1 - File IO with promises.js

var fs = require("uxp").storage.localFileSystem;

function loadStyleguideFile(selection) {
    return fs.getFileForOpening()
    .then(function (file) {
        console.log("User chose file: " + file);
        return file.read();
    })
    .then(function (text) {
        var json = JSON.parse(text);
        createFrame(selection, json.padding.vertical, json.padding.horizontal);
    });
}

2 - sample styles.json

{
    "padding": {
        "horizontal": 10,
        "vertical": 10
    }
}

3 - FileIO with await.js

var fs = require("uxp").storage.localFileSystem;

async function loadStyleguideFile(selection) {
    var file = await fs.getFileForOpening();
    console.log("User chose file: " + file);

    var text = await file.read();
    var json = JSON.parse(text);
    createRectangle(selection, json.padding.vertical, json.padding.horizontal);
}

4 - FileIO with await and cancel handling.js

var fs = require("uxp").storage.localFileSystem;

async function loadStyleguideFile(selection) {
    var file = await fs.getFileForOpening();
    if (!file) {
        return;  // picker was canceled
    }
    console.log("User chose file: " + file);

    var text = await file.read();
    var json = JSON.parse(text);
    createRectangle(selection, json.padding.vertical, json.padding.horizontal);
}