vue.js $watch array of objects

You should pass an object instead of boolean as options, so:

mounted: function () {
  this.$watch('things', function () {
    console.log('a thing changed')
  }, {deep:true})
}

Or you could set the watcher into the vue instance like this:

new Vue({
  ...
  watch: {
    things: {
      handler: function (val, oldVal) {
        console.log('a thing changed')
      },
      deep: true
    }
  },
  ...
})

[demo]