OG Images from an Express Server

Generate OG images in Express with FastOG's credit-based API — sign up free for 100 credits (create your API key in the dashboard). Preview any template in the Free OG Image Tester, then serve dynamic Open Graph images from your Express server with HMAC-signed URLs.

Setup

bash
npm install express

Implementation

javascript
// app.js
import express from "express";
import crypto from "crypto";
import { URL } from "url";

const app = express();

function generateSignedOgUrl(params, options = {}) {
    const {
        baseUrl = "https://fastog.com/api/v1/og",
        apiKey = process.env.FASTOG_API_KEY,
        hmacSecret = process.env.FASTOG_HMAC_SECRET,
    } = options;

    const timestamp = Math.floor(Date.now() / 1000);
    const url = new URL(baseUrl);

    for (const [key, value] of Object.entries(params)) {
        url.searchParams.set(key, value);
    }

    const apiKey = apiKey;
    const path = url.pathname;
    const message = `GET${path}${timestamp}${apiKey}`;
    const signature = crypto
        .createHmac("sha256", hmacSecret)
        .update(message)
        .digest("hex");

    url.searchParams.set("key", apiKey);
    url.searchParams.set("s", signature);
    url.searchParams.set("t", timestamp);

    return url.toString();
}

// Blog post route
app.get("/blog/:slug", async (req, res) => {
    const post = await getPost(req.params.slug);

    const ogImageUrl = generateSignedOgUrl({
        template: "blog",
        title: post.title,
        subtitle: post.excerpt,
    });

    res.render("blog/post", {
        post,
        ogImageUrl,
    });
});

// API endpoint for dynamic OG images
app.get("/api/og", (req, res) => {
    const { title, template = "default" } = req.query;

    if (!title) {
        return res.status(400).json({ error: "Missing title" });
    }

    const ogImageUrl = generateSignedOgUrl({
        template,
        title,
    });

    res.json({ ogImageUrl });
});

app.listen(3000, () => {
    console.log("Server running on port 3000");
});

EJS Template

ejs
<!-- views/blog/post.ejs -->
<!DOCTYPE html>
<html>
<head>
    <meta property="og:image" content="<%= ogImageUrl %>" />
    <meta property="og:title" content="<%= post.title %>" />
    <meta property="og:description" content="<%= post.excerpt %>" />
</head>
<body>
    <h1><%= post.title %></h1>
</body>
</html>

Pug Template

pug
// views/blog/post.pug
doctype html
html
  head
    meta(property="og:image" content=ogImageUrl)
    meta(property="og:title" content=post.title)
    meta(property="og:description" content=post.excerpt)
  body
    h1= post.title

Middleware Approach

javascript
// middleware/og-image.js
import { generateSignedOgUrl } from "../lib/og-image-signer.js";

export function injectOgImage(template, titleFn) {
    return (req, res, next) => {
        const originalRender = res.render.bind(res);

        res.render = function (view, locals) {
            const title = titleFn(locals);
            locals.ogImageUrl = generateSignedOgUrl({
                template,
                title,
            });
            return originalRender(view, locals);
        };

        next();
    };
}

// Usage
app.get(
    "/blog/:slug",
    injectOgImage("blog", (locals) => locals.post.title),
    async (req, res) => {
        const post = await getPost(req.params.slug);
        res.render("blog/post", { post });
    },
);

Environment Variables

bash
# .env
FASTOG_API_KEY=sk_your_full_api_key_here
FASTOG_HMAC_SECRET=your_64_char_hex_secret_here

Full API Reference

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