Shows how the content-length can be unknown

fetch.js

'use strict';

const FormData = require('form-data'),
    fetch = require('node-fetch');

Promise.all([
    fetch('http://httpbin.org/stream/10'),
    fetch('http://httpbin.org/stream/10')
]).then(res => {
    let data = new FormData();
    // I have put 2 streams to show that I need FormData and its multipart support
    data.append('a', res[0].body);
    data.append('b', res[1].body);
    data.append('c', 'something else');

    // In this version the an incorrect content-length header is set because `form-data` skips over the streams
    // In this case the server will not see the data
    // Removing getLengthSync prevents content-length from being set and the server sees the data correctly
    data.getLengthSync = null;

    return fetch('http://httpbin.org/post', {
        method: 'POST',
        body: data
    })
        .then(res => res.text())
        .then(text => {
            console.log(text);
        });
})
.catch(err => {
    console.error(err);
});

http.js

'use strict';

const FormData = require('form-data'),
    fetch = require('node-fetch'),
    http = require('http');

http.request('http://httpbin.org/stream/10', (res) => {
    let data = new FormData();
    data.append('a', res);
    data.append('c', 'something else');
    
    // In this version the error is thrown because `form-data` correctly finds that it cannot
    // find the length of the stream
    // Removing getLengthSync prevents content-length from being set and the server sees the data correctly
    data.getLengthSync = null;

    return fetch('http://httpbin.org/post', {
        method: 'POST',
        body: data
    })
        .then(res => res.text())
        .then(text => {
            console.log(text);
        })
        .catch(err => {
            console.error(err);
        });
}).end();