Discussion on: The Difference Between Null and Undefined in JavaScript

[Zahra Khan]

This is incredible feedback and so good to learn!

Yeah I've watched some videos on the topic where it's best practice to use undefined over null but I love your explanation!

Thank you!

[peerreynders]

One caveat about undefined:

const obj = {
  nil: null,
  nothing: undefined,
};

console.log(obj.nil == undefined);          // true  (non-strict equality)
console.log(obj.nothing === undefined);     // true
console.log(obj.zilch === undefined);       // true

console.log(obj.hasOwnProperty('nil'));     // true
console.log(obj.hasOwnProperty('nothing')); // true
console.log(obj.hasOwnProperty('zilch'));   // false

const array = [null, ,undefined];

console.log(array[0] == undefined);         // true  (non-strict equality)
console.log(array[1] === undefined);        // true
console.log(array[2] === undefined);        // true
console.log(array[3] === undefined);        // true

console.log(array.hasOwnProperty(0));       // true  (value `null`)
console.log(array.hasOwnProperty(1));       // false (hole, empty)
console.log(array.hasOwnProperty(2));       // true  (value `undefined`)
console.log(array.hasOwnProperty(3));       // false (out of bounds)

An undefined value is inconclusive with regards to the existence of the property/element within an object/array.

Also note that browser developer tools will mark "holes" in arrays with empty.

Chromium:

> console.log([0,,2]);
    (3) [0, empty, 2]

Firefox:

> console.log([0,,2]);
    Array(3) [ 0, <1 empty slot>, 2 ]

ECMAScript 6: holes in Arrays

---

The void operator returns undefined.

let count = 0;
const result = void(count += 1);
console.log(count);  // 1
console.log(result);  // undefined