Before understanding the differences between undefined and null we must understand the similarities between them.

• They belong to JavaScript's 7 primitive types.

 let primitiveTypes = ['string','number','null','undefined','boolean','symbol', 'bigint'];

• They are falsy values. Values that evaluated to false when converting it to boolean using Boolean(value) or !!value.

   console.log(!!null); //logs false
   console.log(!!undefined); //logs false

   console.log(Boolean(null)); //logs false
   console.log(Boolean(undefined)); //logs false

Ok, let's talk about the differences.

• undefined is the default value of a variable that has not been assigned a specific value. Or a function that has no explicit return value ex. console.log(1). Or a property that does not exist in an object. The JavaScript engine does this for us the assigning of undefined value.

  let _thisIsUndefined;
  const doNothing = () => {};
  const someObj = {
    a : "ay",
    b : "bee",
    c : "si"
  };

  console.log(_thisIsUndefined); //logs undefined
  console.log(doNothing()); //logs undefined
  console.log(someObj["d"]); //logs undefined

• null is "a value that represents no value". null is value that has been explicitly defined to a variable. In this example we get a value of null when the fs.readFile method does not throw an error.

  fs.readFile('path/to/file', (e,data) => {
     console.log(e); //it logs null when no error occurred
     if(e){
       console.log(e);
     }
     console.log(data);
   });

When comparing null and undefined we get true when using == and false when using ===. You can read the reason here.

   console.log(null == undefined); // logs true
   console.log(null === undefined); // logs false