How to use js to modify the content in a certain json?

Hi @dublove have a study of this:

/**
 * Example of reading, parse and writing JSON file.
 *
 * @author m1b
 * @version 2025-06-25
 * @discussion https://community.adobe.com/t5/indesign-discussions/how-to-use-js-to-modify-the-content-in-a-certain-json/m-p/15387727
 */
//@include '../../../../Lib/json3.js'
(function () {

    // read the json file's contents
    var myJSON = readFile('my.json');

    if (!myJSON)
        return alert('Could not read JSON file.');

    // parse as json
    myJSON = JSON.parse(myJSON);

    alert('oldCode = ' + myJSON.oldCode);

    // change the json
    myJSON.oldCode = Math.floor(Math.random() * 999999);

    // write the json file
    var f = writeFile('my.json', JSON.stringify(myJSON, undefined, '\t'));

})();

/**
 * Read a file and return contents.
 * Will ask for file if no path is supplied.
 * @author m1b
 * @version 2023-08-17
 * @param {File|CSFile|String} [file] - file or path.
 * @param {String} [fileExtension] - the file extension (default: '').
 * @param {Number} [lineCount] - the number of lines to read (default: all).
 * @returns {String} file contents.
 */
function readFile(file, fileExtension, lineCount) {

    fileExtension = (fileExtension || '').replace('.', '');
    lineCount = lineCount || 0;

    var f,
        data = '';

    if (file != undefined) {
        if (file.constructor.name == 'String') {
            if (file.indexOf('/') < 0)
                file = File($.fileName).parent + '/' + file;
            f = File(file);
        }
        else if (file.constructor.name == 'File')
            f = file;
    }

    if (
        f == undefined
        || typeof f.open !== 'function'
    )
        f = Utility.chooseFile(fileExtension, false, file);

    if (!f || !f.exists)
        return;

    $.appEncoding = 'UTF-8';

    f.open('r');

    if (lineCount == 0)
        // read whole file
        data = f.read();

    else
        // read first line(s)
        while (lineCount--)
            data += f.readln() + '\n';

    f.close();

    if (data == undefined)
        throw Error('readFile: Could not read data.');

    return data;

};

/**
 * Write text to file.
 * @author m1b
 * @version 2023-05-30
 * @param {File|String} file - the file|path to write to.
 * @param {String} data - the data to write.
 * @param {Boolean} append - whether to append to end of file (default: false).
 * @returns {File} the written file
 */
function writeFile(file, data, append) {

    var success,
        writeMode = append == true ? 'a' : 'w',
        bom = "\uFEFF";

    if (file.constructor.name == 'String') {
        if (file.indexOf('/') < 0)
            file = File($.fileName).parent + '/' + file;
        file = File(file);
    }

    try {
        file.open(writeMode);
        file.encoding = "UTF-8";
        success = file.write(bom + data);
        file.close();
    }

    catch (error) {
        success = false;
        alert('writeFile failed: ' + error.message);
    }

    if (success)
        return file;

};

Note: this script uses the json3.js library. It won't work without it (or a similar library).