Custom Components

Evidence supports custom Svelte components, allowing you to extend the built-in component library with your own interactive visualizations and UI elements.

Why Custom Components?

Custom components are useful when you need to:

Create specialized visualizations not available in the built-in library
Integrate third-party JavaScript libraries
Build reusable UI patterns specific to your organization
Implement custom business logic or calculations
Add advanced interactivity beyond standard inputs

Creating a Custom Component

Custom components are Svelte files (.svelte) placed in your project’s components directory.

Project Structure

my-evidence-project/
├── components/
│   ├── CustomChart.svelte
│   ├── MetricCard.svelte
│   └── DataFilter.svelte
├── pages/
│   └── index.md
└── sources/

Basic Component Example

Create components/MetricCard.svelte:

<script>
  // Props that can be passed to the component
  export let title = "Metric";
  export let value;
  export let change;
  export let fmt = "num0";
  
  // Determine color based on change
  $: changeColor = change >= 0 ? 'text-green-600' : 'text-red-600';
  $: changeIcon = change >= 0 ? '↑' : '↓';
</script>

<div class="metric-card">
  <h3 class="text-sm font-medium text-gray-600">{title}</h3>
  <div class="flex items-baseline gap-2">
    <span class="text-3xl font-bold">
      {value.toLocaleString()}
    </span>
    {#if change !== undefined}
      <span class={changeColor}>
        {changeIcon} {Math.abs(change).toFixed(1)}%
      </span>
    {/if}
  </div>
</div>

<style>
  .metric-card {
    padding: 1.5rem;
    background: white;
    border-radius: 0.5rem;
    border: 1px solid #e5e7eb;
  }
</style>

Using the Component

In your markdown pages:

```sql revenue_metrics
SELECT 
  SUM(revenue) as total_revenue,
  12.5 as revenue_change_pct
FROM orders

## Working with Query Data

Custom components can accept and process query results:

### Component Accepting Data Prop

```svelte
<!-- components/SimpleTable.svelte -->
<script>
  export let data = [];
  export let columns = [];
  
  // Auto-detect columns if not provided
  $: displayColumns = columns.length > 0 
    ? columns 
    : (data.length > 0 ? Object.keys(data[0]) : []);
</script>

<div class="overflow-x-auto">
  <table>
    <thead>
      <tr>
        {#each displayColumns as column}
          <th>{column}</th>
        {/each}
      </tr>
    </thead>
    <tbody>
      {#each data as row}
        <tr>
          {#each displayColumns as column}
            <td>{row[column]}</td>
          {/each}
        </tr>
      {/each}
    </tbody>
  </table>
</div>

<style>
  table {
    width: 100%;
    border-collapse: collapse;
  }
  th, td {
    padding: 0.75rem;
    text-align: left;
    border-bottom: 1px solid #e5e7eb;
  }
  th {
    font-weight: 600;
    background-color: #f9fafb;
  }
</style>

Usage

```sql orders
SELECT 
  order_id,
  customer_name,
  order_date,
  total
FROM orders
LIMIT 10

## Integrating Third-Party Libraries

You can use npm packages in custom components.

### Installing Dependencies

First, install the package:

```bash
npm install d3-scale

Using the Library

<!-- components/ColorScale.svelte -->
<script>
  import { scaleLinear } from 'd3-scale';
  
  export let value;
  export let min = 0;
  export let max = 100;
  export let colors = ['#fee', '#f00'];
  
  $: colorScale = scaleLinear()
    .domain([min, max])
    .range(colors);
  
  $: backgroundColor = colorScale(value);
</script>

<div class="color-cell" style="background-color: {backgroundColor}">
  {value}
</div>

<style>
  .color-cell {
    padding: 0.5rem 1rem;
    border-radius: 0.25rem;
    font-weight: 500;
  }
</style>

Interactive Components

Create components with internal state and interactions:

