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Mohd Ahshan Danish
Mohd Ahshan Danish

Posted on • Edited on

null !== undefined

#javascript

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

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Top comments (2)

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joolsmcfly profile image
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
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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
}
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Above example is not much but it shows how to use undefined.

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kaamkiya profile image
Kaamkiya •

This is pretty good.

The best explanation I have ever seen:

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