CODEWITHSUNDEEP
javascriptcomparisonequality
Alan Kis
Alan Kis

Reputation: 1910

Comparison operators !== against 0

I think that it is obvious what my code does.

Why does my code return a whole string if I use the !== operator? I know that arrays in Javascript start from at index 0, and here I'm entering the whole filename as the argument, so indexOf(".") will always be greater then 0. No, I'm not passing an .htaccess file here.

function getFileExtension(i) {

    // return the file extension (with no period) if it has one, otherwise false
   if(i.indexOf(".") !== 0) { // 
       return i.slice(i.indexOf(".") + 1, i.length);
   } else {
       return false;
   }
}

// here we go! Given a filename in a string (like 'test.jpg'),

getFileExtension('pictureofmepdf'); return given string
// both operand are same type and value

But if I change comparasion to 

(i.indexOf(".") > 0) // logs false

P.S. I case that you are asking, this is form usvsth3m.

Upvotes: 0

Views: 113

Answers (3)

Alan Kis
Alan Kis

Reputation: 1910

Well, to simplify, I omit that indexOf returns index which is typeof number, or -1, not returning boolean value FALSE in case when given value not found. So in case of comparing -1 to 0, result is true, and that's why I actually geting outputed given string, instead false. Well, MDN is in my bookmark bar now

var z = -1;
console.log(z >= 0); // evaluates false because -1 < 0
console.log(z !== 0); // evaluates true because -1 !== 0
// also if z > 0 it is !== 0, but could be < 0

So next code works like a charm.

function getFileExtension(i) {

    // i will be a string, but it may not have a file extension.
    // return the file extension (with no period) if it has one, otherwise false
   if(i.indexOf(".") >= 0) {
       return i.substring(i.indexOf(".") + 1, i.length);
   } else {
       return false;
   }
}

getFileExtension('pictureofmepdf');

Upvotes: 1

Bucket
Bucket

Reputation: 7521

indexOf() returns the index of the substring, so it can return 0, which would mean that the substring appears at position 0. If the substring is not found, it returns -1 instead, so change your if statement to reflect this logic:

if(i.indexOf(".") >= 0)

Additionally, you should use substring() to extract a substring from a string - slice() is for arrays.

return i.substring(i.indexOf(".") + 1, i.length);

Still, I think a better way to do this is with split():

var fileNameArray = i.split("."); // "foo.txt" --> ["foo" "txt"]
if(fileNameArray.length >= 2) {
    return fileNameArray[1];
} else {
    return false; //maybe you want to return "" instead?
}

Upvotes: 2

Andr&#233;s Torres
Andr&#233;s Torres

Reputation: 733

The String method indexOf returns (if it is founded) the first index of the string you search, and remember, index can be zero, that why you have to do a strict comparision to check if indexOf it is not returning a boolean false.

I will suggest you to use lastIndexOf for this case because a file named as something.min.js will return min.js as an valid extension, and nope.

Upvotes: 2

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