Back to basics: null vs. undefined

null and undefined are two special values in Javascript used to represent non-values. There are two of such non-values because

1. JavaScript was designed with Java's type system in mind and borrowed null to represent not an object.
2. Early JavaScript handles error silently, leading to unexpected bugs. undefined was added to indicate no such value. This is used when a property/value is expected to happen but didn't show up.

So, with these in mind, let's take a look at where we can find these values.

Values and operators

Since undefined is used to indicate missing property and value, all non-existing variables/properties and uninitialised variables/properties would have a value of undefined. A deleted element also has a value of undefined as a placeholder for the element. (Please note that delete array[index] does not remove the element, merely setting it to undefined)

undefined is also a property on the global object (global in Node environment and window in browser environment).

void operator is used to return undefined for any value provided as the operand. There are a couple of reasons why you would want to use void instead of undefined

1. undefined value could be overshadowed by other values.
2. Before ES5, there was no read-only property, so value of undefined can be overridden.

null, on the other hand, is used where no object is expected. You can find null as the result of Object.getPrototypeOf(Object.prototype)

Types and value

Both undefined and null are primitive values. undefined is a type of itself, while null indicates emptiness

Coercion

undefined represents values that are missing, so when coerced, or converted to numeric type, it will become NaN, whereas null represents no value and when coerced, it will become 0.

Source: http://speakingjs.com/es5/ch08.html