Converting JSONML to XML (without sanity checks)

stringifyJsonml.js

function isObject(obj) {
  if (!obj) return false;
  if (typeof obj !== "object") return false;
  if (Array.isArray(obj)) return false;
  return true;
}

function* stringifyElement(element, indentLevel, depth = 0) {
  const indent =
    typeof indentLevel === "string"
      ? indentLevel
      : " ".repeat(indentLevel * depth);
  const nl = indentLevel ? "\n" : "";
  if (Array.isArray(element)) {
    const [tagName, ...children] = element;
    const attributes = isObject(children[0]) ? children.shift() : {};
    yield `${indent}<${tagName}`;
    for (const key of Object.keys(attributes).sort()) {
      const value = attributes[key];
      yield ` ${key}="${value.replace(/"/g, "&quot;")}"`;
    }
    if (!children.length) {
      yield ` />${nl}`;
    } else if (children.length === 1 && !Array.isArray(children[0])) {
      yield `>${String(children[0])}</${tagName}>${nl}`;
    } else {
      yield `>${nl}`;
      for (const child of children) {
        yield* stringifyElement(child, indentLevel, depth + 1);
      }
      yield `${indent}</${tagName}>${nl}`;
    }
  } else if (!nl) {
    yield `${indent}${String(element)}${nl}`;
  } else {
    for (const line of String(element).split("\n")) {
      yield `${indent}${line}${nl}`;
    }
  }
}

function stringify(jsonml, indentLevel = 0) {
  if (!jsonml.length) return "";
  return jsonml.reduce(
    (str, element) =>
      str + [...stringifyElement(element, indentLevel)].join(""),
    ""
  );
}