how to read ts file and update code dynamically using fs?

Question

i am scaffolding new project using yeoman generator it is creating all the directories and running dependencies, now once files are generated i want to update js class same as appName, first i am trying to read the ts file which i failed to do it throws error TypeError: Cannot read property 'toString' of undefined then i would update the file with appName if there is any better approach to achieve this task i will apprecaite the help.

index.js

 updateTsFile () {
    const npmdir = `${process.cwd()}/${this.props.appName}`;
    const dirPath = `${npmdir}/${"./api.ts"}`;
    console.log("path", dirPath);
    let response;
    _fs.readFile(dirPath, (_err, res) => {
      if (_err) {
        console.error(_err);
      }

      let file = res.toString("utf-8");
      console.log(file);
      response = file;
      let lines = file.split("\n");
      for (let i = 0; i < lines.length; i++) {
        console.log(lines[i]);
      }
    });
    return response;
  }

api.ts

export class CAPIClass extends Wrapper {
    public after = after;
    constructor() {
        super({
            configFileName: "package-name-v1.json"
        });
    }
}

expected output

export class CMyAppNameClass extends Wrapper {
    public after = after;
    constructor() {
        super({
            configFileName: "package-name-v1.json"
        });
    }
}

Answer 1

In case of an error you're just logging the error but continuing with the logic. So it seems like you're running into an error resulting in res being undefined . Since fs exposes a promise-based api nowadays, I would rewrite this as follows instead of using callbacks (also note that you were using utf-8 for the encoding but it should be utf8 ):

async updateTsFile() {
    const npmdir = `${process.cwd()}/${this.props.appName}`;
    const dirPath = `${npmdir}/${"./api.ts"}`;
    console.log("path", dirPath);

    try {
        const fileData = await _fs.promises.readFile(dirPath);
        const fileAsStr = fileData.toString("utf8");

        // replace class-name
        fileAsStr = fileAsStr.replace(/CAPIClass/g, "CMyAppNameClass");
        // (over)write file: setting 'utf8' is not actually needed as it's the default
        await _fs.promises.writeFile(dirPath, fileAsStr, 'utf8');
    } catch (err) {
        console.log(err);
        // handle error here
    }

}