Here's a good, better, best approach to convert a JavaScript object to "application/x-www-form-urlencoded" postable form data.

formData.js

const formData = { foo: "foo", bar: "M&Ms" };

// Good 🔥
const postBody = Object.keys(formData)
  .map((key) => encodeURIComponent(key) + "=" + encodeURIComponent(body[key]))
  .join("&");

// Better 🔥🔥
const postBody = Object.entries(formData)
  .map(([key, value]) => `${encodeURIComponent(key)}=${encodeURIComponent(value)}`)
  .join("&");

// Best 🔥🔥🔥🔥🔥
const postBody = new URLSearchParams(formData).toString();