How to Add Address Autocomplete in Node.js / Express

What the API returns

The API wraps every response in a standard envelope. The address suggestions live in the Result field, which for autocomplete is a plain array of strings:

{
  "Id": "3f2504e0-4f89-11d3-9a0c-0305e82c3301",
  "Result": [
    "123 Main St APT 1, Andover, MA 01810-3816",
    "123 Main St APT 1, Delhi, NY 13753-1257",
    "123 Main St STE 1, Caldwell, ID 83605-5476",
    "123 Main St STE 1, Corinth, NY 12822-1010",
    "123 Main St STE 1, Delhi, NY 13753-1258"
  ],
  "ClientSessionId": null,
  "StatusCode": 200,
  "IsError": false,
  "Errors": []
}

Each suggestion includes the full street, the unit designation (APT, STE, UNIT), city, state code, and ZIP+4. The API handles abbreviations (St, Ave, Blvd) and directional prefixes (N, S, E, W) on the way in, and returns clean, standardized output. In Node you read body.Result and hand that array back to the browser.

Get your API key

1. Sign up at sthan.io and subscribe to the free Address Autocomplete tier
2. Open your dashboard and create an API key
3. Copy the key - it looks like sthan_live_xxxxxxxxxxxxxxxx

You get the key immediately, with no approval queue. An API key is the simplest way to authenticate: you send it as a Bearer token on every request and there is no separate login step. (If you prefer a short-lived token, there is a JWT flow covered later.)

Set up the project

npm init -y
npm install express dotenv

Store the key in a .env file at the project root - never in source you commit:

# .env
STHAN_API_KEY=sthan_live_xxxxxxxxxxxxxxxx

Write the fetch wrapper

Node 18+ ships a global fetch, so no HTTP library is needed. This small function sends the API key as a Bearer token, URL-encodes the query, and returns the Result array:

// sthan.js
const API_BASE = "https://api.sthan.io";
const API_KEY = process.env.STHAN_API_KEY;

export async function autocomplete(query) {
  const text = (query || "").trim();
  if (text.length < 3) return [];

  const res = await fetch(
    `${API_BASE}/AutoComplete/USA/Address/${encodeURIComponent(text)}`,
    { headers: { Authorization: `Bearer ${API_KEY}` } }
  );

  if (!res.ok) {
    throw new Error(`Autocomplete failed: ${res.status}`);
  }

  const body = await res.json();
  if (body.IsError) {
    throw new Error(body.Errors.join(", "));
  }

  // The envelope wraps the data — suggestions are in Result
  return body.Result ?? [];
}

The whole integration is this one request. Everything else is plumbing to keep the key on the server and to debounce the front end.

Expose a proxy route

The browser should call your server, and your server calls sthan.io. There are two reasons for this. First, the API does not enable CORS for browser requests, so a direct call from the page would be blocked. Second, and more important, putting your API key in client-side JavaScript would expose it to anyone who opens the network tab. The Express route keeps the key on the server.

// server.js
import "dotenv/config";
import express from "express";
import { autocomplete } from "./sthan.js";

const app = express();
app.use(express.static("public"));

app.get("/api/address/autocomplete", async (req, res) => {
  const query = (req.query.query || "").toString();
  try {
    const suggestions = await autocomplete(query);
    res.json(suggestions);
  } catch (err) {
    console.error(err);
    res.status(502).json({ error: "lookup_failed" });
  }
});

app.listen(3000, () => console.log("Listening on http://localhost:3000"));

Your front end now has a clean URL to call: /api/address/autocomplete?query=123 main st returns a JSON array of addresses, and the API key never leaves the server.

Wire up the front-end input

The last piece is a debounced input that calls your proxy. Debouncing matters: without it, "123 main st" fires eleven requests, one per keystroke. With a 250ms debounce, it fires one request after the user pauses. Add this to a page in your public/ folder:

<input type="text" id="address" autocomplete="off"
       placeholder="Start typing your address..." />
<ul id="suggestions"></ul>

const input = document.getElementById("address");
const list = document.getElementById("suggestions");
let timer;

input.addEventListener("input", () => {
  clearTimeout(timer);
  const query = input.value.trim();

  if (query.length < 3) {
    list.innerHTML = "";
    return;
  }

  // Wait 250ms after the last keystroke before calling the server
  timer = setTimeout(async () => {
    const res = await fetch(
      `/api/address/autocomplete?query=${encodeURIComponent(query)}`);
    const items = await res.json();
    list.innerHTML = items.map((a) => `<li>${a}</li>`).join("");
  }, 250);
});

The browser only ever talks to /api/address/autocomplete on your own domain. No key, no CORS, no third-party script. From here you can style the list, add keyboard navigation, and fill the form fields when a user clicks a suggestion.

Alternative: JWT authentication

An API key is the simplest option and is all most apps need. If your security policy prefers short-lived credentials, the platform also supports a 2-step JWT flow. You call GET /Auth/Token once with your profileName and profilePassword headers, receive a token valid for up to 60 minutes, then send that token as the Bearer value on subsequent calls. Cache it and refresh on expiry:

let cached = { token: null, expires: 0 };

async function getToken() {
  if (cached.token && Date.now() < cached.expires - 30_000) {
    return cached.token;
  }

  const res = await fetch("https://api.sthan.io/Auth/Token", {
    headers: {
      profileName: process.env.STHAN_PROFILE_NAME,
      profilePassword: process.env.STHAN_PROFILE_PASSWORD,
    },
  });
  const body = await res.json();

  cached.token = body.Result.access_token;
  cached.expires = new Date(body.Result.expiration).getTime();
  return cached.token;
}

You would then build the Authorization header per request from the cached token instead of a static key. Everything else - the endpoint, the envelope, the parsing - stays the same.

Handle errors

Two status codes are worth handling explicitly so a hiccup never crashes your form:

• 401 - The key or token was rejected. Check the value and, on the JWT flow, refresh and retry once.
• 429 - Rate limit reached. Back off and return what the user has typed so far rather than throwing.

export async function safeAutocomplete(query) {
  try {
    return await autocomplete(query);
  } catch (err) {
    // Degrade gracefully — an empty list beats a broken form
    return [];
  }
}

In the proxy route, a non-2xx from your own endpoint should still return a small JSON body so the front end can render an empty list rather than choke on unexpected HTML.

What's next: confirm the address is deliverable

Autocomplete gets the user to a clean, well-formed address fast. It does not, on its own, confirm that mail or a package will actually arrive there - a suggestion can be correctly formatted yet point at a unit that no longer accepts delivery.

The natural next step is to run the chosen address through the Address Verification API at the moment the user submits the form. It returns a Delivery Point Validation (DPV) result and a deliverable status, standardizes the address to standard postal format, and appends ZIP+4 and county. The call is the same pattern you already built - one GET, the same envelope:

const res = await fetch(
  `https://api.sthan.io/v2/address-verification/usa/speculative/${encodeURIComponent(selected)}`,
  { headers: { Authorization: `Bearer ${API_KEY}` } });
const { Result } = await res.json();
// Read Result.deliverableStatus and Result.dpvConfirmation

Address Verification has its own free tier of 100 requests/month, with paid plans from $12/month. Pairing autocomplete (volume, real-time, as the user types) with verification (one confirming call at submit) keeps your costs low and your delivery data clean. For a deeper walkthrough of the address APIs, see the complete guide to address verification.