JavaScript Navigator registerProtocolHandler() Method

const lines = [
  "registerProtocolHandler: " +
    (typeof navigator.registerProtocolHandler === "function"
      ? "available"
      : "missing"),
  "isSecureContext: " + window.isSecureContext,
  "origin: " + location.origin
];
console.log(lines.join("\n"));

function registerMailtoHandler() {
  if (typeof navigator.registerProtocolHandler !== "function") {
    return "API not supported";
  }
  if (!window.isSecureContext) {
    return "Needs HTTPS / localhost";
  }
  try {
    // Handler URL must be same-origin as this page in real apps
    const handler =
      location.origin + "/mail/compose?to=%s";
    navigator.registerProtocolHandler("mailto", handler);
    return "Registration requested for mailto → " + handler;
  } catch (err) {
    return "Error: " + err.name + " — " + err.message;
  }
}

console.log(registerMailtoHandler());

function registerCustomScheme() {
  if (typeof navigator.registerProtocolHandler !== "function") {
    return "API not supported";
  }
  try {
    const handler = location.origin + "/open?item=%s";
    navigator.registerProtocolHandler("web+demo", handler);
    return "Requested web+demo → " + handler;
  } catch (err) {
    return "Error: " + err.name;
  }
}

console.log(registerCustomScheme());
// Link example: <a href="web+demo:cheeseburger">open</a>

function previewHandlerUrl(template, activatedHref) {
  if (!template.includes("%s")) {
    return { ok: false, reason: "missing-%s" };
  }
  return {
    ok: true,
    result: template.replace("%s", encodeURIComponent(activatedHref))
  };
}

const template = "https://mail.example.org/?to=%s";
const activated = "mailto:webmaster@example.com";
console.log(JSON.stringify(previewHandlerUrl(template, activated)));

function registerProtocolHandlerSafe(scheme, url) {
  if (typeof navigator.registerProtocolHandler !== "function") {
    return { ok: false, reason: "unsupported" };
  }
  if (!window.isSecureContext) {
    return { ok: false, reason: "insecure-context" };
  }
  if (!url.includes("%s")) {
    return { ok: false, reason: "missing-%s" };
  }
  try {
    navigator.registerProtocolHandler(scheme, url);
    return { ok: true, reason: "requested" };
  } catch (err) {
    return { ok: false, reason: err.name };
  }
}

// Demo: intentionally missing %s → SyntaxError path / pre-check
console.log(JSON.stringify(
  registerProtocolHandlerSafe("mailto", location.origin + "/mail?to=")
));
console.log(JSON.stringify(
  registerProtocolHandlerSafe("mailto", location.origin + "/mail?to=%s")
));