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Glossary

This glossary is intended to provide some guidance about the meanings of technical terms that are in common usage when talking about Vue. It is intended to be descriptive of how terms are commonly used, not a prescriptive specification of how they must be used. Some terms may have slightly different meanings or nuances depending on the surrounding context.

async component

An async component is a wrapper around another component that allows for the wrapped component to be lazy loaded. This is typically used as a way to reduce the size of the built .js files, allowing them to be split into smaller chunks that are loaded only when required.

Vue Router has a similar feature for the lazy loading of route components, though this does not use Vue's async components feature.

For more details see:

compiler macro

A compiler macro is special code that is processed by a compiler and converted into something else. They are effectively a clever form of string replacement.

Vue's SFC compiler supports various macros, such as defineProps(), defineEmits() and defineExpose(). These macros are intentionally designed to look like normal JavaScript functions so that they can leverage the same parser and type inference tooling around JavaScript / TypeScript. However, they are not actual functions that are run in the browser. These are special strings that the compiler detects and replaces with the real JavaScript code that will actually be run.

Macros have limitations on their use that don't apply to normal JavaScript code. For example, you might think that const dp = defineProps would allow you to create an alias for defineProps, but it'll actually result in an error. There are also limitations on what values can be passed to defineProps(), as the 'arguments' have to be processed by the compiler and not at runtime.

For more details see:

component

The term component is not unique to Vue. It is common to many UI frameworks. It describes a chunk of the UI, such as a button or checkbox. Components can also be combined to form larger components.

Components are the primary mechanism provided by Vue to split a UI into smaller pieces, both to improve maintainability and to allow for code reuse.

A Vue component is an object. All properties are optional, but either a template or render function is required for the component to render. For example, the following object would be a valid component:

js
const HelloWorldComponent = {
  render() {
    return 'Hello world!'
  }
}

In practice, most Vue applications are written using Single-File Components (.vue files). While these components may not appear to be objects at first glance, the SFC compiler will convert them into an object, which is used as the default export for the file. From an external perspective, a .vue file is just an ES module that exports a component object.

The properties of a component object are usually referred to as options. This is where the Options API gets its name.

The options for a component define how instances of that component should be created. Components are conceptually similar to classes, though Vue doesn't use actual JavaScript classes to define them.

The term component can also be used more loosely to refer to component instances.

For more details see:

The word 'component' also features in several other terms:

composable

The term composable describes a common usage pattern in Vue. It isn't a separate feature of Vue, it's just a way of using the framework's Composition API.

  • A composable is a function.
  • Composables are used to encapsulate and reuse stateful logic.
  • The function name usually begins with use, so that other developers know it's a composable.
  • The function is typically expected to be called during the synchronous execution of a component's setup() function (or, equivalently, during the execution of a <script setup> block). This ties the invocation of the composable to the current component context, e.g. via calls to provide(), inject() or onMounted().
  • Composables typically return a plain object, not a reactive object. This object usually contains refs and functions and is expected to be destructured within the calling code.

As with many patterns, there can be some disagreement about whether specific code qualifies for the label. Not all JavaScript utility functions are composables. If a function doesn't use the Composition API then it probably isn't a composable. If it doesn't expect to be called during the synchronous execution of setup() then it probably isn't a composable. Composables are specifically used to encapsulate stateful logic, they are not just a naming convention for functions.

See Guide - Composables for more details about writing composables.

Composition API

The Composition API is a collection of functions used to write components and composables in Vue.

The term is also used to describe one of the two main styles used to write components, the other being the Options API. Components written using the Composition API use either <script setup> or an explicit setup() function.

See the Composition API FAQ for more details.

custom element

A custom element is a feature of the Web Components standard, which is implemented in modern web browsers. It refers to the ability to use a custom HTML element in your HTML markup to include a Web Component at that point in the page.

Vue has built-in support for rendering custom elements and allows them to be used directly in Vue component templates.

