post-based-redirect.js

var redirectUri = "https://example.com/after-auth?code=123&state=456";
post(redirectUri);

// Thanks to https://stackoverflow.com/a/133997/318206
function post(path, params, method) {
    method = method || "post";
    var form = document.createElement("form");
    form.setAttribute("method", method);
    form.setAttribute("action", path);

    for(var key in params) {
        if(params.hasOwnProperty(key)) {
            var hiddenField = document.createElement("input");
            hiddenField.setAttribute("type", "hidden");
            hiddenField.setAttribute("name", key);
            hiddenField.setAttribute("value", params[key]);

            form.appendChild(hiddenField);
         }
    }

    document.body.appendChild(form);
    form.submit();
}