{"id":16836,"date":"2019-12-15T11:16:05","date_gmt":"2019-12-15T11:16:05","guid":{"rendered":"https:\/\/kiencang.net\/?p=14023"},"modified":"2019-12-15T11:16:05","modified_gmt":"2019-12-15T11:16:05","slug":"tai-javascript-defer-async","status":"publish","type":"post","link":"https:\/\/kiencang.net\/tai-javascript-defer-async\/","title":{"rendered":"T\u1ea3i JavaScript hi\u1ec7u qu\u1ea3 v\u1edbi defer v\u00e0 async"},"content":{"rendered":"\n<p>Khi b\u1ea1n t\u1ea3i JavaScript tr\u00ean trang HTML, b\u1ea1n c\u1ea7n c\u1ea9n th\u1eadn \u0111\u1ec3 kh\u00f4ng l\u00e0m \u1ea3nh h\u01b0\u1edfng x\u1ea5u \u0111\u1ebfn <a href=\"https:\/\/kiencang.net\/tang-toc-wordpress\/\" target=\"_blank\" rel=\"noreferrer noopener\">hi\u1ec7u su\u1ea5t (t\u1ed1c \u0111\u1ed9) c\u1ee7a trang<\/a>. Ph\u1ee5 thu\u1ed9c v\u00e0o vi\u1ec7c b\u1ea1n \u0111\u1eb7t \u0111o\u1ea1n m\u00e3 JavaScript <strong>\u1edf \u0111\u00e2u<\/strong> v\u00e0 <strong>c\u00e1ch<\/strong> \u0111\u1eb7t n\u00f3 tr\u00ean trang HTML, s\u1ef1 k\u1ebft h\u1ee3p n\u00e0y s\u1ebd \u1ea3nh h\u01b0\u1edfng \u0111\u1ebfn th\u1eddi gian t\u1ea3i trang (loading time).<\/p>\n\n\n<p>M\u1ed9t \u0111o\u1ea1n m\u00e3 JavaScript th\u00f4ng th\u01b0\u1eddng s\u1ebd \u0111\u01b0\u1ee3c \u0111\u1eb7t nh\u01b0 th\u1ebf n\u00e0y:<\/p>\n\n\n<pre class=\"wp-block-code language-html\"><code>&lt;script src=\"script.js\"&gt;&lt;\/script&gt;<\/code><\/pre>\n\n\n<p>B\u1ea5t c\u1ee9 khi n\u00e0o tr\u00ecnh ph\u00e2n t\u00edch (parser) HTML t\u00ecm th\u1ea5y d\u00f2ng n\u00e0y, m\u1ed9t y\u00eau c\u1ea7u s\u1ebd \u0111\u01b0\u1ee3c th\u1ef1c hi\u1ec7n \u0111\u1ec3 t\u00ecm n\u1ea1p (fetch \/ t\u1ea3i xu\u1ed1ng) JavaScript, v\u00e0 \u0111o\u1ea1n m\u00e3 s\u1ebd \u0111\u01b0\u1ee3c th\u1ef1c thi (executed).<\/p>\n\n\n<p>M\u1ed9t khi qu\u00e1 tr\u00ecnh n\u00e0y ho\u00e0n th\u00e0nh, tr\u00ecnh ph\u00e2n t\u00edch c\u00fa ph\u00e1p m\u1edbi c\u00f3 th\u1ec3 ti\u1ebfp t\u1ee5c (resume), v\u00e0 ph\u1ea7n c\u00f2n l\u1ea1i c\u1ee7a HTML (c\u00f3 th\u1ec3) \u0111\u01b0\u1ee3c ph\u00e2n t\u00edch ti\u1ebfp [nh\u01b0 cu\u1ed9c ch\u1ea1y ti\u1ebfp s\u1ee9c, khi t\u1eebng ng\u01b0\u1eddi ch\u1ec9 \u0111\u01b0\u1ee3c ch\u1ea1y khi \u0111\u1ebfn l\u01b0\u1ee3t c\u1ee7a h\u1ecd, di\u1ec5n ra v\u00e0o l\u00fac trao g\u1eady].<\/p>\n\n\n<p>Nh\u01b0 b\u1ea1n c\u00f3 th\u1ec3 \u0111o\u00e1n \u0111\u01b0\u1ee3c r\u1ed3i, ki\u1ec3u ho\u1ea1t \u0111\u1ed9ng n\u00e0y c\u00f3 th\u1ec3 \u1ea3nh h\u01b0\u1edfng l\u1edbn l\u00ean th\u1eddi gian t\u1ea3i c\u1ee7a trang.