Nodejs wrap async function in synchronous function

I am writing a spoof of the fs module; the actual storage mechanism is not the the filesystem but rather a database. Overall, however, the api will function exactly like fs.

So far my code is comparable to the following:

let underlyingDb = require('module-for-accessing-db');

let mockFs = {
  
  promises: {
    
    readFile: async (fp, opts) => {
      let pk = convertFpToPk(fp);
      let res = await underlyingDb.query('SELECT * FROM "MockFs" WHERE "pk"=$1', [ pk ]);
      
      // Mock potential fs error:
      if (!res.rows.length) throw new Error(`ENOENT: no such file or directory...`);
      
      // TODO: check `opts` to see which encoding is desired
      return res.rows[0].fileContent;
    },
    writeFile: async (fp, data, opts) => {
      // Same idea as readFile, but with an INSERT ON CONFLICT statement
      // instead of SELECT...
    }
    
  },
  
  readFileSync: () => { /* ... */ }
  
};

As you can see it is straightforward for me to mock async file reading and writing.

I am now wondering, however, if it is possible to mock the *Sync methods in fs. Importantly, module-for-accessing-db only provides async access methods!

My initial instinct was to try the following (warning: gross!):

{
  readFileSync: (fp, data) => {
    let result = null;
    let err = null;
    
    (async () => {
      try {
        result = await mockFs.promises.readFile(fp, data);
      } catch(err0) {
        err = err0;
      }
    })();
    
    while (result === null && err === null) {} // Ew!!
    
    if (err) throw err;
    return result;
  }
}

Of course, this doesn't work. The while loop chokes out the entire event loop making it impossible for result or err to ever become set, even if the async database queries complete.

What are my options here? Do I need to switch the module I'm using to access the db? Do I need to write C code? Or is there a really clever technique to directly solve this problem, using only nodejs?

posted about 4 years ago

CC BY-SA 4.0

4y ago

Gershy‭

46 reputation 2 0 9 1