Svelte Preprocessors

List of some useful preprocessors for svelte

Usage

you can install this package as devDependency of your svelte based project.

npm install -D @ubeac/svelte-preprocessors

then edit your svelte.config.js file:

import {ifProcessor, previewProcessor} from '@ubeac/svelte-preprocessors'

export default {
    preprocessors: [
        ifProcessor(), // to enable if preprocessor
        previewProcessor() // to enable Preview preprocessor
    ],
    ...other configs
    kit: {
        ...sveltekit configs
    }
}

if Processor

if processor transforms if={...} props to {#if ...} in svelte templates for all Components and DOM Elements.

after installation and enabling if Processor you should be able to use if prop in your .sevlte files.

<Button if={2 + 2 === 4}>
    My Button
</Button>

<p if={!!user}>
    name: {user.name}
    email: {user.email}
</p>

after build process above code will be changed to

{#if 2 + 2 === 4}
<Button>
    My Button
</Button>
{/if}

{#if !!user}
<p>
    name: {user.name}
    email: {user.email}
</p>
{/if}

Example

<div class="actions">
    <button if={user.hasAccess('add-item')}>
        Add
    </button>
    <button if={user.hasAccess('edit-item')}>
        Edit
    </button>
    <button if={user.isAdmin()}>
        Remove
    </button>
</div>

after if processor:

<div class="actions">
    {#if user.hasAccess('add-item')}
    <button>
        Add
    </button>
    {/if}
    {#if user.hasAccess('edit-item')}
    <button>
        Edit
    </button>
    {/if}
    {#if user.isAdmin()}
    <button>
        Remove
    </button>
    {/if}
</div>

typescript support:

you can add if as an attribute for all HTML Elements by adding below code in *.d.ts file in your project.

declare    namespace svelte.JSX {
    interface HTMLAttributes<T> {
        if?: boolean
    }
}

also you need to ignore your editor's Intellisense errors for if={...} prop in Components. everything works in dev-mode and after build. only editor doesn't know that you use ifProcessor.

Alternatively, you can add typing for if prop to your component.

<script lang="ts">

type $$Props = {
    color: string;
    size: string;
    // other props
    if: boolean
}

export let color: string = 'primary'
export let size: string = 'md'
// other props
</script>

<button ...>
    <slot/>
</button>

limitations:

one problem is that you cannot use ifProcessor with slot prop in same element:

<Card>
    <div if={hasHeader} slot="header">
        Header
    </div>
    Body
</Card>

<Card>
    {#if hasHeader}
        <div slot="header"> <!-- cannot have slot="**" inside if conditions -->
            Header
        </div>
    {/if}
    Body
</Card>

Preview Processor

using this processor you can extract source code of svelte components as string.

to get started you should have a Preview component which have some props:

Preview.svelte

<script lang="ts">
    export let markup: string | undefined = undefined;
    export let script: string | undefined = undefined;
    export let style: string | undefined = undefined;
    export let isTypescript: boolean | undefined = undefined;

    $: finalScript = isTypescript ? script.replace('<script>', '<script lang="ts">')
</script>

{finalScript}
<br/>
{markup}
<br />
{style}

here is an example of Preview.svelte component which uses this preprocessor.

this processor is useful when you want to show source code of examples alongside live demo. Example