Utility function for getting the image URL for a Messenger-style emoji

DOCS.md

@@ -14,6 +14,7 @@
 * [`api.forwardAttachment`](#forwardAttachment)
 * [`api.getAppState`](#getAppState)
 * [`api.getCurrentUserID`](#getCurrentUserID)
+* [`api.getEmojiUrl`](#getEmojiUrl)
 * [`api.getFriendsList`](#getFriendsList)
 * [`api.getThreadHistory`](#getThreadHistory)
 * [`api.getThreadInfo`](#getThreadInfo)
@@ -440,6 +441,41 @@ Returns the currently logged-in user's Facebook user ID.
 
 ---------------------------------------
 
+<a name="getEmojiUrl"></a>
+### api.getEmojiUrl(c, size[, pixelRatio])
+
+Returns the URL to a Facebook Messenger-style emoji image asset.
+
+__note__: This function will return a URL regardless of whether the image at the URL actually exists.
+This can happen if, for example, Messenger does not have an image asset for the requested emoji.
+
+__Arguments__
+
+* `c` - The emoji character
+* `size` - The width and height of the emoji image; supported sizes are 32, 64, and 128
+* `pixelRatio` - The pixel ratio of the emoji image; supported ratios are '1.0' and '1.5' (default is '1.0')
+
+__Example__
+
+```js
+const fs = require("fs");
+const login = require("facebook-chat-api");
+
+login({appState: JSON.parse(fs.readFileSync('appstate.json', 'utf8'))}, (err, api) => {
+    if(err) return console.error(err);
+
+    // Prints https://static.xx.fbcdn.net/images/emoji.php/v8/z9c/1.0/128/1f40d.png
+    console.log('Snake emoji, 128px (128x128 with pixel ratio of 1.0');
+    console.log(api.getEmojiUrl('\ud83d\udc0d', 128));
+
+    // Prints https://static.xx.fbcdn.net/images/emoji.php/v8/ze1/1.5/128/1f40d.png
+    console.log('Snake emoji, 192px (128x128 with pixel ratio of 1.5');
+    console.log(api.getEmojiUrl('\ud83d\udc0d', 128, '1.5'));
+});
+```
+
+---------------------------------------
+
 <a name="getFriendsList"></a>
 ### api.getFriendsList(callback)
 

index.js

@@ -85,6 +85,7 @@ function buildAPI(globalOptions, html, jar) {
     'deleteThread',
     'forwardAttachment',
     'getCurrentUserID',
+    'getEmojiUrl',
     'getFriendsList',
     'getThreadHistory',
     'getThreadInfo',

src/getEmojiUrl.js

@@ -0,0 +1,24 @@
+"use strict";
+
+const util = require("util");
+
+module.exports = function() {
+  return function getEmojiUrl(c, size, pixelRatio) {
+    /*
+     Resolves Facebook Messenger emoji image asset URL for an emoji character.
+     Supported sizes are 32, 64, and 128.
+     Supported pixel ratios are '1.0' and '1.5' (possibly more; haven't tested)
+     */
+    const baseUrl = 'https://static.xx.fbcdn.net/images/emoji.php/v8/z%s/%s';
+    pixelRatio = pixelRatio || '1.0';
+
+    let ending = util.format('%s/%s/%s.png', pixelRatio, size, c.codePointAt(0).toString(16));
+    let base = 317426846;
+    for (let i = 0; i < ending.length; i++) {
+      base = (base << 5) - base + ending.charCodeAt(i);
+    }
+
+    let hashed = (base & 255).toString(16);
+    return util.format(baseUrl, hashed, ending);
+  };
+};