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Customizing feature colors with callbacks and plugins

If you have a color callback that has a lot of logic in it, then using jexl to express all that logic may be hard. Instead, you can make a small plugin which adds a function to the jexl language, and call that function in your jexl callback.

For example, create a file named "myplugin.js" (see also Footnote 1)

// myplugin.js
;(function () {
class MyPlugin {
install() {}
configure(pluginManager) {
pluginManager.jexl.addFunction('colorFeature', feature => {
let type = feature.get('type')
if (type === 'CDS') {
return 'red'
} else if (type === 'exon') {
return 'green'
} else {
return 'purple'

// the plugin will be included in both the main thread and web worker, so
// install plugin to either window or self (webworker global scope)
;(typeof self !== 'undefined' ? self : window).JBrowsePluginMyPlugin = {
default: MyPlugin,

Then put myplugin.js in the same folder as your config file, and then you can use the custom jexl function in your config callbacks as follows:

"plugins": [
"name": "MyPlugin",
"umdLoc": { "uri": "myplugin.js" }
"tracks": [
"type": "FeatureTrack",
"trackId": "my_track",
"name": "my track",
"assemblyNames": ["hg19"],
"adapter": {
"type": "Gff3TabixAdapter",
"gffLocation": {
"uri": "volvox.filtered.gff"
"displays": [
"type": "LinearBasicDisplay",
"displayId": "mytrack-LinearBasicDisplay",
"renderer": {
"type": "SvgFeatureRenderer",
"color1": "jexl:colorFeature(feature)"

The feature in the callback is a "SimpleFeature" type object, and you can call feature.get('start'), feature.get('end'), feature.get('refName'), or feature.get('other_attribute') for e.g. maybe a field in a GFF3 column 9

See our no-build plugin tutorial for more info on setting up a simple plugin for doing these customizations.

Footnote 1

myplugin.js does not have to use the jbrowse-plugin-template if it is small and self contained like this, and does not import other modules. if you import other modules from your plugin, then it can be worth it to use the jbrowse-plugin-template.

Footnote 2

If you are using embedded, there are also other methods of including plugins, see