Un poco de manipulación básica de cadenas funcionará bien.

 const fs = require("fs"); const path = require("path"); // get paths to file const file1 = path.join(__dirname, "path/to/file1.csv"), file2 = path.join(__dirname, "path/to/file2.csv"); // read the csv files const csv1 = fs.readFileSync(file1), csv2 = fs.readFileSync(file2); // split the second csv by a new line then remove the first line (remove the header) const [, ...rest] = csv2.split("\n"); const result = csv1 + rest.join("\n"); // combine csv fs.writeFileSync(path.join(__dirname, "output.csv"), result); // write result to file

También puedes hacer esto dinámicamente:

 // let's pretend you have an array of all the CSV text that you read from the file system named 'csv' for(let i = 1; i < csv.length; i++) { const [, ...spl] = csv[i].split("\n"); // instead of doing [, ...spl] we could also call: spl.shift(); csv[i] = spl.join("\n"); } const result = csv.join("\n"); // our combined csv