Custom Components

Custom components allow you to extend the functionality of Evidence, as well as to make your code more reusable.

In Evidence, you can build your own components and use them anywhere in your app. This is made possible through Svelte, the JavaScript framework Evidence is built on. These components can include the charts used for visualization, custom components created completely from scratch, or adaptations of existing UI components such as the header, sidebar, menu, etc.

Below is a short guide on building a simple component in Evidence.

For a fuller guide, Svelte offers a great interactive tutorial that you can complete in your browser in about an hour: Svelte Tutorial

Built a great component?

Let us know in our Slack community!

We'd love to see what you've built, and may add generally applicable components to the Evidence core component library!

Example custom component

If you were creating a component called <Hello />, which included some text and a BarChart, to use in index.md, you could do so like this:

Add a folder called components/ in the root of your project. This is where Evidence looks for your Svelte components:

Folder structure

.
|-- pages/
|   `-- index.md
`-- components/
    `-- Hello.svelte

Hello.svelte is your component. Add the following code to these two files:

File contents


```sql sales_by_country 
select 'Canada' as country, 100 as sales_usd 
union all 
select 'USA' as country, 200 as sales_usd 
union all 
select 'UK' as country, 300 as sales_usd 
```

<!-- To use data in the component, pass it to the component as a prop
     You can use multiple queries, and name the props anything you like -->
<Hello myData={sales_by_country} />

<!-- To allow the component to accept data, you need to use the 'export let' syntax
     If you need any Evidence components inside your custom component, you must import them explicitly -->
<script>
	export let myData;
	import { BarChart } from '@evidence-dev/core-components';
</script>

<p>
	Here is a BarChart in a Component, with some accompanying text. Components stored in the
	/components/ folder will be included in your app.
</p>

<BarChart data={myData} />

Building your own component: Checklist

If you're building a component, here are some things to keep in mind.

In your markdown file:

Pass any data as props if you need to access query results in the component

In the custom component:

Use Svelte (HTML + extra features) syntax in this component - it will not support Markdown
Use the /components/ folder for your .svelte files
export any props you want to use in the component
Import any Evidence components you want to use in the custom component

import checkInputs from '@evidence-dev/component-utilities/checkInputs';

data: the query result you need to check
reqCols: a list of columns that are required for your component (e.g., x or y for a chart)
optCols: a list of optional columns

ErrorChart `<ErrorChart error={error} chartType="My Chart"/>`

A component used to display an error state on the page

import { ErrorChart } from '@evidence-dev/core-components';

error: the error message to display
chartType: the title that appears at the top of the error component

Data Manipulation

getDistinctValues `getDistinctValues(data, column)`

Returns an array of distinct values from a specified column in a dataset

import getDistinctValues from '@evidence-dev/component-utilities/getDistinctValues';

data: query result to pull the values from
column: name of column to use

getDistinctCount `getDistinctCount(data, column)`

Returns the count of distinct values in a column

import getDistinctCount from '@evidence-dev/component-utilities/getDistinctCount';

data: query result to pull the values from
column: name of column to use

getSortedData `getSortedData(data, col, isAsc)`

Returns the original dataset, sorted by the specific column and direction

import getSortedData from '@evidence-dev/component-utilities/getSortedData';

data: query result to pull the values from
col: name of column to sort
isAsc: if true, will sort ascending; otherwise, descending

getColumnSummary `getColumnSummary(data, returnType = "object")`

Returns an object with information about each column (title, min, max, format, etc.)

import getColumnSummary from '@evidence-dev/component-utilities/getColumnSummary';

data: query result to summarize
returnType: "object" or "array"

getCompletedData `getCompletedData(data, x, y, series, nullsZero = false, fillX = false)`

Returns the original dataset with rows filled in as needed to complete a continuous number series

import getCompletedData from '@evidence-dev/component-utilities/getCompletedData';

data: query result which requires completed data
x: name of column for x-axis
y: name of column for y-axis
series: name of column for series
nullsZero: if true, will treat nulls as zeroes; otherwise, will leave as nulls
fillX: if true, will find the smallest increment in the x-axis values and create rows as needed to create a continous series

Value & Label Formatting

formatValue `formatValue(value, columnFormat = undefined, columnUnitSummary = undefined)`

Returns a formatted value

import { formatValue } from '@evidence-dev/component-utilities/formatting';

value: value to format
columnFormat: a format object for column being formatted (can be obtained from the getColumnSummary function)
columnUnitSummary: an object containing the extents of the column, used for unit summary formatting like "M" or "B" (can be obtained from the getColumnSummary function)

fmt `fmt(value, format)`

Simpler version of the formatValue function which does not require a format object

import { fmt } from '@evidence-dev/component-utilities/formatting';

value: value to be formatted
format: a string representing a format tag name or an Excel-style format code

formatTitle `formatTitle(column, columnFormat)`

Returns a formatted column title (with proper letter casing)

import formatTitle from '@evidence-dev/component-utilities/formatTitle';

column: name of column to be formatted
columnFormat: a format object for column being formatted (can be obtained from the getColumnSummary function)

Adding Queries

Custom components can execute queries rather than requiring data to be passed to them. See component queries for details