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Configuration model basics

This guide will introduce some critical concepts to understanding the configuration model. The configuration model is used to structure the features and data available in a given JBrowse session, and each new pluggable element you create will need its own configuration schema.

Configuration slot types

Our configuration system is "typed" to facilitate graphical editing of the configuration. Each configuration has a "schema" that lists what "configuration slots" it has. Each configuration slot has a name, description, a type, and a value.

Here is a mostly comprehensive list of config types:

  • stringEnum - allows assigning one of a limited set of entries, becomes a dropdown box in the GUI
  • color - allows selecting a color, becomes a color picker in the GUI
  • number - allows entering any numeric value
  • string - allows entering any string
  • integer - allows entering a integer value
  • `boolean
  • frozen - an arbitrary JSON can be specified in this config slot, becomes textarea in the GUI
  • fileLocation - refers to a URL, local file path on desktop, or file blob object in the browser
  • text - allows entering a string, becomes textarea in the GUI
  • stringArray - allows entering a list of strings, becomes a "todolist" style editor in the GUI where you can add or delete things
  • stringArrayMap - allows entering a list of key-value entries

Let's examine the PileupRenderer configuration as an example.

Example config with multiple slot types

This PileupRenderer config contains an example of several different slot types:

// plugins/alignments/src/PileupRenderer/configSchema.ts

import { types } from 'mobx-state-tree'
export default ConfigurationSchema('PileupRenderer', {
color: {
type: 'color',
description: 'the color of each feature in a pileup alignment',
defaultValue: `jexl:get(feature,'strand') == - 1 ? '#8F8FD8' : '#EC8B8B'`,
contextVariable: ['feature'],
displayMode: {
type: 'stringEnum',
model: types.enumeration('displayMode', ['normal', 'compact', 'collapse']),
description: 'Alternative display modes',
defaultValue: 'normal',
minSubfeatureWidth: {
type: 'number',
description: `the minimum width in px for a pileup mismatch feature. use for
increasing mismatch marker widths when zoomed out to e.g. 1px or
defaultValue: 0,
maxHeight: {
type: 'integer',
description: 'the maximum height to be used in a pileup rendering',
defaultValue: 600,

Accessing config values

So instead of accessing config.displayMode, we say,

readConfObject(config, 'displayMode')

You might also see in the code like this:

getConf(track, 'maxHeight')

Which would be equivalent to calling,

readConfObject(track.configuration, 'maxHeight')`

Using config callbacks

Config callbacks allow you to have a dynamic color based on some function logic you provide. All config slots can actually become config callback. The arguments that are given to the callback are listed by the 'contextVariable' but must be provided by the calling code (the code reading the config slot). To pass arguments to the a callback we say

readConfObject(config, 'color', { feature })

That implies the color configuration callback will be passed a feature, so the config callback can be a complex function determining the color to use based on various feature attributes.

Example of a config callback

We use Jexl to express callbacks. See for more details.

There are also more examples and information in our config guide.

If you had a variant track in your config, and wanted to make a custom config callback for color, it might look like this:

"type": "VariantTrack",
"trackId": "variant_colors",
"name": "volvox filtered vcf (green snp, purple indel)",
"category": ["VCF"],
"assemblyNames": ["volvox"],
"adapter": {
"type": "VcfTabixAdapter",
"vcfGzLocation": {
"uri": "volvox.filtered.vcf.gz",
"locationType": "UriLocation"
"index": {
"location": {
"uri": "volvox.filtered.vcf.gz.tbi",
"locationType": "UriLocation"
"displays": [
"type": "LinearVariantDisplay",
"displayId": "volvox_filtered_vcf_color-LinearVariantDisplay",
"renderer": {
"type": "SvgFeatureRenderer",
"color1": "jexl:get(feature,'type')=='SNV'?'green':'purple'" # here we call our jexl function

This draws all SNV (single nucleotide variants) as green, and other types as purple (insertion, deletion, other structural variant).

You can also write your own jexl function and call it in the same way in the configuration.


It can be extremely useful to utilize a custom jexl function in the default configuration for a pluggable element type, as you can observe in one of the examples above, the default value of the color slot of the renderer is a jexl function. If you configure your plugin with a custom jexl function, you can use that function as a default value in your various pluggable elements.

Configuration internals

A configuration is a type of mobx-state-tree model, in which leaf nodes are ConfigSlot types, and other nodes are ConfigurationSchema types.

/ | \
Slot Schema Slot
| \
Slot Slot

Configurations are all descendants of a single root configuration, which is root.configuration.

Configuration types should always be created by the ConfigurationSchema factory, e.g.:

import { ConfigurationSchema } from '@jbrowse/core/utils/configuration'
const ThingStateModel = types.model('MyThingsState', {
foo: 42,
configuration: ConfigurationSchema('MyThing', {
backgroundColor: {
defaultValue: 'white',
type: 'string',

An example of a config schema with a sub-config schema is the BamAdapter, with the index sub-config schema:

bamLocation: {
type: 'fileLocation',
defaultValue: { uri: '/path/to/my.bam', locationType: 'UriLocation' },
// this is a sub-config schema
index: ConfigurationSchema('BamIndex', {
indexType: {
model: types.enumeration('IndexType', ['BAI', 'CSI']),
type: 'stringEnum',
defaultValue: 'BAI',
location: {
type: 'fileLocation',
defaultValue: {
uri: '/path/to/my.bam.bai',
locationType: 'UriLocation',
{ explicitlyTyped: true },

Reading the sub-config schema is as follows

const indexType = readConfObject(config, ['index', 'indexType'])

Alternatively can use

const indexConf = readConfObject(config, ['index'])

However, this may miss default values from the slot, the readConfObject has special logic to fill in the default value.