ELI5: How does facebook could get new notification without us to refresh the page?

Hmm.. except:

<script src="https://fbstatic-a.akamaihd.net/rsrc.php/v2/yd/r/I33VCk7I0ai.js" crossorigin="anonymous"></script> <script> (require("ServerJSDefine")).handleDefines([ ["BootloaderConfig", [], {}, 329], ["CSSLoaderConfig", [], { "timeout": 5000 }, 619] ]); new(require("ServerJS"))().handle({ "require": [ ["Bootloader", "loadEarlyResources", [], [{ "O26Xt": { "type": "js", "crossOrigin": 1, "src": "https://fbstatic-a.akamaihd.net/rsrc.php/v2/yw/r/Dq9mR4mb3VY.js" }, "by1mQ": { "type": "js", "crossOrigin": 1, "src": "https://fbstatic-a.akamaihd.net/rsrc.php/v2/yZ/r/ohrIEvHgnrS.js" }, "m6/d9": { "type": "js", "crossOrigin": 1, "src": "https://fbstatic-a.akamaihd.net/rsrc.php/v2/ym/r/syVeUiErL9P.js" }, "muRv2": { "type": "js", "crossOrigin": 1, "src": "https://fbstatic-a.akamaihd.net/rsrc.php/v2/ym/r/199Yegj3F-g.js" }, "WEnmS": { "type": "js", "crossOrigin": 1, "src": "https://fbstatic-a.akamaihd.net/rsrc.php/v2/yA/r/zrrnFLgaZ9i.js" }, "9JtNC": { "type": "js", "crossOrigin": 1, "src": "https://fbstatic-a.akamaihd.net/rsrc.php/v2/ya/r/S9tdZBp_wUC.js" }, "2tsAG": { "type": "js", "crossOrigin": 1, "src": "https://fbstatic-a.akamaihd.net/rsrc.php/v2/yK/r/x1WKvwyByXR.js" }, "+aj6/": { "type": "js", "crossOrigin": 1, "src": "https://fbstatic-a.akamaihd.net/rsrc.php/v2/y3/r/Lj4-S7lZW1S.js" } }] ] ] }); </script> <script></script>

/r/explainlikeimfive Thread