For now, just know that all Vue components are also Vue instances, and so accept the same options object (except for a few root-specific options). When a Vue instance is created, it adds all the properties found in its data object to Vue’s reactivity system. When the values of those properties change, the view will “react”, updating to match the new values. When this data changes, the view will re-render. It should be noted that properties in data are only reactive if they existed when the instance was created. That means if you add a new property, like:. Then changes to b will not trigger any view updates. If you know you’ll need a property later, but it starts out empty or non-existent, you’ll need to set some initial value. The only exception to this being the use of Object.freeze(), which prevents existing properties from being changed, which also means the reactivity system can’t track changes. In addition to data properties, Vue instances expose a number of useful instance properties and methods. These are prefixed with $ to differentiate them from user-defined properties. In the future, you can consult the API reference for a full list of instance properties and methods. Each Vue instance goes through a series of initialization steps when it’s created - for example, it needs to set up data observation, compile the template, mount the instance to the DOM, and update the DOM when data changes. Along the way, it also runs functions called lifecycle hooks, giving users the opportunity to add their own code at specific stages. For example, the created hook can be used to run code after an instance is created:. There are also other hooks which will be called at different stages of the instance’s lifecycle, such as mounted, updated, and destroyed. All lifecycle hooks are called with their this context pointing to the Vue instance invoking it. Don’t use arrow functions on an options property or callback, such as created: () => console.log(this.a) or vm.$watch('a', newValue => this.myMethod()). Since an arrow function doesn’t have a this, this will be treated as any other variable and lexically looked up through parent scopes until found, often resulting in errors such as Uncaught TypeError: Cannot read property of undefined or Uncaught TypeError: this.myMethod is not a function. Below is a diagram for the instance lifecycle. You don’t need to fully understand everything going on right now, but as you learn and build more, it will be a useful reference.