Custom elements should not be confused with the ability to include Vue components as tags within another Vue component's template. Custom elements are used to create Web Components, not Vue components.

For more details see:

directive

The term directive refers to template attributes beginning with the v- prefix, or their equivalent shorthands.

Built-in directives include v-if, v-for, v-bind, v-on and v-slot.

Vue also supports creating custom directives, though they are typically only used as an 'escape hatch' for manipulating DOM nodes directly. Custom directives generally can't be used to recreate the functionality of the built-in directives.

For more details see:

dynamic component

The term dynamic component is used to describe cases where the choice of which child component to render needs to be made dynamically. Typically, this is achieved using <component :is="type">.

A dynamic component is not a special type of component. Any component can be used as a dynamic component. It is the choice of component that is dynamic, rather than the component itself.

For more details see:

effect

See reactive effect and side effect.

event

The use of events for communicating between different parts of a program is common to many different areas of programming. Within Vue, the term is commonly applied to both native HTML element events and Vue component events. The v-on directive is used in templates to listen for both types of event.

For more details see:

fragment

The term fragment refers to a special type of VNode that is used as a parent for other VNodes, but which doesn't render any elements itself.

The name comes from the similar concept of a DocumentFragment in the native DOM API.

Fragments are used to support components with multiple root nodes. While such components might appear to have multiple roots, behind the scenes they use a fragment node as a single root, as a parent of the 'root' nodes.

Fragments are also used by the template compiler as a way to wrap multiple dynamic nodes, e.g. those created via v-for or v-if. This allows for extra hints to be passed to the VDOM patching algorithm. Much of this is handled internally, but one place you may encounter this directly is using a key on a <template> tag with v-for. In that scenario, the key is added as a prop to the fragment VNode.

Fragment nodes are currently rendered to the DOM as empty text nodes, though that is an implementation detail. You may encounter those text nodes if you use $el or attempt to walk the DOM with built-in browser APIs.

functional component

A component definition is usually an object containing options. It may not appear that way if you're using <script setup>, but the component exported from the .vue file will still be an object.

A functional component is an alternative form of component that is declared using a function instead. That function acts as the render function for the component.

A functional component cannot have any state of its own. It also doesn't go through the usual component lifecycle, so lifecycle hooks can't be used. This makes them slightly lighter than normal, stateful components.

For more details see:

hoisting

The term hoisting is used to describe running a section of code before it is reached, ahead of other code. The execution is 'pulled up' to an earlier point.

JavaScript uses hoisting for some constructs, such as var, import and function declarations.

In a Vue context, the template compiler applies static hoisting to improve performance. When converting a template to a render function, VNodes that correspond to static content can be created once and then reused. These static VNodes are described as hoisted because they are created outside the render function, before it runs. A similar form of hoisting is applied to static objects or arrays that are generated by the template compiler.

For more details see:

in-DOM template

There are various ways to specify a template for a component. In most cases the template is provided as a string.

The term in-DOM template refers to the scenario where the template is provided in the form of DOM nodes, instead of a string. Vue then converts the DOM nodes into a template string using innerHTML.

Typically, an in-DOM template starts off as HTML markup written directly in the HTML of the page. The browser then parses this into DOM nodes, which Vue then uses to read off the innerHTML.

For more details see:

inject

See provide / inject.

lifecycle hooks

A Vue component instance goes through a lifecycle. For example, it is created, mounted, updated, and unmounted.

The lifecycle hooks are a way to listen for these lifecycle events.

With the Options API, each hook is provided as a separate option, e.g. mounted. The Composition API uses functions instead, such as onMounted().

For more details see:

macro

See compiler macro.

named slot

A component can have multiple slots, differentiated by name. Slots other than the default slot are referred to as named slots.

For more details see:

Options API

Vue components are defined using objects. The properties of these component objects are known as options.

