toJSON(...args) {
        return io.toJSON(this, ...args);
    }
Convert the DataFrame into a json string. You can also save the file if you are using nodejs.
@param {Boolean} [asCollection=false] Writing the JSON as collection of Object.
@param {String} [path] The path to save the file. /!\ Works only on node.js, not into the browser.
@returns {String} The json file in raw string.
@example
df.toJSON()
// From node.js only
df.toJSON('/my/absolute/path/dataframe.json')