Example JavaScript code to consume a custom JSON API in a Wordpress plugin

custom-route-script.js

document.addEventListener('DOMContentLoaded', async () => {
  const data = {
    key1: 'data1',
    key2: 'data2',
  };

  const response = await fetch('/wp-json/custom-api-route/v1/submit', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
    },
    body: JSON.stringify(data),
  });

  const result = await response.json();
  console.log(result);
});