null !== undefined

I have noticed that many developers struggle with the appropriate use of "null" and "undefined" in JavaScript.

"Null" refers to the absence of any object value. It is primarily used when an object is not present.

console.log(typeof null) // 'object'

"Undefined" is used when something is not defined. If any attribute in an API is missing, it will be considered as "undefined".

let i;
console.error(typeof i) // undefined

Note: It is important to always compare attribute values with "undefined" when processing API responses.

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Comments

[Julien Dephix]

Hi.

It's tricky indeed.

I would suggest using back ticks when talking about undefined because typeof returns string "undefined" and not undefined. Beginners might struggle on that too.

let i;
console.log(typeof i);  // "undefined" and not undefined

So when you say one should compare attribute values to "undefined" I think you should say: compare with undefined:

const myAPIResponse = {id: 12, name: "Jools"}

if (myAPIResponse.status === undefined) {
    // bla bla
}

Above example is not much but it shows how to use undefined.

[Kaamkiya]

This is pretty good.

The best explanation I have ever seen: