Vue Instance & Dynamic Data

We've just seen the most basic thing you can do with Vue. And if you think of Vue as a templating engine like Twig, it makes a lot of sense: we instantiated a new Vue instance, told it where on the page to render and passed it a template. And that totally worked. Booya!

The data Option

When you instantiate Vue, you control it by passing a number of different options... and a lot of this tutorial will be about learning what options are possible. One of the most important ones is data. Unlike el and template, data is a function. It returns an array - or, really, this is an "object" in JavaScript - of variables that you want to pass into the template.

Notice that ESLint is angry - it's because this line is empty. Sometimes you need to ignore it until you finish: it's a bit overeager. Let's create one new "data" - one new "variable" - to pass into the template: firstName set to Ryan. That's me!

And now that we're passing a firstName variable into the template, we can say "Hello" and {{ firstName }}. Yes, by complete coincidence, Vue uses the same syntax as Twig to render things.

Before we try this, ESLint is still mad at me. Sheesh! It says:

Expected method shorthand

As I mentioned, some of the options you pass to Vue are set to values - like el and template, while others are functions. When you have a method in an object like this, you can use a shorthand: data() {... which is just a lot more attractive.

Oh, and at the bottom, temporarily add window.app = app. That will set our Vue app as a global variable, which will let us play with it in our console. Ready?

Refresh! It rendered! But it gets better! In your browser's console, type app.firstName. You can already see that this equals Ryan! Any data key becomes accessible as a property on our instance. Set this to Beckett - my son's name... who is hopefully napping right now. Boom! The template immediately updates for the new data. And we can change this over and over again.

So Vue is a lot like Twig - it renders templates and can pass variables into the templates - but with this crazy-cool extra power that when we change a piece of data, it automatically re-renders... which, of course, is exactly what we want.

How the Vue Instance Rendering Really Works

And... at a high level... that's Vue! Yes, we're going to talk about so much more, but you already understand its main purpose. Remove the global variable and the app = code - we don't need that. ESLint will temporarily get mad because it thinks it's weird that we're instantiating an object and not setting to a variable, but that's fine... and it'll go away in a little while.

Behind the scenes, when Vue renders, it actually calls a render() method on the object, which you don't normally need to worry or care about. But to help this all make more sense, I want you to see what this method looks like one time. Stick with me, we're going to do some temporary experimentation.

I'm going to add a new render() function. As soon as I add this, when Vue renders it will call our function instead of rendering it internally for us. But inside, I'm going to put the exact code that Vue normally runs: return Vue.compile(this.$options.template) - that's a special way to reference the template option here - .render.call(this, h).

I know, that's totally crazy, and you will never need to type this in a real project. What this shows us is that Vue has a compile() function where you can "compile" a template string and then call render() on it. The .render.call thing is a fancy way of basically calling .render() and passing it the h variable, which is another object that's good at dealing with DOM elements... and that you don't need to worry about.

If we refresh now, it works exactly like before because our render() method does exactly what Vue normally does. If you look at this, you might start to wonder: why do we need a template option at all? We're just reading it in render()... so it could live anywhere. Let's try that! Remove the option, and, at the top, add const template = the template string. In render, reference that local variable.

That should work, right? Let's find out. It totally does!

Ok, so let me tell you the big important point I'm trying to make. Vue... is simple: it's a system where you can take a template string, render it, and pass this data into the template... just like Twig.

Of course, as we start adding more to our app, this template variable is going to get huge and ugly. To help with that, Vue has a very special organizational concept called single file components. Let's create one next.