How to use the fs.exists function in fs

'or empty arguments to dotenv-assert. Please see the official README.md for details.';
      throw new OptionsError(errorMessage);
    }

    assertFilePath = options.filePath || assertFilePath;

    if (typeof callback !== 'function') {
      errorMessage = '    The callback is not a function';
      throw new CallbackError(errorMessage);
    }
  }

  if (assertFilePath.indexOf('/') > -1) {

    // look for explicit file location
    fs.exists(assertFilePath, function(exists) {
      if (!exists) {
        errorMessage = assertFilePath + ' does not exist.';
        throw new FileNotFoundError(errorMessage);
      }

      settings = getSettingsFromFile({
        'assertFilePath': assertFilePath
      });
      assertSettings(settings);

      callback();
    });
  } else {

    // find the file in $CWD or the nearest parent directory
    findParentDir(CURRENT_WORKING_DIRECTORY, assertFilePath, function(error, directory) {

function tryExistsForbidden(fn) {
    try {
      let rc = fs.exists(fn);
      fail();
    }
    catch (err) {
      assertEqual(arangodb.ERROR_FORBIDDEN, err.errorNum, 'stat-access to ' + fn + ' wasn\'t forbidden');
    }
    try {
      let rc = fs.mtime(fn);
      fail();
    }
    catch (err) {
      assertEqual(arangodb.ERROR_FORBIDDEN, err.errorNum, 'stat-access to ' + fn + ' wasn\'t forbidden');
    }
  }
  function tryExistsAllowed(fn, exists) {

fs.exists(firstLowerCase(path.join(__dirname, theRequireContentA)), function (e85, v85) {
fs.exists(firstLowerCase(__dirname + path.sep + theRequireContentB), function (e86, v86) {
fs.exists(firstLowerCase(__dirname + '/' + theRequireContentB), function (e87, v87) {
fs.exists(firstUpperCase(path.join(__dirname, theRequireContentA)), function (e88, v88) {
fs.exists(firstUpperCase(__dirname + path.sep + theRequireContentB), function (e89, v89) {
fs.exists(firstUpperCase(__dirname + '/' + theRequireContentB), function (e90, v90) {
//

fs.exists(path.join(__dirname, theRequireContentA + '-no-such'), function (e91, v91) {
fs.exists(__dirname + path.sep + theRequireContentB + '-no-such', function (e92, v92) {
fs.exists(__dirname + '/' + theRequireContentB + '-no-such', function (e93, v93) {
fs.exists(firstLowerCase(path.join(__dirname, theRequireContentA + '-no-such')), function (e94, v94) {
fs.exists(firstLowerCase(__dirname + path.sep + theRequireContentB + '-no-such'), function (e95, v95) {
fs.exists(firstLowerCase(__dirname + '/' + theRequireContentB + '-no-such'), function (e96, v96) {
fs.exists(firstUpperCase(path.join(__dirname, theRequireContentA + '-no-such')), function (e97, v97) {
fs.exists(firstUpperCase(__dirname + path.sep + theRequireContentB + '-no-such'), function (e98, v98) {
fs.exists(firstUpperCase(__dirname + '/' + theRequireContentB + '-no-such'), function (e99, v99) {
//

/*if(!phantom.args[1]){
        fileid = phantom.args[0].replace('http://','').replace('https://','').replace(/\//g,'');
        fileid = fileid.split('?')[0];
        myjson = 'speedreports/' + fileid + '.js';
        myfile = 'speedreports/' + fileid + '.' + extension;
    }else{
        fileid = phantom.args[1];
        myjson = 'speedreports/' + fileid + '.js';
        myfile = 'speedreports/' + fileid + '.' + extension;
    }*/

    if(myfile!==null){
        try {
            data = "var reportdata = " + data + ";";
            if(fs.exists(myfile)){
                fs.remove(myfile);
            }
            if(!fs.exists('templates/speedreport.html')){
                console.error("Unable to find template: %s", 'templates/speedreport.html');
                phantom.exit();
            }else{
                html = fs.read('templates/speedreport.html');
            }
            html=html.replace('{{REPORT_DATA_URI}}', fileid + '.js');
            if(phantom.args[1]){
                html=html.replace('{{url}}', phantom.args[1]);
            }else{
                html=html.replace('{{url}}', phantom.args[0]);
            }

            f = fs.open(myfile, "w");

function loadBrowser(filename)
{
	var param="http://localhost:1032/" + filename;
	var browserName = explorer.config.browser;
    var browser = explorer.config[browserName];
    if(browserName=='')
    {
      browser = explorer.config.chrome;
      browserName='Chrome';
    }
    
    console.log('Loading browser: '+ browser);

	fs.exists(browser, function(result) {
		if(!result) { console.log("File not found " + browser); return 0; }
		var command = browser +  " " + param;
		console.log("Running " + command);
		runner.exec(command, function(err, stdout, stderr) { 
        console.log("Terminated. "+ stderr); 
    });
	});

}

register: function(callback) {
			    		fs.exists(widgetFilename, function(exists) {
			    			if ( exists ) {
			    				tools.log('Register widget :: '+'widget.'+data+'.js') ;
			    				self.get('assets').js.push(widgetFilename.split(root_path)[1]) ;
			    				widget = new (require(widgetFilename).widget)(_.extend({}, self.attributes, {templates: self.templates}, widget)) ;
			    			}
			    			callback(null, widget) ;
			    		}); 	    				
	    			},
	    			template: function(callback) {

function generateGridsJson(locs, obj) {
    fs.exists('myjsonfile.json', function(exists) {
        if (exists) {
            console.log("yes file exists");
        } else {
            console.log("file not exists");

            var json = JSON.stringify(obj);
            fs.writeFile('myjsonfile.json', json);
        }
    });
}

function existsFile(path, fn) {
    fs.exists(path, function (exists) {
        fn(exists);
    });
}

fs.existsAsync = BBPromise.promisify(function (path, callback) {
    fs.exists(path, function (exists) {
        callback(null, exists);
    });
});

return new Promise((resolve,reject) => {
      nfs.exists(path, (ex) => resolve(ex));
    });
  }