Code to set Firebase's `.runtimeconfig.json` to Firebase's environmental variables.

traverse.mjs

import { readFileSync, writeFileSync } from "fs";

/**
 * A function which traverses and object, retrieving keys and values.
 * @param {Object} o the object to be traversed
 * @param {string} p the dot notation for the object, o
 * @returns {string[]} the keys and values of the object
 */
const traverse = (o, p = "") => {
    return Object.entries(o)
        .map(([k, v]) => {
            if (typeof v === "object") {
                return traverse(v, `${p}${k}_`);
            }

            return `${(p + k).toUpperCase()}="${v}"`;
        })
        .flat(Infinity);
};

console.log("Parsing config.json");
const runtimeConfig = JSON.parse(readFileSync("config.json"));

console.log("Traversing config");
const runtimeConfigKeyValues = traverse(runtimeConfig);

console.log("Writing .env file");
writeFileSync(".env", runtimeConfigKeyValues.join("\n") + "\n");