<!-- components/ExpandableCard.svelte -->
<script>
  export let title;
  export let data;
  
  let expanded = false;
  
  function toggle() {
    expanded = !expanded;
  }
</script>

<div class="card">
  <button class="header" on:click={toggle}>
    <h3>{title}</h3>
    <span class="icon">{expanded ? '−' : '+'}</span>
  </button>
  
  {#if expanded}
    <div class="content">
      <slot />
    </div>
  {/if}
</div>

<style>
  .card {
    border: 1px solid #e5e7eb;
    border-radius: 0.5rem;
    overflow: hidden;
  }
  
  .header {
    width: 100%;
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 1rem;
    background: white;
    border: none;
    cursor: pointer;
  }
  
  .header:hover {
    background: #f9fafb;
  }
  
  .content {
    padding: 1rem;
    border-top: 1px solid #e5e7eb;
  }
</style>

Using Slots

Slots allow you to pass content into components:

<!-- components/Panel.svelte -->
<script>
  export let title;
  export let variant = 'default';
</script>

<div class="panel panel-{variant}">
  <div class="panel-header">
    <h2>{title}</h2>
    {#if $$slots.actions}
      <div class="panel-actions">
        <slot name="actions" />
      </div>
    {/if}
  </div>
  
  <div class="panel-body">
    <slot />
  </div>
  
  {#if $$slots.footer}
    <div class="panel-footer">
      <slot name="footer" />
    </div>
  {/if}
</div>

Usage with Slots

<Panel title="Revenue Analysis" variant="primary">
  <div slot="actions">
    <button>Export</button>
  </div>
  
  <!-- Default slot content -->
  <BarChart data={revenue} x="month" y="total"/>
  
  <div slot="footer">
    Last updated: January 15, 2024
  </div>
</Panel>

Component Props Validation

Validate and provide defaults for props:

<script>
  export let data;
  export let title = "Chart";
  export let width = 600;
  export let height = 400;
  export let colorScheme = 'default';
  
  // Validation
  $: if (!data || data.length === 0) {
    console.warn('No data provided to component');
  }
  
  $: if (width < 200 || height < 200) {
    console.warn('Chart dimensions may be too small');
  }
  
  // Computed values
  $: aspectRatio = width / height;
</script>

Accessing Evidence Context

Evidence provides context that components can access:

<script>
  import { getContext } from 'svelte';
  
  // Access Evidence theme
  const { theme } = getContext('evidence');
  
  export let data;
  
  $: primaryColor = $theme.colors.primary;
</script>

<div style="color: {primaryColor}">
  <!-- Component content -->
</div>

Exporting Components as a Package

Create reusable component libraries:

Package Structure

my-components/
├── package.json
├── src/
│   ├── index.js
│   ├── MetricCard.svelte
│   └── CustomChart.svelte
└── README.md

package.json

{
  "name": "@myorg/evidence-components",
  "version": "1.0.0",
  "svelte": "src/index.js",
  "exports": {
    ".": "./src/index.js"
  },
  "peerDependencies": {
    "svelte": "^4.0.0"
  }
}

src/index.js

export { default as MetricCard } from './MetricCard.svelte';
export { default as CustomChart } from './CustomChart.svelte';

Best Practices

Props over hardcoding - Make components flexible with props
Provide defaults - Set sensible default values for optional props
Validate inputs - Check for required props and valid values
Use TypeScript - Add type safety with TypeScript
Document components - Include JSDoc comments for props
Keep it simple - Start simple, add complexity as needed
Responsive design - Make components work on all screen sizes
Accessibility - Follow ARIA guidelines for interactive components

TypeScript Support

Use TypeScript for type-safe components:

<script lang="ts">
  export let data: Array<{date: string, value: number}>;
  export let title: string = "Chart";
  export let showLegend: boolean = true;
  
  interface ChartConfig {
    width: number;
    height: number;
    margin: { top: number; right: number; bottom: number; left: number };
  }
  
  const config: ChartConfig = {
    width: 600,
    height: 400,
    margin: { top: 20, right: 20, bottom: 30, left: 40 }
  };
</script>

Debugging Components

Use Svelte’s reactive debugging:

<script>
  export let data;
  