<\/p>\n\n\n<p>N\u1ebfu JavaScript m\u1ea5t nhi\u1ec1u th\u1eddi gian h\u01a1n \u0111\u1ec3 t\u1ea3i so v\u1edbi d\u1ef1 \u0111\u1ecbnh, v\u00ed d\u1ee5 <a href=\"https:\/\/kiencang.net\/cdn-may-chu-vietnam\/\" target=\"_blank\" rel=\"noreferrer noopener\">n\u1ebfu m\u1ea1ng h\u01a1i ch\u1eadm<\/a> ho\u1eb7c n\u1ebfu tr\u00ean thi\u1ebft b\u1ecb di \u0111\u1ed9ng v\u00e0 k\u1ebft n\u1ed1i kh\u00f4ng \u1ed5n \u0111\u1ecbnh (sloppy), ng\u01b0\u1eddi xem c\u00f3 kh\u1ea3 n\u0103ng th\u1ea5y m\u1ed9t trang tr\u1ed1ng tr\u01a1n (blank page) cho \u0111\u1ebfn khi \u0111o\u1ea1n m\u00e3 \u0111\u01b0\u1ee3c t\u1ea3i v\u00e0 th\u1ef1c thi.<\/p>\n\n\n<hr class=\"wp-block-separator\"\/>\n\n\n<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_60 counter-hierarchy ez-toc-counter ez-toc-light-blue ez-toc-container-direction\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title \" >M\u1ee5c l\u1ee5c<\/p>\n<span class=\"ez-toc-title-toggle\"><a href=\"#\" class=\"ez-toc-pull-right ez-toc-btn ez-toc-btn-xs ez-toc-btn-default ez-toc-toggle\" aria-label=\"Toggle Table of Content\" role=\"button\"><label for=\"item-6577d6cc1a12c\" ><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">Toggle<\/span><span class=\"ez-toc-icon-toggle-span\"><svg style=\"fill: #999;color:#999\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewBox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #999;color:#999\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewBox=\"0 0 24 24\" version=\"1.2\" baseProfile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/span><\/label><input aria-label=\"Toggle\" aria-label=\"item-6577d6cc1a12c\"  type=\"checkbox\" id=\"item-6577d6cc1a12c\"><\/a><\/span><\/div>\n<nav><ul class='ez-toc-list ez-toc-list-level-1 ' ><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/kiencang.net\/tai-javascript-defer-async\/#Cac_vi_tri_quan_trong\" title=\"C\u00e1c v\u1ecb tr\u00ed quan tr\u1ecdng\">C\u00e1c v\u1ecb tr\u00ed quan tr\u1ecdng<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/kiencang.net\/tai-javascript-defer-async\/#Async_va_Defer\" title=\"Async v\u00e0 Defer\">Async v\u00e0 Defer<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/kiencang.net\/tai-javascript-defer-async\/#So_sanh_hieu_suat\" title=\"So s\u00e1nh hi\u1ec7u su\u1ea5t\">So s\u00e1nh hi\u1ec7u su\u1ea5t<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/kiencang.net\/tai-javascript-defer-async\/#Chan_phan_tich_cu_phap\" title=\"Ch\u1eb7n ph\u00e2n t\u00edch c\u00fa ph\u00e1p\">Ch\u1eb7n ph\u00e2n t\u00edch c\u00fa ph\u00e1p<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/kiencang.net\/tai-javascript-defer-async\/#Chan_hien_thi\" title=\"Ch\u1eb7n hi\u1ec3n th\u1ecb\">Ch\u1eb7n hi\u1ec3n th\u1ecb<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"https:\/\/kiencang.net\/tai-javascript-defer-async\/#domInteractive\" title=\"domInteractive\">domInteractive<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-7\" href=\"https:\/\/kiencang.net\/tai-javascript-defer-async\/#Giu_moi_thu_theo_thu_tu\" title=\"Gi\u1eef