Shows how the content-length can be unknown

file.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();
    data.append('a', res[0].body);
    data.append('b', res[1].body);
    data.append('c', 'something else');

    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);
});