Passing an object with a custom toString() as a 'class' attribute

[BenoitZugmeyer]

I am writing a library using objects with a custom toString() method to render class names. Simplified example:

const object = {
  toString() {
    return computeClassNames();
  },
  //  ... other data
};

element.className = object;

It works great on plain DOM and React, but can't work with Preact because if a class attribute is an object, it will use the object enumerable keys associated with a truthy value to format the classname.

I understand why you chose to do this, and I find it quite useful. But would you consider changing the condition to format the class objects to something like:

if (lastSimple && lastSimple.toString === Object.prototype.toString) {
    attributes.class = hashToClassName(lastSimple);
}

or (might be more robust)

if (lastSimple && String(lastSimple) === '[object Object]') {
    attributes.class = hashToClassName(lastSimple);
}

or (not the cleanest but might be more efficient)

if (lastSimple && Object.prototype.toString.call(lastSimple) === '[object Object]') {
    attributes.class = hashToClassName(lastSimple);
}

I could change my library to return an object with keys as class names, but I'd prefer not to add code specific to preact.

Thank you

[developit]

@BenoitZugmeyer I'm actually toying with the idea of dropping support for Object class values. If I did, would this method begin to work? It would simply be removing these three lines:

https://github.com/developit/preact/blob/master/src/dom/index.js#L27-L29

[BenoitZugmeyer]

Just tried commenting those lines, and yes it works as I expect. (Sorry my fix suggestions were on an older version of preact)

[developit]

heh yeah things are moving quickly around here 😅
I'll add this to my list of nails in the coffin of built-in object class support. Removing it saves some bytes (~150) and speeds things up a tad.

[amccloud]

@developit the change @BenoitZugmeyer is proposing should also allow glamor to be used with preact. https://github.com/threepointone/glamor/blob/master/src/index.js#L235-L237

[developit]

Oooh that's huge, I am actually looking into glamor right now haha. We might need to have a 7.1 and then immediately an 8.x release though, dropping object class prop support is definitely a breaking change.

[wyze]

Came here looking for this since I am using glamor on my personal site. If this change lands, it would be great to get this over on preact-render-to-string also to support SSR.

[developit]

Absolutely, should be seeing this land very soon. Cheers!

[BenoitZugmeyer]

So, I tested Preact 8 in a small project of mine, and I found a single issue, related to this ticket. The class attribute is ignored if its value is an object and the element is SVG.

Test case:

  <svg class={{ toString() { return "pwet" } }} />

This is because of this test. I tried to search the history the reason of this condition, it has been introduced in 8ff9e32 without any explaination. Would it be possible to remove the typeof value!=='object' && part?

[NekR]

I'm not even sure why function test is there. Just do toString() (variable + '') to everything and pass to attribute (unless void/null/false of course).

[BenoitZugmeyer]

Yes the function test is probably useless too. No need to convert the value to string, setAttribute is doing it anyway (see document.body.setAttribute("bleh", { toString() { return "bleh" } }))

[NekR]

setAttribute is doing it anyway

I guess some IE or older Android or whatever webkit may reject such direct objects passing and that's why there is a check. Assuming browser does everything right could be a wrong path :-)