m\u1ecdi th\u1ee9 theo th\u1ee9 t\u1ef1\">Gi\u1eef m\u1ecdi th\u1ee9 theo th\u1ee9 t\u1ef1<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-8\" href=\"https:\/\/kiencang.net\/tai-javascript-defer-async\/#Hay_noi_cho_toi_biet_cach_tot_nhat\" title=\"H\u00e3y n\u00f3i cho t\u00f4i bi\u1ebft c\u00e1ch t\u1ed1t nh\u1ea5t!\">H\u00e3y n\u00f3i cho t\u00f4i bi\u1ebft c\u00e1ch t\u1ed1t nh\u1ea5t!<\/a><\/li><\/ul><\/nav><\/div>\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Cac_vi_tri_quan_trong\"><\/span>C\u00e1c v\u1ecb tr\u00ed quan tr\u1ecdng<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n<p>Khi b\u1ea1n m\u1edbi h\u1ecdc HTML, b\u1ea1n \u0111\u01b0a th\u1ebb JavaScript v\u00e0o trong th\u1ebb <code>&lt;head><\/code><em> <\/em>nh\u01b0 d\u01b0\u1edbi \u0111\u00e2y:<\/p>\n\n\n<pre class=\"wp-block-code language-html\"><code>&lt;html&gt;\n  &lt;head&gt;\n    &lt;title&gt;Ti\u1ec1u \u0111\u1ec1&lt;\/title&gt;\n    &lt;script src=\"script.js\"&gt;&lt;\/script&gt;\n  &lt;\/head&gt;\n  &lt;body&gt;\n    ...\n  &lt;\/body&gt;\n&lt;\/html&gt;<\/code><\/pre>\n\n\n<p>Nh\u01b0 t\u00f4i n\u00f3i \u1edf ph\u1ea7n tr\u00ean, khi tr\u00ecnh ph\u00e2n t\u00edch t\u00ecm th\u1ea5y d\u00f2ng n\u00e0y, n\u00f3 s\u1ebd t\u00ecm n\u1ea1p \u0111o\u1ea1n m\u00e3 v\u00e0 th\u1ef1c thi m\u00e3. Sau \u0111\u00f3, khi tr\u00ecnh ph\u00e2n t\u00edch l\u00e0m xong nhi\u1ec7m v\u1ee5 n\u00e0y, n\u00f3 s\u1ebd ti\u1ebfp t\u1ee5c ph\u00e2n t\u00edch ph\u1ea7n th\u00e2n (<code>body<\/code>).<\/p>\n\n\n<p>\u0110i\u1ec1u n\u00e0y l\u00e0 t\u1ec7 b\u1edfi v\u00ec n\u00f3 l\u00e0 nguy\u00ean nh\u00e2n g\u00e2y tr\u00ec ho\u00e3n, ch\u1eadm tr\u1ec5 (delay). M\u1ed9t gi\u1ea3i ph\u00e1p ph\u1ed5 bi\u1ebfn cho v\u1ea5n \u0111\u1ec1 n\u00e0y l\u00e0 \u0111\u01b0a th\u1ebb <code>&lt;script><\/code> v\u00e0o ph\u1ea7n cu\u1ed1i c\u1ee7a trang, ngay tr\u01b0\u1edbc th\u1ebb \u0111\u00f3ng <code>&lt;\/body><\/code>.<\/p>\n\n\n<p>Nh\u1edd v\u1eady, JavaScript \u0111\u01b0\u1ee3c t\u1ea3i v\u1ec1 v\u00e0 th\u1ef1c thi (loaded and executed) sau khi to\u00e0n b\u1ed9 trang \u0111\u00e3 s\u1eb5n s\u00e0ng ph\u00e2n t\u00edch v\u00e0 t\u1ea3i xong, \u0111i\u1ec1u n\u00e0y l\u00e0 m\u1ed9t <strong>c\u1ea3i ti\u1ebfn l\u1edbn<\/strong> so v\u1edbi vi\u1ec7c \u0111\u01b0a m\u00e3 v\u00e0o th\u1ebb <code>head<\/code>.<\/p>\n\n\n<p>\u0110\u00e2y l\u00e0 c\u00e1ch t\u1ed1t nh\u1ea5t m\u00e0 ch\u00fang ta c\u00f3 th\u1ec3 th\u1ef1c hi\u1ec7n n\u1ebfu b\u1ea1n c\u1ea7n h\u1ed7 tr\u1ee3 cho c\u00e1c tr\u00ecnh duy\u1ec7t c\u0169 (older browsers) kh\u00f4ng h\u1ed7 tr\u1ee3 hai t\u00ednh n\u0103ng m\u1edbi c\u1ee7a HTML l\u00e0: <code>async<\/code> v\u00e0 <code>defer<\/code>.