`This` is undefined in vue.js watcher

You will have to use function syntax here, as warned in the docs here:

Note that you should not use an arrow function to define a watcher (e.g. searchQuery: newValue => this.updateAutocomplete(newValue)). The reason is arrow functions bind the parent context, so this will not be the Vue instance as you expect and this.updateAutocomplete will be undefined.

So your code should be:

watch: {
    propShow: {
        handler: function(val, oldVal) {
            this.show = val;
        }
    }
}