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Thanks Bento :)

To me as a developer something that is prefixed with is, will most likely be a boolean - Here, isPercentage() is a boolean function which tells whether a value is a percentage or not. For that, we are checking if val is in the range of 0-100 & it is the only validation we doing here.

In case we would have more validations and function starts to get bigger then we can create separate functions for that with the names like the one which you made checkInRange() and others like isNegative() etc.

Apoorv Tyagi

Indeed isPercentage() is but isPercentage isn't. In my opinion functions should be named to the actions they are taking and not what their return values will be, for example getUser, checkNaming, otherwise if you name the function isPercentage what will you name the return value?

function isPercentage(val) {
  var isInRange = (val >= 0 && val <= 100);
  return isInRange;
} 

const ??? = isPercentage(9);

So the naming for the function above is bad 👎 as it implies a boolean type but is a function.

Also, that function does not check if a value is a percentage, I'd expect that if a function is called isPercentage it would test if a string contains a value like % or a logic around it.

Naming a function like this will lead to confusion and the point of the article is to leave things clean and with good practices, you advocate good practices and explain them in other points but this is not one of them.

There's no such thing as boolean functions, it's just a function that returns a boolean.

typeof(isPercentage) is function and not boolean.