<\/p>\n\n\n<hr class=\"wp-block-separator\"\/>\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Async_va_Defer\"><\/span>Async v\u00e0 Defer<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n<p>C\u1ea3 <em>async<\/em> v\u00e0 <em>defer<\/em> l\u00e0 c\u00e1c thu\u1ed9c t\u00ednh boolean. C\u00e1ch s\u1eed d\u1ee5ng c\u1ee7a ch\u00fang l\u00e0 t\u01b0\u01a1ng t\u1ef1 nhau:<\/p>\n\n\n<pre class=\"wp-block-code language-html\"><code>&lt;script async src=\"script.js\"&gt;&lt;\/script&gt;<\/code><\/pre>\n\n\n<pre class=\"wp-block-code language-html\"><code>&lt;script defer src=\"script.js\"&gt;&lt;\/script&gt;<\/code><\/pre>\n\n\n<p>N\u1ebfu b\u1ea1n ch\u1ec9 \u0111\u1ecbnh c\u1ea3 hai, <em>async<\/em> s\u1ebd \u0111\u01b0\u1ee3c \u01b0u ti\u00ean tr\u00ean c\u00e1c tr\u00ecnh duy\u1ec7t hi\u1ec7n \u0111\u1ea1i, trong khi v\u1edbi c\u00e1c tr\u00ecnh duy\u1ec7t c\u0169 h\u1ed7 tr\u1ee3 <em>defer <\/em>nh\u01b0ng kh\u00f4ng h\u1ed7 tr\u1ee3 <em>async&nbsp;<\/em>th\u00ec n\u00f3&nbsp;s\u1ebd \u0111\u01b0\u1ee3c chuy\u1ec3n th\u00e0nh <em>defer<\/em>.<\/p>\n\n\n<blockquote class=\"wp-block-quote\"><p>\u0110\u1ec3 ki\u1ec3m tra c\u00e1c phi\u00ean b\u1ea3n tr\u00ecnh duy\u1ec7t h\u1ed7 tr\u1ee3 <strong>async<\/strong>, b\u1ea1n c\u00f3 th\u1ec3 ki\u1ec3m tra th\u00f4ng qua trang Can I Use: <a href=\"https:\/\/caniuse.com\/#feat=script-async\">https:\/\/caniuse.com\/#feat=script-async<\/a> c\u00f2n \u0111\u00e2y l\u00e0 cho thu\u1ed9c t\u00ednh <strong>defer<\/strong>: <a href=\"https:\/\/caniuse.com\/#feat=script-defer\">https:\/\/caniuse.com\/#feat=script-defer<\/a><\/p><\/blockquote>\n\n\n<p>C\u00e1c thu\u1ed9c t\u00ednh n\u00e0y ch\u1ec9 c\u00f3 \u00fd ngh\u0129a khi s\u1eed d\u1ee5ng JavaScript trong ph\u1ea7n <em>head<\/em> c\u1ee7a trang, v\u00e0 ch\u00fang s\u1ebd v\u00f4 t\u00e1c d\u1ee5ng n\u1ebfu b\u1ea1n \u0111\u1ec3 m\u00e3 trong ph\u1ea7n cu\u1ed1i c\u1ee7a <em>body<\/em> nh\u01b0 ch\u00fang ta \u0111\u00e3 th\u1ea5y \u1edf tr\u00ean.<\/p>\n\n\n<hr class=\"wp-block-separator\"\/>\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"So_sanh_hieu_suat\"><\/span>So s\u00e1nh hi\u1ec7u su\u1ea5t<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n<h3 class=\"wp-block-heading\">1. Kh\u00f4ng c\u00f3 defer ho\u1eb7c async, n\u1eb1m trong th\u1ebb head<\/h3>\n\n\n<p>D\u01b0\u1edbi \u0111\u00e2y l\u00e0 c\u00e1ch trang t\u1ea3i m\u00e3 m\u00e0 kh\u00f4ng c\u00f3 c\u1ea3 defer ho\u1eb7c async, khi n\u00f3 \u0111\u01b0\u1ee3c \u0111\u1eb7t trong th\u1ebb <em>head<\/em> c\u1ee7a trang:<\/p>\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter\"><img decoding=\"async\" src=\"https:\/\/kiencang.net\/wp-content\/uploads\/2019\/12\/without-defer-async-head.png\" alt=\"kh\u00f4ng