Node (Express) - Trying to save a PDF from an API call
Your code doesn't work because the underlying request library (used by request-promise) requires the option encoding set to null for binary data - see https://github.com/request/request#requestoptions-callback.
Here's how you download binary data using that module -
app.post("/getPayslipURL", function(client_request, res) { const NI_NUMBER_REQUEST = db_api.createRequestTemplate({ body: JSON.stringify(client_request.body), encoding: null }); requestPromise(NI_NUMBER_REQUEST) .then((db_response) => { const PAY_API_OPTIONS = /*Code to generate options based on furhter DB info (Includes dates etc)*/ return requestPromise(PAY_API_OPTIONS); // Call pay API }) .then((pay_pdf_data) => { fs.writeFile("./test.pdf", pay_pdf_data, 'binary', (err) => { if(err) { return console.log(err); } console.log("The file was saved!"); }); }) .catch(err => `Error caught: ${console.log}`) // Catch any errors on our request chain });}