  // Log when data changes
  $: console.log('Data updated:', data);
  
  // Reactive statement with multiple dependencies
  $: {
    console.log('Recalculating chart with:', {
      dataLength: data?.length,
      width,
      height
    });
  }
</script>

Example: Custom Chart Component

Complete example of a custom chart component:

<!-- components/ProgressChart.svelte -->
<script>
  export let data;
  export let targetColumn = 'target';
  export let actualColumn = 'actual';
  export let nameColumn = 'name';
  export let height = 400;
  
  $: maxValue = Math.max(
    ...data.map(d => Math.max(d[targetColumn], d[actualColumn]))
  );
  
  function getPercentage(value) {
    return (value / maxValue) * 100;
  }
  
  function getColor(actual, target) {
    const pct = (actual / target) * 100;
    if (pct >= 100) return '#10b981';
    if (pct >= 75) return '#f59e0b';
    return '#ef4444';
  }
</script>

<div class="progress-chart" style="height: {height}px">
  {#each data as item}
    <div class="progress-row">
      <div class="label">{item[nameColumn]}</div>
      <div class="bars">
        <div class="bar-container">
          <div 
            class="bar target" 
            style="width: {getPercentage(item[targetColumn])}%"
          >
            <span class="bar-label">Target: {item[targetColumn]}</span>
          </div>
          <div 
            class="bar actual" 
            style="
              width: {getPercentage(item[actualColumn])}%; 
              background-color: {getColor(item[actualColumn], item[targetColumn])}
            "
          >
            <span class="bar-label">Actual: {item[actualColumn]}</span>
          </div>
        </div>
      </div>
    </div>
  {/each}
</div>

<style>
  .progress-chart {
    display: flex;
    flex-direction: column;
    gap: 1rem;
    padding: 1rem;
  }
  
  .progress-row {
    display: flex;
    gap: 1rem;
    align-items: center;
  }
  
  .label {
    flex: 0 0 150px;
    font-weight: 500;
  }
  
  .bars {
    flex: 1;
  }
  
  .bar-container {
    position: relative;
    height: 40px;
  }
  
  .bar {
    position: absolute;
    height: 20px;
    border-radius: 4px;
    transition: width 0.3s;
  }
  
  .bar.target {
    top: 0;
    background-color: #e5e7eb;
  }
  
  .bar.actual {
    top: 22px;
  }
  
  .bar-label {
    padding: 0 0.5rem;
    font-size: 0.75rem;
    line-height: 20px;
    white-space: nowrap;
  }
</style>

Get Started

Core Concepts

Data Sources

Components

Deployment

Guides

Custom Components

Why Custom Components?

Creating a Custom Component

Project Structure

Basic Component Example

Using the Component

Usage

Using the Library

Interactive Components

Using Slots

Usage with Slots

Component Props Validation

Accessing Evidence Context

Exporting Components as a Package

Package Structure

package.json

src/index.js

Best Practices

TypeScript Support

Debugging Components

Example: Custom Chart Component

Next Steps

Svelte Documentation

Component Examples

Build docs developers (and LLMs) love

Get Started

Core Concepts

Data Sources

Components

Deployment

Guides

​Why Custom Components?

​Creating a Custom Component

​Project Structure

​Basic Component Example

​Using the Component

​Usage

​Using the Library

​Interactive Components

​Using Slots

​Usage with Slots

​Component Props Validation

​Accessing Evidence Context

​Exporting Components as a Package

​Package Structure

​package.json

​src/index.js

​Best Practices

​TypeScript Support

​Debugging Components

​Example: Custom Chart Component

​Next Steps

Svelte Documentation

Component Examples

Build docs developers (and LLMs) love

Why Custom Components?

Creating a Custom Component

Project Structure

Basic Component Example

Using the Component

Usage

Using the Library

Interactive Components

Using Slots

Usage with Slots

Component Props Validation

Accessing Evidence Context

Exporting Components as a Package

Package Structure

package.json

src/index.js

Best Practices

TypeScript Support

Debugging Components

Example: Custom Chart Component

Next Steps