c\u00f3 async v\u00e0 defer\" class=\"wp-image-14024\"\/><\/figure><\/div>\n\n\n<ul><li>start parsing HTML: b\u1eaft \u0111\u1ea7u ph\u00e2n t\u00edch HTML;<\/li><li>wait: \u0111\u1ee3i;<\/li><li>fetch script: t\u00ecm n\u1ea1p m\u00e3 JS;<\/li><li>excute script: th\u1ef1c thi m\u00e3 JS;<\/li><li>resume parsing HTML: ti\u1ebfp t\u1ee5c ph\u00e2n t\u00edch c\u00fa ph\u00e1p HTML;<\/li><\/ul>\n\n\n<p>Qu\u00e1 tr\u00ecnh ph\u1ea7n t\u00edch HTML d\u1eebng l\u1ea1i cho \u0111\u1ebfn khi <strong>m\u00e3 JS \u0111\u01b0\u1ee3c t\u00ecm n\u1ea1p v\u00e0 th\u1ef1c thi xong<\/strong>. Khi qu\u00e1 tr\u00ecnh n\u00e0y ho\u00e0n th\u00e0nh, qu\u00e1 tr\u00ecnh ph\u00e2n t\u00edch HTML m\u1edbi l\u1ea1i ti\u1ebfp t\u1ee5c.<\/p>\n\n\n<p>T\u00f3m l\u1ea1i: v\u1edbi c\u00e1ch n\u00e0y JS s\u1ebd l\u00e0m gi\u00e1n \u0111o\u1ea1n ph\u00e2n t\u00edch HTML trong c\u1ea3 2 pha l\u00e0 t\u00ecm n\u1ea1p &amp; th\u1ef1c thi. N\u00f3i c\u00e1ch kh\u00e1c n\u00f3 l\u00e0m website b\u1ecb ch\u1eadm \u0111i nhi\u1ec1u nh\u1ea5t.<\/p>\n\n\n<hr class=\"wp-block-separator\"\/>\n\n\n<h3 class=\"wp-block-heading\">2. Kh\u00f4ng defer ho\u1eb7c async, n\u1eb1m trong th\u1ebb body<\/h3>\n\n\n<p>D\u01b0\u1edbi \u0111\u00e2y l\u00e0 c\u00e1ch trang t\u1ea3i m\u00e3 m\u00e0 kh\u00f4ng c\u00f3 defer ho\u1eb7c async, \u0111\u1eb7t t\u1ea1i ph\u1ea7n cu\u1ed1i c\u1ee7a th\u1ebb <em>body<\/em>, ngay tr\u01b0\u1edbc th\u1ebb \u0111\u00f3ng:<\/p>\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter\"><img decoding=\"async\" src=\"https:\/\/kiencang.net\/wp-content\/uploads\/2019\/12\/without-defer-async-body.png\" alt=\"kh\u00f4ng c\u00f3 defer v\u00e0 async trong body\" class=\"wp-image-14025\"\/><\/figure><\/div>\n\n\n<p>Qu\u00e1 tr\u00ecnh ph\u00e2n t\u00edch \u0111\u01b0\u1ee3c th\u1ef1c hi\u1ec7n m\u00e0 kh\u00f4ng b\u1ecb t\u1ea1m d\u1eebng l\u1ea7n n\u00e0o trong su\u1ed1t giai \u0111o\u1ea1n n\u00e0y (parse HTML), v\u00e0 khi n\u00f3 ho\u00e0n th\u00e0nh, ti\u1ebfp theo JS s\u1ebd \u0111\u01b0\u1ee3c t\u00ecm n\u1ea1p, v\u00e0 th\u1ef1c thi. Ph\u00e2n t\u00edch c\u00fa ph\u00e1p \u0111\u01b0\u1ee3c ho\u00e0n th\u00e0nh th\u1eadm ch\u00ed l\u00e0 tr\u01b0\u1edbc khi JS t\u1ea3i v\u1ec1, v\u00ec th\u1ebf trang s\u1ebd xu\u1ea5t hi\u1ec7n tr\u01b0\u1edbc c\u00e1ch tr\u00ean.<\/p>\n\n\n<p>V\u1edbi c\u00e1ch n\u00e0y c\u1ea3 2 pha t\u00ecm n\u1ea1p &amp; th\u1ef1c thi kh\u00f4ng c\u1ea3n tr\u1edf qu\u00e1 tr\u00ecnh ph\u00e2n t\u00edch HTML.