discordAttachmentMessage.html

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title>Find Message URL of Discord Attachment</title>
</head>
<body>

    <script type="text/javascript">
        function strictParseInt(num) {
            try {
                const result = JSON.parse(num);
                return Number.isInteger(result) ? result : NaN;
            } catch {
                return NaN;
            }
        }

        function getMessageUrl(attachmentUrl, serverId) {
            const attachPrefix = "https://cdn.discordapp.com/attachments/";
            const messagePrefix = "https://discord.com/channels/";
            if (!attachmentUrl.startsWith(attachPrefix)) {
                throw new Error("Invalid attachment URL");
            }
            if (!strictParseInt(serverId)) {
                throw new Error("Invalid server ID");
            }
            attachmentUrl = new URL(attachmentUrl);
            let [channelId, messageId] = 
                attachmentUrl.pathname.split("/").slice(2, 4);
            let messagePath = [serverId, channelId, messageId].join("/");
            return messagePrefix + messagePath;
        }

        function generate() {
            let attachmentUrl = document.getElementById("url-input").value;
            let serverId = document.getElementById("id-input").value;
            let linkElem = document.getElementById("message-link");
            let errorElem = document.getElementById("error");
            let messageUrl = "";
            try {
                messageUrl = getMessageUrl(attachmentUrl, serverId);
                errorElem.innerText = "";
            } catch(e) {
                errorElem.innerText = e.message;
            }
            linkElem.href = messageUrl;
            linkElem.innerText = messageUrl;
        }
    </script>

    <h1>Find Message URL of Discord Attachment</h1>
    <p>
        This tool generates the message URL associated with the given 
        Discord attachment URL and server ID.
    </p>
    <div id="box1">
        <label for="url-input">Attachment URL:</label><br>
        <input type="text" id="url-input" value="" style="width:60em" />
    </div>
    <br>
    <div id="box2">
        <label for="id-input">Server ID:</label><br>
        <input type="text" id="id-input" value="" style="width:10em" />
    </div>
    <br>
    <button onclick="generate();">Generate Message URL</button>
    <br><br>
    <div id="link-area">
        <a id="message-link" href=""></a>
        <div id="error" style="color: red"></div>
    </div>
    
</body>
</html>