How to use the jju.update function in jju

it('handles valid case', () => {
    // call this before mocking fs since it internally uses a dynamic require,
    // which doesn't work while fs is mocked (but if it's called before mocking,
    // the required file will be cached)
    jju.update('{}', {});

    mockfs({
      root1: { 'rush.json': rushJsonStrNoProjects }, // no projects yet
      root2: { 'rush.json': rushJsonStr } // already has a project
    });

    rushAddPackage('a', 'root1');
    expect(fs.readFileSync('root1/rush.json').toString()).toBe(rushJsonStr);

    rushAddPackage('b', 'root2');
    expect(fs.readFileSync('root2/rush.json').toString()).toBe(rushJsonUpdatedStr);
  });

public static updateString(
    previousJson: string,
    newJsonObject: Object,
    options?: IJsonFileStringifyOptions
  ): string {
    if (!options) {
      options = {};
    }

    JsonFile.validateNoUndefinedMembers(newJsonObject);

    let stringified: string;

    if (previousJson !== '') {
      // NOTE: We don't use mode=json here because comments aren't allowed by strict JSON
      stringified = jju.update(previousJson, newJsonObject, {
        mode: 'cjson',
        indent: 2
      });
    } else if (options.prettyFormatting) {
      stringified = jju.stringify(newJsonObject, {
        mode: 'json',
        indent: 2
      });
    } else {
      stringified = JSON.stringify(newJsonObject, undefined, 2);
    }

    // Add the trailing newline
    stringified = Text.ensureTrailingNewline(stringified);

    if (options && options.newlineConversion) {

function writeConfig(file, newValue) {
  file = findConfig(file);
  if (!file) {
    return false;
  }

  const oldContents = fs.readFileSync(file, 'utf8');
  const newContents = jju.update(oldContents, newValue, {
    mode: 'cjson',
    indent: 2
  });
  fs.writeFileSync(file, newContents);
  return true;
}

function update(file, old_data, new_data) {
 if (file === "package.json5") {
		if (!old_data) {
			return json2json(new_data)
		} else {
			return require("jju").update(old_data, JSON.parse(new_data))
		}

	} else {
		if (!old_data) {
			return new_data
		} else {
			return require("jju").update(old_data, JSON.parse(new_data), {mode: "json"})
		}
	}
}

const rushJson = _parseRushJson(oldContents);
  if (!rushJson) {
    logger.error(`Couldn't read rush.json under ${rootPath}. Not adding package.`);
    return;
  }

  if (!rushJson.projects) {
    rushJson.projects = [];
  }
  rushJson.projects.push({
    packageName,
    projectFolder: `packages/${packageName}`
  });

  try {
    const newContents = jju.update(oldContents, rushJson, { mode: 'cjson', indent: 2 });
    fs.writeFileSync(rushJsonPath, newContents);
  } catch (ex) {
    logger.error(`Couldn't update rush.json under ${rootPath}. Not adding package.`);
    logger.error('Error:', ex);
  }
}

function on_input_update(name, value) {
 var old_text = $('textarea').val()
 var json = jju.parse(old_text)
 
 if (value === '') {
  delete json[name]
 } else {
  json[name] = value
 }
 $('textarea').val(jju.update(old_text, json))
}
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function update(file, old_data, new_data) {
 if (file === "package.json5") {
		if (!old_data) {
			return json2json(new_data)
		} else {
			return require("jju").update(old_data, JSON.parse(new_data))
		}

	} else {
		if (!old_data) {
			return new_data
		} else {
			return require("jju").update(old_data, JSON.parse(new_data), {mode: "json"})
		}
	}
}