<\/p>\n\n\n<hr class=\"wp-block-separator\"\/>\n\n\n<h3 class=\"wp-block-heading\">3. V\u1edbi async, n\u1eb1m trong th\u1ebb head<\/h3>\n\n\n<p>D\u01b0\u1edbi \u0111\u00e2y l\u00e0 c\u00e1ch trang t\u1ea3i JavaScript v\u1edbi <em>async<\/em>, \u1edf trong th\u1ebb <em>head<\/em>:<\/p>\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter\"><img decoding=\"async\" src=\"https:\/\/kiencang.net\/wp-content\/uploads\/2019\/12\/with-async.png\" alt=\"v\u1edbi async trong th\u1ebb head\" class=\"wp-image-14026\"\/><\/figure><\/div>\n\n\n<p>\u0110o\u1ea1n m\u00e3 JavaScript \u0111\u01b0\u1ee3c t\u00ecm n\u1ea1p <strong>kh\u00f4ng \u0111\u1ed3ng b\u1ed9 <\/strong>(asynchronously), v\u00e0 khi n\u00f3 s\u1eb5n s\u00e0ng, tr\u00ecnh ph\u00e2n t\u00edch s\u1ebd t\u1ea1m d\u1eebng \u0111\u1ec3 th\u1ef1c thi JavaScript, sau khi ho\u00e0n th\u00e0nh, n\u00f3 l\u1ea1i ti\u1ebfp t\u1ee5c tr\u1edf l\u1ea1i.<\/p>\n\n\n<p>V\u1edbi c\u00e1ch n\u00e0y ch\u1ec9 pha th\u1ef1c thi c\u1ea3n tr\u1edf ph\u00e2n t\u00edch HTML, c\u00f2n qu\u00e1 tr\u00ecnh t\u00ecm n\u1ea1p th\u00ec kh\u00f4ng, n\u00f3 \u0111\u01b0\u1ee3c th\u1ef1c hi\u1ec7n song song v\u1edbi ph\u00e2n t\u00edch HTML.<\/p>\n\n\n<hr class=\"wp-block-separator\"\/>\n\n\n<h3 class=\"wp-block-heading\">4. V\u1edbi defer trong head<\/h3>\n\n\n<p>D\u01b0\u1edbi \u0111\u00e2y l\u00e0 c\u00e1ch trang t\u1ea3i m\u00e3 v\u1edbi <em>defer<\/em>, \u0111\u01b0\u1ee3c \u0111\u1eb7t trong th\u1ebb <em>head<\/em>:<\/p>\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter\"><img decoding=\"async\" src=\"https:\/\/kiencang.net\/wp-content\/uploads\/2019\/12\/with-defer.png\" alt=\"defer trong th\u1ebb head\" class=\"wp-image-14027\"\/><\/figure><\/div>\n\n\n<p>\u0110o\u1ea1n m\u00e3 \u0111\u01b0\u1ee3c t\u00ecm n\u1ea1p kh\u00f4ng \u0111\u1ed3ng b\u1ed9, v\u00e0 n\u00f3 \u0111\u01b0\u1ee3c th\u1ef1c thi ch\u1ec9 khi tr\u00ecnh ph\u00e2n t\u00edch l\u00e0m vi\u1ec7c xong v\u1edbi HTML.<\/p>\n\n\n<p>Tr\u00ecnh ph\u00e2n t\u00edch ho\u00e0n th\u00e0nh vi\u1ec7c n\u00e0y c\u0169ng gi\u1ed1ng nh\u01b0 khi ch\u00fang ta \u0111\u1ec3 JS t\u1ea1i ph\u1ea7n cu\u1ed1i c\u1ee7a th\u1ebb body, nh\u01b0ng n\u00f3i chung m\u00e3 \u0111\u01b0\u1ee3c th\u1ef1c thi s\u1edbm h\u01a1n, b\u1edfi v\u00ec \u0111o\u1ea1n m\u00e3 <em>\u0111\u01b0\u1ee3c t\u1ea3i song song v\u1edbi qu\u00e1 tr\u00ecnh ph\u00e2n t\u00edch HTML<\/em>.<\/p>\n\n\n<p>V\u00ec th\u1ebf \u0111\u00e2y l\u00e0 gi\u1ea3i ph\u00e1p t\u1ed1t nh\u1ea5t khi x\u00e9t v\u1ec1 m\u1eb7t t\u1ed1c \u0111\u1ed9.<\/p>\n\n\n<hr class=\"wp-block-separator\"\/>\n\n\n<p><strong>K\u1ebft lu\u1eadn<\/strong>: \u0111\u1ec3 defer JS trong th\u1ebb head l\u00e0 c\u00e1ch t\u1ed1t nh\u1ea5t v\u1ec1 m\u1eb7t t\u1ed1c \u0111\u1ed9, c\u00e1ch t\u1ed1t th\u1ee9 nh\u00ec l\u00e0 \u0111\u1ec3 tr\u01b0\u1edbc th\u1ebb \u0111\u00f3ng body.