undefined means a variable has been declared but has not yet been assigned a value:

var testVar;
console.log(testVar); //shows undefined
console.log(typeof testVar); //shows undefined

null is a assignment value. It can ve assigned to a variable as representation of no value:

var testVar = null;
console.log(testVar); //shows null
console.log(typeof testVar); //shows object

Por esto see entiende queue no son el mismo valor, pero si representan falsy los dos

console.log(null === undefined) // false (not the same type)
console.log(null == undefined) // true (but the "same falsy value")
console.log(null === null) // true (both type and value are the same)

Como extra:

null = 'value' // Uncaught SyntaxError: invalid assignment left-hand side
undefined = 'value' // 'value'