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]

If someone needs to get an item that was changed inside the array, please, check it:

JSFiddle Example

The post example code:

new Vue({
  ...
  watch: {
    things: {
      handler: function (val, oldVal) {
        var vm = this;
        val.filter( function( p, idx ) {
            return Object.keys(p).some( function( prop ) {
                var diff = p[prop] !== vm.clonethings[idx][prop];
                if(diff) {
                    p.changed = true;                        
                }
            })
        });
      },
      deep: true
    }
  },
  ...
})