<\/p>\n\n\n<hr class=\"wp-block-separator\"\/>\n\n\n<p>Gi\u1ea3i th\u00edch d\u1ec5 hi\u1ec3u h\u01a1n: ch\u1ea1y ti\u1ebfp s\u1ee9c kh\u00f4ng ph\u1ea3i l\u00e0 v\u00ed d\u1ee5 t\u1ed1t \u0111\u1ec3 minh h\u1ecda ph\u1ea7n n\u00e0y. T\u00f4i s\u1ebd l\u1ea5y v\u00ed d\u1ee5 kh\u00e1c. H\u00e3y t\u01b0\u1edfng t\u01b0\u1ee3ng b\u1ea1n ph\u1ea3i l\u00e0m 2 m\u00f3n \u0103n, m\u1ed9t m\u00f3n ch\u00ednh v\u00e0 m\u1ed9t m\u00f3n \u0103n ph\u1ee5. M\u00f3n ch\u00ednh l\u00e0 lem r\u00e1n (parse HTML), v\u00e0 m\u00f3n ph\u1ee5 l\u00e0 hoa qu\u1ea3 tr\u00e1ng mi\u1ec7ng. Tuy nhi\u00ean tr\u01b0\u1edbc khi g\u1ecdt (execute) hoa qu\u1ea3 th\u00ec b\u1ea1n ph\u1ea3i ng\u00e2m (fetch) cho s\u1ea1ch. Th\u1ebf th\u00ec c\u00e1ch l\u00e0m ch\u1eadm ch\u1ea1p l\u00e0 l\u00e0m m\u00f3n ch\u00ednh, l\u00e0m xong th\u00ec m\u1edbi ng\u00e2m hoa qu\u1ea3 cho s\u1ea1ch, \u0111\u1ee3i ng\u00e2m xong m\u1edbi g\u1ecdt hoa qu\u1ea3. C\u00e1ch l\u00e0m nhanh h\u01a1n l\u00e0 trong khi b\u1ea1n l\u00e0m m\u00f3n ch\u00ednh, hoa qu\u1ea3 \u0111\u01b0\u1ee3c ng\u00e2m (ch\u1eafc ch\u1eafn b\u1ea1n l\u00e0m vi\u1ec7c n\u00e0y nhi\u1ec1u r\u1ed3i \u0111\u00fang kh\u00f4ng!), qu\u00e1 tr\u00ecnh di\u1ec5n ra song song, v\u00e0 l\u00e0m xong m\u00f3n lem r\u00e1n th\u00ec b\u1ea1n g\u1ecdt \u0111\u01b0\u1ee3c lu\u00f4n hoa qu\u1ea3 ch\u1ee9 kh\u00f4ng ph\u1ea3i ch\u1edd th\u00eam th\u1eddi gian \u0111\u1ec3 ng\u00e2m n\u1eefa.<\/p>\n\n\n<hr class=\"wp-block-separator\"\/>\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Chan_phan_tich_cu_phap\"><\/span>Ch\u1eb7n ph\u00e2n t\u00edch c\u00fa ph\u00e1p<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n<p><em>async<\/em> ch\u1eb7n tr\u00ecnh ph\u00e2n t\u00edch trang trong khi <em>defer<\/em> kh\u00f4ng l\u00e0m nh\u01b0 v\u1eady.<\/p>\n\n\n<hr class=\"wp-block-separator\"\/>\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Chan_hien_thi\"><\/span>Ch\u1eb7n hi\u1ec3n th\u1ecb<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n<p>C\u1ea3 <em>async<\/em> l\u1eabn <em>defer<\/em> \u0111\u1ec1u kh\u00f4ng \u0111\u1ea3m b\u1ea3o b\u1ea5t c\u1ee9 \u0111i\u1ec1u g\u00ec v\u1ec1 ch\u1eb7n hi\u1ec3n th\u1ecb (blocking rendering). \u0110i\u1ec1u n\u00e0y ph\u1ee5 thu\u1ed9c v\u00e0o b\u1ea1n v\u00e0 m\u00e3 c\u1ee7a b\u1ea1n (v\u00ed d\u1ee5, \u0111\u1ea3m b\u1ea3o r\u1eb1ng m\u00e3 JavaScript c\u1ee7a b\u1ea1n ch\u1ea1y sau <em>onLoad<\/em>).<\/p>\n\n\n<hr class=\"wp-block-separator\"\/>\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"domInteractive\"><\/span>domInteractive<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n<p>JS \u0111\u01b0\u1ee3c \u0111\u00e1nh d\u1ea5u v\u1edbi <em>defer<\/em> s\u1ebd \u0111\u01b0\u1ee3c th\u1ef1c thi ngay sau s\u1ef1 ki\u1ec7n <em>domInteractive,&nbsp;<\/em>c\u00e1i x\u1ea3y ra sau khi HTML t\u1ea3i, ph\u00e2n t\u00edch v\u00e0 x\u00e2y d\u1ef1ng DOM.