Jan 10, 2020 See the Pen Slots and styling web components by Chris Coyier (@chriscoyier) on CodePen.::part and::theme I didn’t investigate this too much because this is a spec that’s still being worked on I guess, but will likely ultimately play a large role here. Sep 19, 2017 With Shadow DOM, we can create Web Components that are a composition between the component’s internals and elements provided at author-time by the user of the custom element. This is similar with using either option or optgroup elements inside a native select element. Web components; 7th December 2020. If there are multiple elements in light DOM with the same slot name, they are appended into the slot, one after another. Dec 13, 2020 In practice, components that have “slots” are probably pretty likely to be minimal and quick to render, anyway, and therefore less likely to need performance optimizations. If you run into a situation where you do need to optimize performance of a “slotted” component, consider extracting the slow-performing part into a separate.

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What are Components?

Components are one of the most powerful features of Vue.js. They help you extend basic HTML elements to encapsulate reusable code. At a high level, Components are custom elements that Vue.js’ compiler would attach specified behavior to. In some cases, they may also appear as a native HTML element extended with the special is attribute.

Using Components

Registration

We’ve learned in the previous sections that we can create a component constructor using Vue.extend():

To use this constructor as a component, you need to register it with Vue.component(tag, constructor):

Note that Vue.js does not enforce the W3C rules for custom tag-names (all-lowercase, must contain a hyphen) though following this convention is considered good practice.

Once registered, the component can now be used in a parent instance’s template as a custom element, <my-component>. Make sure the component is registered before you instantiate your root Vue instance. Here’s the full example:

Which will render:

Note the component’s template replaces the custom element, which only serves as a mounting point. This behavior can be configured using the replace instance option.

Also note that components are provided a template instead of mounting with the el option! Only the root Vue instance (defined using new Vue) will include an el to mount to).

Local Registration

You don’t have to register every component globally. You can make a component available only in the scope of another component by registering it with the components instance option:

The same encapsulation applies for other assets types such as directives, filters and transitions.

Registration Sugar

To make things easier, you can directly pass in the options object instead of an actual constructor to Vue.component() and the component option. Vue.js will automatically call Vue.extend() for you under the hood:

Component Option Caveats

Most of the options that can be passed into the Vue constructor can be used in Vue.extend(), with two special cases: data and el. Imagine we simply pass an object as data to Vue.extend():

The problem with this is that the same data object will be shared across all instances of MyComponent! This is most likely not what we want, so we should use a function that returns a fresh object as the data option:

The el option also requires a function value when used in Vue.extend(), for exactly the same reason.

Template Parsing

Vue.js template engine is DOM-based and uses native parser that comes with the browser instead of providing a custom one. There are benefits to this approach when compared to string-based template engines, but there are also caveats. Templates have to be individually valid pieces of HTML. Some HTML elements have restrictions on what elements can appear inside them. Most common of these restrictions are:

a can not contain other interactive elements (e.g. buttons and other links)
li should be a direct child of ul or ol, and both ul and ol can only contain li
option should be a direct child of select, and select can only contain option (and optgroup)
table can only contain thead, tbody, tfoot and tr, and these elements should be direct children of table
tr can only contain th and td, and these elements should be direct children of tr

In practice these restriction can cause unexpected behavior. Although in simple cases it might appear to work, you can not rely on custom elements being expanded before browser validation. E.g. <my-select><option>...</option></my-select> is not a valid template even if my-select component eventually expands to <select>...</select>.

Another consequence is that you can not use custom tags (including custom elements and special tags like <component>, <template> and <partial>) inside of select, table and other elements with similar restrictions. Custom tags will be hoisted out and thus not render properly.

In case of a custom element you should use the is special attribute:

In case of a <template> inside of a <table> you should use <tbody>, as tables are allowed to have multiple tbody:

Props

Passing Data with Props

Every component instance has its own isolated scope. This means you cannot (and should not) directly reference parent data in a child component’s template. Data can be passed down to child components using props.

A “prop” is a field on a component’s data that is expected to be passed down from its parent component. A child component needs to explicitly declare the props it expects to receive using the props option:

Then, we can pass a plain string to it like so:

Result:

camelCase vs. kebab-case

HTML attributes are case-insensitive. When using camelCased prop names as attributes, you need to use their kebab-case (hyphen-delimited) equivalents:

Dynamic Props

Similar to binding a normal attribute to an expression, we can also use v-bind for dynamically binding props to data on the parent. Whenever the data is updated in the parent, it will also flow down to the child:

It is often simpler to use the shorthand syntax for v-bind:

Result:

Literal vs. Dynamic

A common mistake beginners tend to make is attempting to pass down a number using the literal syntax:

However, since this is a literal prop, its value is passed down as a plain string '1', instead of an actual number. If we want to pass down an actual JavaScript number, we need to use the dynamic syntax to make its value be evaluated as a JavaScript expression:

Prop Binding Types

By default, all props form a one-way-down binding between the child property and the parent one: when the parent property updates, it will flow down to the child, but not the other way around. This default is meant to prevent child components from accidentally mutating the parent’s state, which can make your app’s data flow harder to reason about. However, it is also possible to explicitly enforce a two-way or a one-time binding with the .sync and .oncebinding type modifiers:

Compare the syntax:

The two-way binding will sync the change of child’s msg property back to the parent’s parentMsg property. The one-time binding, once set up, will not sync future changes between the parent and the child.

Note that if th

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