<!doctype htmtl> <html> <head> <meta charset=utf-8> <title>Forms</title> </head> <body> <h3>Submit Json</h3> <button onclick="submitJson()">Submit</button> <script> let payload = { timestamp: 12345, kind: "this is a kind", tags: ['tag1', 'tag2', 'tag3'], } function submitJson() { fetch('http://localhost:8080/event', { method: 'POST', headers: { 'Accept': 'application/json', 'Content-Type': 'application/json' }, body: JSON.stringify(payload) }) .then(function (res) { return res.json(); }) .then(function (data) { let expected = { ...payload, id: 1 }; console.log("expected: ", expected); console.log("received: ", data); }); } </script> </body> </html>