This is expected because a dom prop is casted by the browser into a string

@posva What browser casts dom props into strings? They're not attributes and not expressed as such, and I've been using domProps to attach data to elements without issues across all browsers. There are, in fact, plenty of properties (not attributes!) on HTMLElement instances that are not strings. Please reopen this issue.

It seems that now when domProps are updated, undefined and null will be converted to '' (empty string).

@posva
@MayaWolf
I have tested this link in v2.6.9 with minor modifications.
(https://jsfiddle.net/pistis/5whnyt0q/6/)

When the x value of domProps is set to undefined
Test Scenario 1

1. Click the 'Click me' button
2. The value of document.getElementById('test').x is set to 'undefined'

Test Scenario 2

1. Click the 'Update' button
2. Click the 'Click me' button
3. The value of document.getElementById('test').x is 2 when this is done.

Also, if x value of domProps is set to null, the same result as above is displayed.

I estimate the following.

1. The value of the initial domProps is set correctly.
2. When DOM with domProps is updated via render, undefined and null are converted to empty string in vnode -> Real DOM conversion.