v-model on <select> sets the value to undefined if the <select> is lacking the matching <option>

[kordian-kowalski]

Let's say we have a variable called someVar in our component - it is properly defined and its initial value is 1:

...
data: {
    someVar: 1
}
...

In our template there is a select tag with 3 options bound to someVar with v-model:

<select v-model="someVar">
    <option value="1">Option 1</option>
    <option value="3">Option 3</option>
    <option value="5">Option 5</option>
</select>

At this point everything works as expected. However, if we then change the value of someVar to a value that is not covered by any of the option tags (2 in this example), v-model will set someVar to undefined instead:

...
created: function() {
  setTimeout(function(){
    this.someVar = 2;
    // this.someVar becomes undefined instead
  }.bind(this), 2000);
}
...

Here is a jsfiddle showing the issue:
https://jsfiddle.net/0vpqny9y/4/

What I expected to happen was that the value is set properly and the select tag becomes empty.

While the setTimeout example may be silly, where it's becoming a pain to me is when I load both the options and the model that has a property bound to the select at the same time, which results in a race condition:

1. Options load first, model loads second, the option is present so everything works fine.
2. Model loads first, the bound value is set to whatever was loaded through AJAX, but since there are no options yet the value is instead set to undefined, then the options load by this time the original value is lost

[defcc]

Thanks @kordian-kowalski , I think this is an expected behavior.
Native html select also behaves like that, the value will be empty if you set an 'invalid' value to it.
Vue.js will force to update the select and calculate the value when you assign a new value to it, so it becomes undefined.

You might need to process it in the userland.

[kordian-kowalski]

Hmm, any chance to make a feature request for this to be handled by Vue instead? I think it would make the templates nicer if there was no need to work around such a common (I think?) quirk :)
Especially since just initializing the value without changing it later works fine:
https://jsfiddle.net/4h6ysyvf/
The value is preserved here. And of course if the initial value happens to be the same as the one I'm trying to assign to it, nothing breaks:
https://jsfiddle.net/4h6ysyvf/1/

[mgrinspan]

Those examples work because no events are triggered when the variable is initialized with a value, and when it's updated to it's current value. I don't think there's a perfect way to handle this consistently, so I'd suggest following @defcc and implementing it yourself on a case by case basis.

[fenduru]

Native <select> does not fire a change event when setting the property to an invalid value (jsfiddle), so I think it is a bit unexpected that setting a property in Vue would cause it to immediately overwrite the same property.

[jonjanisch]

I was also confused by this. If this is the expected behavior, it may be helpful to mention this in the Form Input Bindings -> Select section of the documentation.

[Plasmatium]

As @defcc said, you can implement this behavior manually, by watching anotherVal.
"select" bind with "selectedVal" by v-model, "input" bind with "anotherVal" by v-model, "selectedVal" bind "anotherVal" with each other by watch, except selectedVal changes to undefined
jsfiddle

[paulredmond]

Not sure if this is the "right" way, but I got around this issue with async options by using v-if on the component to not render it until the async operation was complete.

https://jsfiddle.net/paulredmond/a085bpmn/2/

[andreynering]

A v-if to check if there's any element in the list solves the issue:

<select v-if="items.length">
  <option v-for="item in items">{{item}}</option>
</select>

I still consider this an issue, though. I think this hack should not be necessary.

[posva]

A good way of having a default value is adding a disabled option:

<select v-model="input">
    <option v-if="input == null" disabled :value="null">Select an option</option>
...
</select>

http://jsfiddle.net/oov159xx/12

We're always open to improve the docs on vuejs/vuejs.org with PR 🙂
Closing this since this it's a documentation issue and because we cannot change this for v2 because it will be a breaking change. We will revisit it for next major release though

[shealan]

This thread saved my life :)

[AaronMul]

@paulredmond Thank you! Was seeing some strange async issues with IE 11 that I could not recreate in Chrome/Edge etc. Using the loaded true/false check on my form before executing v-for fixed it!