JS 如何监听一个变量的变化

var a = { watchValue:0 };
var lastTimeValue=a.watchValue;
Object.defineProperty(a, ‘watchValue‘, {
  get: function() {
    console.log(‘get：‘ + watchValue);
    return watchValue;
  },
  set: function(value) {
    watchValue = value;
    if(lastTimeValue!=watchValue){
        lastTimeValue=watchValue;
        console.log(‘value changed!! set: ‘ + watchValue);
    }
  }
});