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can't use `:srcObject.prop` to set prop to null #9138

freddialpad opened this Issue Dec 3, 2018 · 2 comments


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freddialpad commented Dec 3, 2018



Reproduction link

Steps to reproduce

(See minimal repro)

  1. have a component with an HTMLMediaElement that binds :srcObject.prop to some data property (that starts off as null):
<audio :srcObject.prop="stream">
  1. set it to some valid MediaStream: = new MediaStream();
  1. set it back to null: = null;

What is expected?

I expect the audio.srcObject to be null

What is actually happening?

it stays as the MediaStream and there's a TypeError in the log.

I noticed that it did actually work in my first attempt to write the repro example: (probably because the whole thing is inline?)

Im not sure how helpful this is, but based on the stack trace it's related to this code, found in updateDOMProps:

  for (key in oldProps) {
    if (isUndef(props[key])) {
      elm[key] = '';

isUndef(null) is evaluating true, and so it's trying to set the prop to '', but srcObject specifically needs it to be null or a MediaStream

@posva posva added the improvement label Dec 4, 2018


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posva commented Dec 4, 2018

maybe we shouldn't apply transformation for .prop bindings


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freddialpad commented Dec 13, 2018

Now that I've looked at the code (updateDOMProps in src/platforms/web/runtime/modules/dom-props.js) in question I think I understand a little better what's happening. The associated test case (in unit/modules/vdom/modules/dom-props.spec.js) makes it seem like it's guarding against deletion:

  it('should remove the elements domProps', () => {
    const vnode1 = new VNode('a', { domProps: { src: 'http://localhost/' }})
    const vnode2 = new VNode('a', { domProps: {}})
    patch(null, vnode1)
    const elm = patch(vnode1, vnode2)

...There's no generic way to "delete" an HTMLEmelent property aside from setting it to whatever arbitrary value the setter defines. In that sense, casting to empty string as elm[key] = ''; Is basically the closest there is to a standard way. in the case of an <img> tag, setting the .src prop to anything beside empty string results in some pretty obnoxious behavior:

09:48:05.267 const img = document.createElement('img')
09:48:08.948 img.src
09:48:08.953 ""
09:48:14.469 img.src = null
09:48:17.213 img.src
09:48:17.222 "https://localhost:8080/null"

(I'm sure there's a great reason for that behavior and it has legitimate use cases all over.)

So in that context, empty string seems like as reasonable a default as any for null/undef props. I've experimented with changing the isUndef(props[key]) check to props.hasOwnProperty(key), or even just comparing it directly to undefined... pretty simple to do, but it doesn't really seem viable if users are depending on the ability to delete a prop by setting it to null; it's not backwards compatible. And special-casing property/element combinations in dom-props.js seems too obviously terrible to even mention.

That said, am I making too many assumptions there? is that code purely a guard against missing props in the new vnode? was the author of the test only considering src property because it's what caused a failure? is it reasonable to expect that users supply the correct value from among "", null and undefined when "deleting" a prop?

On the other hand, I guess it doesn't really make sense to talk about deletion when it comes to bound properties... like in the example of <audio :srcObject.prop="stream"> the stream computed property will have to return something, so I'm assuming there's something with the vnode internals that are beyond my understanding that makes it possible for {stream: {some: thing}} to be replaced with {}.

If the above suggestion sounds okay, then I would assume hasOwnProperty is the right check to make there, and the result should be elm[key] = '' (like I said it's as good a "standard" delete as any, and probably the most commonly applicable case); I'll probably write a pull request this week. But I don't want to spend the time if it's not a safe change to make.

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