Just run the script in your terminal like this…
node script-file.js > log-file.txt
This tells the shell to write the standard output of the command node script-file.js to your log file instead of the default, which is printing it to the console.
This is called redirection and its very powerful. Say you wanted to write all errors to a separate file…
node script-file.js >log-file.txt 2>error-file.txt
Now all console.log are written to log-file.txt and all console.error are written to error-file.txt