Components can be written in two styles. One style uses the Composition API in conjunction with setup (either via a setup() option or <script setup>). The other style makes very little direct use of the Composition API, instead using various component options to achieve a similar result. The component options that are used in this way are referred to as the Options API.

The Options API includes options such as data(), computed, methods and created().

Some options, such as props, emits and inheritAttrs, can be used when authoring components with either API. As they are component options, they could be considered part of the Options API. However, as these options are also used in conjunction with setup(), it is usually more useful to think of them as shared between the two component styles.

The setup() function itself is a component option, so it could be described as part of the Options API. However, this is not how the term 'Options API' is normally used. Instead, the setup() function is considered to be part of Composition API.

plugin

While the term plugin can be used in a wide variety of contexts, Vue has a specific concept of a plugin as a way to add functionality to an application.

Plugins are added to an application by calling app.use(plugin). The plugin itself is either a function or an object with an install function. That function will be passed the application instance and can then do whatever it needs to do.

For more details see:

prop

There are three common uses of the term prop in Vue:

  • Component props
  • VNode props
  • Slot props

Component props are what most people think of as props. These are explicitly defined by a component using either defineProps() or the props option.

The term VNode props refers to the properties of the object passed as the second argument to h(). These can include component props, but they can also include component events, DOM events, DOM attributes and DOM properties. You'd usually only encounter VNode props if you're working with render functions to manipulate VNodes directly.

Slot props are the properties passed to a scoped slot.

In all cases, props are properties that are passed in from elsewhere.

While the word props is derived from the word properties, the term props has a much more specific meaning in the context of Vue. You should avoid using it as an abbreviation of properties.

For more details see:

provide / inject

provide and inject are a form of inter-component communication.

When a component provides a value, all descendants of that component can then choose to grab that value, using inject. Unlike with props, the providing component doesn't know precisely which component is receiving the value.

provide and inject are sometimes used to avoid prop drilling. They can also be used as an implicit way for a component to communicate with its slot contents.

provide can also be used at the application level, making a value available to all components within that application.

For more details see:

reactive effect

A reactive effect is part of Vue's reactivity system. It refers to the process of tracking the dependencies of a function and re-running that function when the values of those dependencies change.

watchEffect() is the most direct way to create an effect. Various other parts of Vue use effects internally. e.g. component rendering updates, computed() and watch().

Vue can only track reactive dependencies within a reactive effect. If a property's value is read outside a reactive effect it'll 'lose' reactivity, in the sense that Vue won't know what to do if that property subsequently changes.

The term is derived from 'side effect'. Calling the effect function is a side effect of the property value being changed.

For more details see:

reactivity

In general, reactivity refers to the ability to automatically perform actions in response to data changes. For example, updating the DOM or making a network request when a data value changes.

In a Vue context, reactivity is used to describe a collection of features. Those features combine to form a reactivity system, which is exposed via the Reactivity API.

There are various different ways that a reactivity system could be implemented. For example, it could be done by static analysis of code to determine its dependencies. However, Vue doesn't employ that form of reactivity system.

Instead, Vue's reactivity system tracks property access at runtime. It does this using both Proxy wrappers and getter/setter functions for properties.

For more details see:

Reactivity API

The Reactivity API is a collection of core Vue functions related to reactivity. These can be used independently of components. It includes functions such as ref(), reactive(), computed(), watch() and watchEffect().

The Reactivity API is a subset of the Composition API.

For more details see:

ref

This entry is about the use of ref for reactivity. For the ref attribute used in templates, see template ref instead.

A ref is part of Vue's reactivity system. It is an object with a single reactive property, called value.

There are various different types of ref. For example, refs can be created using ref(), shallowRef(), computed(), and customRef(). The function isRef() can be used to check whether an object is a ref, and isReadonly() can be used to check whether the ref allows the direct reassignment of its value.

For more details see:

render function

A render function is the part of a component that generates the VNodes used during rendering. Templates are compiled down into render functions.

