Why does javascript output the same, even when I negate the condition? [duplicate]

Question

I am a student, and I am doing a project for school. While I was working on it, I encountered a problem and discovered something weird.

I needed to check if a something is null or a empty string, but for some reason it didn't work. Here's the code snippet: if(!data.data == null && !data.data == 'null' && !data.data == ''){ // Some code here... }

It didn't work, so I tried this:

console.log(!data.data == null) console.log(!data.data == 'null') console.log(!data.data == '')

and when the data.data wasn't empty, the output was this:Console output: false false true

So i change it to:

console.log(data.data == null) console.log(data.data == 'null') console.log(data.data == '')

(the data.data value is the same) the output changed to this:Console output: false false false

So I tried to test this in consoleJavascript console

Here's it copied:

var data = 'aaa' console.log(data) aaa console.log(data == null) false console.log(!data == null) false console.log(!data == '') true console.log(data == '') false console.log(!data === null) false console.log(data === null) false data = null null console.log(data === null) true console.log(!data === null)

So the question is, When the value is not null, why does it always output false, even when I negate it, but when the value is null, negating it works?

PS: I am sorry if this is a stupid question, or not too clear, but I am not that good, and really confused right now

basically !data is not part of operator == , so you are negating the var only. which is true and true == null is false — Ahmed Sunny
– Ahmed Sunny, Commented Oct 17, 2024 at 15:00
Maybe you meant !(data == null) instead of !data == null? — 001
– 001, Commented Oct 17, 2024 at 15:04
evolutionxbox yes, I am sorry for the confusion, but I am still confused why it works with == '' — Typek834
– Typek834, Commented Oct 17, 2024 at 15:05
!"aaa" is false and In javascript, is an empty string always false as a boolean? — 001
– 001, Commented Oct 17, 2024 at 15:10

Tim Lewis · Accepted Answer · 2024-10-17 15:38:09Z

The fundamental issue here is where you are putting the exclamation marks in your code.

When you write !data == null, javascript does two things:

It evaluates the expression !data - which will return false if data is truthy (generally speaking - if it has a value that is not false or zero). It applies the logical NOT operator
Then, it takes that value and checks if it equals null. It will never equal null - because the expression !data will be a boolean - either true or false - but NEVER null.

It will do the first action (negating the value) first per JavaScript's operation precedence. You could reverse that precedence by writing !(data == null), which ill give a result more similar to what you are trying to do.

What you probably meant to do was data != null - which will check wether data is not equal null. This is called the inequality operator.

developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… is relevant here

VietTN · Accepted Answer · 2024-10-17 15:10:12Z

You need to understand how data behaves when it has a value, such as data = 'aaa'. In this case, data will hold the value 'aaa'. However, if you check !data, it will evaluate whether data has a value; if there is no value, !data will return false.

When you use the comparison operator == with false, all values like 0, "0", "" will be coerced into false. Therefore, when comparing 0 == false or "" == false, the result will be true.

When you use the strict equality operator ===, it not only compares the values but also checks the data types.

"it will evaluate whether data has a value;" no, just checks truthyness and inverses it
"if there is no value, !data will return false" not at all. Try data = "aaa"; console.log(!data)
"all values like 0, "0", """ incomplete - like what? What is the common thing between them and how would one know what other values belong to this group?

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