Nightwatch: Custom assertion checking file exists

Is there a way to write my own custom assertion which can check if the file exists or not ? I have tried every implementations as the docs said but it seems not working !

Here is my code:

const fs = require('fs');

exports.assertion = function (path) {
  this.message = 'Testing if file exists';

  this.expected = true;

  this.pass = (value) => {
    return value === this.expected;
  };

  this.value = (result) => {
    return result;
  };

  this.command = (callback) => {
    return this.api.execute(function (filePath) {
      try {
        fs.existsSync(filePath);
        return true;
      } catch (err) {
        return false;
      }
    }, [path],
    function (result) {
      callback(result.value);
    });
  };
};

browser
    .click('#export-postgres-btn')
    .assert.checkFileExists(`~/Downloads/${diagramName}.sql`);

The result is always false even when the file does exist !