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

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

Tags:

2

Answers


  1. 0

    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.

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  2. 0

    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.

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