There are several ways to concatenate JavaScript files into one file. Here are a few methods:

1. Manually concatenate the files: You can open each JavaScript file in a text editor and copy and paste the code into a new file, in the order that you want the code to execute. Once you have all the code in one file, save it with a new name.

2. Use a build tool like Gulp or Grunt: These tools automate the process of concatenating files and can also perform other tasks such as minification and code optimization. You will need to install the tool and create a configuration file that specifies the input files and output file.

3. Use a bundler like Webpack or Parcel: These tools allow you to use modules in your JavaScript code and then bundle all the modules into a single file for deployment. You will need to install the tool and create a configuration file that specifies the entry point and output file.

Here is an example of how to concatenate files using Node.js and the built-in file system module:

const fs = require('fs');

// Array of file names to concatenate
const files = ['file1.js', 'file2.js', 'file3.js'];

// Loop through the files and concatenate them
let concatenatedCode = '';
for (let i = 0; i < files.length; i++) {
  concatenatedCode += fs.readFileSync(files[i], 'utf8') + '\n';
}

// Write the concatenated code to a new file
fs.writeFileSync('output.js', concatenatedCode);

This code reads in each file, concatenates them together with a newline character, and writes the concatenated code to a new file called “output.js”. Note that this is a very simple example and does not perform any error handling or optimization.