => String
21typeof Symbol("foo"); // => Symbol
22
23// Objects/Functions/Arrays
24typeof {}; // => Object
25...
26
27// Warning note for default parameters
28let sendTelemetryData = (x = 11111) => {
29  console.log(x);
30}
31sendTelemetryData(333); // => 333
32sendTelemetryData(null); // => null   !!!
33sendTelemetryData(); // => 11111
34sendTelemetryData(undefined); // => 11111
35