Used to express 'nothing' in JavaScript.

If we declare a variable, and don't set anything to it, then it has value undefined.

  let dog;
  console.log(dog); // undefined

When we try to access a variable that is created but not defined/set a value, we get undefined.

null: value of nothing

  const somethingNull = null;
  // we have explicitly set the value to be nothing with null