For more details see:

scheduler

The scheduler is the part of Vue's internals that controls the timing of when reactive effects are run.

When reactive state changes, Vue doesn't immediately trigger rendering updates. Instead, it batches them together using a queue. This ensures that a component only re-renders once, even if multiple changes are made to the underlying data.

Watchers are also batched using the scheduler queue. Watchers with flush: 'pre' (the default) will run before component rendering, whereas those with flush: 'post' will run after component rendering.

Jobs in the scheduler are also used to perform various other internal tasks, such as triggering some lifecycle hooks and updating template refs.

scoped slot

The term scoped slot is used to refer to a slot that receives props.

Historically, Vue made a much greater distinction between scoped and non-scoped slots. To some extent they could be regarded as two separate features, unified behind a common template syntax.

In Vue 3, the slot APIs were simplified to make all slots behave like scoped slots. However, the use cases for scoped and non-scoped slots often differ, so the term still proves useful as a way to refer to slots with props.

The props passed to a slot can only be used within a specific region of the parent template, responsible for defining the slot's contents. This region of the template behaves as a variable scope for the props, hence the name 'scoped slot'.

For more details see:

SFC

See Single-File Component.

side effect

The term side effect is not specific to Vue. It is used to describe operations or functions that do something beyond their local scope.

For example, in the context of setting a property like user.name = null, it is expected that this will change the value of user.name. If it also does something else, like triggering Vue's reactivity system, then this would be described as a side effect. This is the origin of the term reactive effect within Vue.

When a function is described as having side effects, it means that the function performs some sort of action that is observable outside the function, aside from just returning a value. This might mean that it updates a value in state, or triggers a network request.

The term is often used when describing rendering or computed properties. It is considered best practice for rendering to have no side effects. Likewise, the getter function for a computed property should have no side effects.

Single-File Component

The term Single-File Component, or SFC, refers to the .vue file format that is commonly used for Vue components.

See also:

slot

Slots are used to pass content to child components. Whereas props are used to pass data values, slots are used to pass richer content consisting of HTML elements and other Vue components.

For more details see:

template ref

The term template ref refers to using a ref attribute on a tag within a template. After the component renders, this attribute is used to populate a corresponding property with either the HTML element or the component instance that corresponds to the tag in the template.

If you are using the Options API then the refs are exposed via properties of the $refs object.

With the Composition API, template refs populate a reactive ref with the same name.

Template refs should not be confused with the reactive refs found in Vue's reactivity system.

For more details see:

VDOM

See virtual DOM.

virtual DOM

The term virtual DOM (VDOM) is not unique to Vue. It is a common approach used by several web frameworks for managing updates to the UI.

Browsers use a tree of nodes to represent the current state of the page. That tree, and the JavaScript APIs used to interact with it, are referred to as the document object model, or DOM.

Manipulating the DOM is a major performance bottleneck. The virtual DOM provides one strategy for managing that.

Rather than creating DOM nodes directly, Vue components generate a description of what DOM nodes they would like. These descriptors are plain JavaScript objects, known as VNodes (virtual DOM nodes). Creating VNodes is relatively cheap.

Every time a component re-renders, the new tree of VNodes is compared to the previous tree of VNodes and any differences are then applied to the real DOM. If nothing has changed then the DOM doesn't need to be touched.

Vue uses a hybrid approach that we call Compiler-Informed Virtual DOM. Vue's template compiler is able to apply performance optimizations based on static analysis of the template. Rather than performing a full comparison of a component's old and new VNode trees at runtime, Vue can use information extracted by the compiler to reduce the comparison to just the parts of the tree that can actually change.

For more details see:

VNode

A VNode is a virtual DOM node. They can be created using the h() function.

See virtual DOM for more information.

Web Component

The Web Components standard is a collection of features implemented in modern web browsers.

Vue components are not Web Components, but defineCustomElement() can be used to create a custom element from a Vue component. Vue also supports the use of custom elements inside Vue components.

For more details see:

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