Node.js comes bundled with the file system module which allows the developers to interact with the file system of the operating system. This module is commonly used to create, read, update, delete and rename a file. However, before we start performing the above-mentioned operations it is always a better idea to check if the file we're trying to access exists in the file system. And the file system module provides multiple ways to perform this check.
Using existsSync method

The easiest way to test if a file exists in the file system is by using the existsSync method. All you need to do is pass the path of the file to this method.
const fs = require('fs');
const path = './my-awesome-file.txt';

if (fs.existsSync(path)) {
  console.log('file exists');
} else {
  console.log('file not found!');
}

The existsSync method will return true if the file exists and else it'll return false.
One thing that we need to keep in mind is this method is synchronous in nature. that means that it’s blocking.

Using exists method (Deprecated)
The exists method checks for the existence of a file asynchronously. It accepts two parameters:
    Path to the file
    Callback function to execute once path is verified passing a boolean parameter value to the function as its argument.
const fs = require('fs');
const path = './my-awesome-file.txt';

fs.exists(path, function (doesExist) {
  if (doesExist) {
    console.log('file exists');
  } else {
    console.log('file not found!');
  }
});

The parameter value will be true when the file exists otherwise it'll be false.

Using accessSync method
The accessSync method is used to verify the user’s permission for the specified path. If the path is not found, it will throw an error that we need to handle using try...catch block.

This method accepts two parameters as follows,
    Path to the file
    Mode of verification

The mode parameter is optional. It specifies the accessibility checks that need to be performed.0

Following constants can be used as the second parameter to specify the mode.
      fs.constants.R_OK to check for read permission
    fs.constants.W_OK to check for write permission
    fs.constants.X_OK to check for execute permission

If the second parameter is not provided it defaults to  fs.constants.F_OK constant.
const fs = require('fs');
const path = './my-awesome-file.txt';

try {
  fs.accessSync(path);
  console.log('file exists');
} catch (err) {
  console.log('file not found');
  console.error(err);
}

The above code verifies the existence of my-awesome-file.txt file:

If you wish to determine the write and read access, you can pass the mode parameter with a bitwise OR operator as follows:
const fs = require('fs');
const path = './my-awesome-file.txt';

try {
  fs.accessSync(path, fs.constants.R_OK | fs.constants.W_OK);
  console.log('read/write access');
} catch (err) {
  console.log('no access:');
  console.error(err);
}

Using access method
The access method is basically the asynchronous version of accessSync method.

The access method allows the developers to verify the existence or permissions of a specified path asynchronously.

The method accepts three parameters, first two parameters are the same as accessSync method parameters, and last parameter is a callback function:
    Path to the file
    Mode of verification
    The callback function to execute

const fs = require('fs')
const path = './my-awesome-file.txt'

fs.access(path, fs.F_OK, (err) => {
  if (err) {
    console.error(err)
    return;
  } else {
    console.log('file exists')
  }
})

existsSync, exists, accessSync, and access are the four methods that allow the developers to verify if the file exists in the file system. It’s recommended to use existsSync method if you only wish to check for the existence of a file. When you wish to check for specific permissions as well as the existence of the file, you can use either accessSync or access method. The exists method is deprecated from node.js and hence needs to be avoided.