Vue 3 Tutorial: Lesson 4 – Template Syntax

Vue.js uses an HTML-based template syntax that allows you to declaratively bind the rendered DOM to the underlying application instance’s data. All Vue.js templates are valid HTML that can be parsed by spec-compliant browsers and HTML parsers.

Under the hood, Vue compiles the templates into Virtual DOM render functions. Combined with the reactivity system, Vue is able to intelligently figure out the minimal number of components to re-render and apply the minimal amount of DOM manipulations when the app state changes.

If you are familiar with Virtual DOM concepts and prefer the raw power of JavaScript, you can also directly write render functions instead of templates, with optional JSX support.

Interpolations

Text
The most basic form of data binding is text interpolation using the “Mustache” syntax (double curly braces):

<span>Content: {{ content }}</span>

The mustache tag will be replaced with the value of the msg property on the corresponding data object. It will also be updated whenever the data object’s msg property changes.

You can also perform one-time interpolations that do not update on data change by using the v-once directive, but keep in mind this will also affect any other bindings on the same node:

<span v-once>This will never change: {{ content }}</span>

Raw HTML

The double mustaches interprets the data as plain text, not HTML. In order to output real HTML, you will need to use the v-html directive:

<p>Using mustaches: {{ rawHtml }}</p>
<p>Using v-html directive: <span v-html="rawHtml"></span></p>

The contents of the span will be replaced with the value of the rawHtml property, interpreted as plain HTML – data bindings are ignored. Note that you cannot use v-html to compose template partials, because Vue is not a string-based templating engine. Instead, components are preferred as the fundamental unit for UI reuse and composition.

Attributes

Mustaches cannot be used inside HTML attributes. Instead, use a v-bind directive:

<div v-bind:id="dynamicId"></div>

In the case of boolean attributes, where their mere existence implies true, v-bind works a little differently. In this example:

<button v-bind:disabled="isButtonDisabled">Button</button>

If isButtonDisabled has the value of null or undefined, the disabled attribute will not even be included in the rendered <button> element.

Using JavaScript Expressions

So far we’ve only been binding to simple property keys in our templates. But Vue.js actually supports the full power of JavaScript expressions inside all data bindings:

{{ number + 1 }}
{{ ok ? 'YES' : 'NO' }}
{{ message.split('').reverse().join('')
}}

<div v-bind:id="'list-' + id"></div>

These expressions will be evaluated as JavaScript in the data scope of the current active instance. One restriction is that each binding can only contain one single expression, so the following will NOT work:

<!-- this is a statement, not an expression: -->
{{ var a = 1 }}

<!-- flow control won't work either, use ternary expressions -->
{{ if (ok) { return message } }}

Directives

Directives are special attributes with the v- prefix. Directive attribute values are expected to be a single JavaScript expression (with the exception of v-for and v-on, which be discussed in Lesson 2 and will be discussed clearly later). A directive’s job is to reactively apply side effects to the DOM when the value of its expression changes. Let’s review the example we saw in the introduction:

<p v-if="seen">Now you see me</p>

Here, the v-if directive would remove/insert the

element based on the truthiness of the value of the expression seen.

Arguments

Some directives can take an “argument”, denoted by a colon after the directive name. For example, the v-bind directive is used to reactively update an HTML attribute:

<a v-bind:href="url"> ... </a>

Here href is the argument, which tells the v-bind directive to bind the element’s href attribute to the value of the expression url.

Another example is the v-on directive, which listens to DOM events:

<a v-on:click="doSomething"> ... </a>

Here the argument is the event name to listen to. We will talk about event handling in more detail too.

Dynamic Arguments

It is also possible to use a JavaScript expression in a directive argument by wrapping it with square brackets:

<!--
Note that there are some constraints to the argument expression, as explained
in the "Dynamic Argument Expression Constraints" section below.
-->
<a v-bind:[attributeName]="url"> ... </a>

Here attributeName will be dynamically evaluated as a JavaScript expression, and its evaluated value will be used as the final value for the argument. For example, if your application instance has a data property, attributeName, whose value is "href", then this binding will be equivalent to v-bind:href.

Similarly, you can use dynamic arguments to bind a handler to a dynamic event name:

<a v-on:[eventName]="doSomething"> ... </a>

In this example, when eventName‘s value is "focus", v-on:[eventName] will be equivalent to v-on:focus.

Modifiers
Modifiers are special postfixes denoted by a dot, which indicate that a directive should be bound in some special way. For example, the .prevent modifier tells the v-on directive to call event.preventDefault() on the triggered event:

<form v-on:submit.prevent="onSubmit">...</form>

You’ll see other examples of modifiers later, for v-on and for v-model, when we explore those features.

Shorthands

The v- prefix serves as a visual cue for identifying Vue-specific attributes in your templates. This is useful when you are using Vue.js to apply dynamic behavior to some existing markup, but can feel verbose for some frequently used directives. At the same time, the need for the v- prefix becomes less important when you are building a SPA, where Vue manages every template. Therefore, Vue provides special shorthands for two of the most often used directives, v-bind and v-on:

v-bind Shorthand

<!-- full syntax -->
<a v-bind:href="url"> ... </a>

<!-- shorthand -->
<a :href="url"> ... </a>

<!-- shorthand with dynamic argument -->
<a :[key]="url"> ... </a>

v-on Shorthand

<!-- full syntax -->
<a v-on:click="doSomething"> ... </a>

<!-- shorthand -->
<a @click="doSomething"> ... </a>

<!-- shorthand with dynamic argument (2.6.0+) -->
<a @[event]="doSomething"> ... </a>

They may look a bit different from normal HTML, but : and @ are valid characters for attribute names and all Vue-supported browsers can parse it correctly. In addition, they do not appear in the final rendered markup. The shorthand syntax is totally optional, but you will likely appreciate it when you learn more about its usage later.

Caveats
Dynamic Argument Value Constraints
Dynamic arguments are expected to evaluate to a string, with the exception of null. The special value null can be used to explicitly remove the binding. Any other non-string value will trigger a warning.

Dynamic Argument Expression Constraints
Dynamic argument expressions have some syntax constraints because certain characters, such as spaces and quotes, are invalid inside HTML attribute names. For example, the following is invalid:

<!-- This will trigger a compiler warning. -->
<a v-bind:['foo' + bar]="value"> ... </a>

We recommend replacing any complex expressions with a computed property, one of the most fundamental pieces of Vue, which we’ll cover shortly.

When using in-DOM templates (templates directly written in an HTML file), you should also avoid naming keys with uppercase characters, as browsers will coerce attribute names into lowercase:

<!--
This will be converted to v-bind:[someattr] in in-DOM templates.
Unless you have a "someattr" property in your instance, your code won't work.
-->
<a v-bind:[someAttr]="value"> ... </a>

JavaScript Expressions

Template expressions are sandboxed and only have access to a whitelist of globals such as Math and Date. You should not attempt to access user defined globals in template expressions.