HTTP Content-Length header calculation in javascript

Question

I am new in javascript and node.js. I have an issue in calling a rest endpoint. My problem is when I try to call the endpoint using Postman everything is ok. I can see the calculated Content-Length header in Postman console. But when I try to call the same endpoint in javascript something is wrong because the calculated Content-Length is a bit lower than the calculated one by Postman. Here is my node.js code that I am using to call the endpoint. Note that the API is multipart API that is used for uploading the files.

file = fs.readFileSync('/Users/.../Documents/doc.pdf')
form = new FormData()
form.append('file', file)
headers = Object.assign({'Content-Length': form.getLengthSync() }, form.getHeaders())
config = {
    method: 'post',
    url: 'https://staging:...',
    headers: headers,
    data : form
};
axios(config).then((res) ->
        console.log("The file is posted successfully. status is : " + res.status)
    ).catch((err) -> 
        console.log("The error occurred in posting file : " + err)
    )

Can you please tell me what is missing here? for example for a specific file that I am testing the postman calculated value is 388 but the code calculated it as 379 for the same file. Note: when I hardcode the value with the value from Postman everything is ok.

Answer 1

Issue #426 in the form-data repository seems to describe a solution for your problem.

I gave it a try locally and managed to get the desired results by applying what described in the issue:

const filePath = '/Users/.../Documents/doc.pdf';
const form = new FormData();
let options = {
  knownLength: fs.statSync(filePath).size
}
form.append(
  'file',
  fs.createReadStream(filePath),
  options,
);

I gather the issue is related to the acclaration made in the getLengthSync specification , which states that stream lengths' aren't calculated, hence the extra step that must be taken in order to provide the correct file length to the FormData instance.