Primitive value returned from constructor will be lost

Question

I created a function:

function CheckHidden(el){ return $(el).css('display')!='none' }

My IDE warns me that:

Primitive value returned from constructor will be lost when called with 'new'

Actually when I call it like this:

var all = $("#catalog-body > div").filter(function(){return  CheckHidden(this)})

it doesn't work and doesn't reduce set of elements to unhidden ones. Please explain to me the issue. I have a giant lack of knowledge.

There are multiple questions in one. Which one describes your problem best? — feeela, Oct 13 '14 at 11:14
I'm not sure what IDE you're using, but this is specific to the IDE and not to JavaScript. If you have a method that you never instantiate with new then disregard the warning. — Konstantin Dinev, Oct 13 '14 at 11:24

score 86 · Accepted Answer · answered Apr 23 '15 at 12:35

86

I ran into this warning myself and if you want to know the cause, it is because your IDE expect function names to start with a lowercase letter. Since your function is called CheckHidden with a capital C, it thinks it's a class declaration.

However, you should still use jQuery's :visible selector to fix your specific issue.

answered Apr 23 '15 at 12:35

Matt234

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If you don't want to change your function name to lowercase, you can avoid the warning with a`/** * @returns {boolean} */` comment line above the function. – terrymorse Apr 21 '17 at 18:35

score 11 · Answer 2 · answered Oct 13 '14 at 11:14

If you create an object using the keyword new (as mentioned in the warning), JS returns an new instance of the object rather than the return value from the constructor.

A function like

function CheckHidden() {
    return false;
}

would return two different values, depending on how it is executed.

// "a" is boolean value
var a = CheckHidden();

// "a" is an object of type "CheckHidden"
var a = new CheckHidden();

score 0 · Answer 3 · answered Oct 13 '14 at 11:13

0

jQuery actually has something like this built in. You can use the :visible selector. You can just do:

var all=$("#catalog-body > div").filter(':visible')

to get the visible elements.

answered Oct 13 '14 at 11:13

Scimonster

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score 0 · Answer 4 · answered Oct 13 '14 at 11:14

If you want to pull all elements which are visible, you can use jQuery's :visible selector:

var all = $("#catalog-body > div:visible")

Conversely, if you want to pull all elements which are hidden, you can combine this with jQuery's :not selector:

$("#catalog-body > div:not(:visible)")

Primitive value returned from constructor will be lost

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