updateSubstringFontAttributes.js

var sketch = require('sketch')
var document = sketch.getSelectedDocument()
var text = document.selectedLayers.layers[0].sketchObject

let textView = text.editingDelegate().textView()
let selectedRange = textView.selectedRange()
let textStorage = textView.textStorage()
let settingsAttribute = getSettingsAttributeForKey_Value(kLowerCaseType, kLowerCaseSmallCapsSelector)

function getSettingsAttributeForKey_Value(key, value) {
    let settingsAttribute = {
        [NSFontFeatureSettingsAttribute]: [{
            [NSFontFeatureTypeIdentifierKey]: key,
            [NSFontFeatureSelectorIdentifierKey]: value
        }]
    }
    return settingsAttribute
}

let font = text.font()
let fontSize = font.pointSize()
let descriptor = font.fontDescriptor().fontDescriptorByAddingAttributes(settingsAttribute)
let newFont = NSFont.fontWithDescriptor_size(descriptor,fontSize)

let attrsDict = NSDictionary.dictionaryWithObject_forKey(newFont,NSFontAttributeName)
textStorage.addAttributes_range(attrsDict,selectedRange)
textView.didChangeText()