<\/p>\n\n\n<p>CSS v\u00e0 \u1ea3nh t\u1ea1i th\u1eddi \u0111i\u1ec3m n\u00e0y s\u1ebd v\u1eabn \u0111\u01b0\u1ee3c ph\u00e2n t\u00edch c\u00fa ph\u00e1p v\u00e0 t\u1ea3i.<\/p>\n\n\n<p>M\u1ed9t khi \u0111i\u1ec1u n\u00e0y ho\u00e0n th\u00e0nh xong, tr\u00ecnh duy\u1ec7t s\u1ebd kh\u1edfi ph\u00e1t s\u1ef1 ki\u1ec7n <em>dotComplete<\/em>, v\u00e0 sau \u0111\u00f3 l\u00e0 <em>onLoad<\/em>.<\/p>\n\n\n<p><em>domInteractive<\/em> l\u00e0 quan tr\u1ecdng b\u1edfi v\u00ec th\u1eddi gian c\u1ee7a n\u00f3 \u0111\u01b0\u1ee3c ghi nh\u1eadn l\u00e0 th\u01b0\u1edbc \u0111o t\u1ed1c \u0111\u1ed9 t\u1ea3i nh\u1eadn th\u1ee9c (perceived loading speed). <a href=\"https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/API\/PerformanceTiming\/domInteractive\" target=\"_blank\" rel=\"noreferrer noopener\">Xem th\u00eam \u1edf \u0111\u00e2y<\/a> \u0111\u1ec3 bi\u1ebft chi ti\u1ebft h\u01a1n.<\/p>\n\n\n<hr class=\"wp-block-separator\"\/>\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Giu_moi_thu_theo_thu_tu\"><\/span>Gi\u1eef m\u1ecdi th\u1ee9 theo th\u1ee9 t\u1ef1<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n<p>M\u1ed9t \u0111i\u1ec3m c\u1ea7n l\u01b0u \u00fd v\u1ec1 \u01b0u \u0111i\u1ec3m c\u1ee7a defer: JavaScript \u0111\u01b0\u1ee3c \u0111\u00e1nh d\u1ea5u v\u1edbi <em>async<\/em> \u0111\u01b0\u1ee3c th\u1ef1c thi ngay khi ch\u00fang s\u1eb5n s\u00e0ng. C\u00f2n JavaScript \u0111\u01b0\u1ee3c \u0111\u00e1nh d\u1ea5u defer \u0111\u01b0\u1ee3c th\u1ef1c thi (sau khi tr\u00ecnh ph\u00e2n t\u00edch c\u00fa ph\u00e1p HTML ho\u00e0n th\u00e0nh) theo th\u1ee9 t\u1ef1 m\u00e0 n\u00f3 \u0111\u01b0\u1ee3c x\u00e1c \u0111\u1ecbnh t\u1eeb tr\u01b0\u1edbc trong m\u00e3 \u0111\u00e1nh d\u1ea5u HTML (do \u0111\u00f3 n\u00f3 h\u1ea1n ch\u1ebf kh\u1ea3 n\u0103ng g\u00e2y ra l\u1ed7i do kh\u00f4ng l\u00e0m \u0111\u1ea3o l\u1ed9n tr\u1eadt t\u1ef1 th\u1ef1c thi m\u00e3 c\u1ee7a ng\u01b0\u1eddi l\u1eadp tr\u00ecnh).<\/p>\n\n\n<hr class=\"wp-block-separator\"\/>\n\n\n<h2 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Hay_noi_cho_toi_biet_cach_tot_nhat\"><\/span>H\u00e3y n\u00f3i cho t\u00f4i bi\u1ebft c\u00e1ch t\u1ed1t nh\u1ea5t!<span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n<p>C\u00e1ch t\u1ed1t nh\u1ea5t \u0111\u1ec3 t\u0103ng t\u1ed1c trang c\u1ee7a b\u1ea1n khi s\u1eed d\u1ee5ng JavaScript l\u00e0 \u0111\u01b0a ch\u00fang v\u00e0o trong ph\u1ea7n <em>&lt;head&gt;<\/em> v\u00e0 th\u00eam thu\u1ed9c t\u00ednh <em>defer<\/em> v\u00e0o th\u1ebb <em>&lt;script&gt;<\/em>:<\/p>\n\n\n<pre class=\"wp-block-code language-html\"><code>&lt;script defer src=\"script.js\"&gt;&lt;\/script&gt;<\/code><\/pre>\n\n\n<p>\u0110\u00e2y l\u00e0 k\u1ecbch b\u1ea3n gi\u00fap k\u00edch ho\u1ea1t s\u1ef1 ki\u1ec7n <em>domInteractive<\/em> nhanh h\u01a1n.<\/p>\n\n\n<p>H\u00e3y \u0111\u1ec3 \u00fd \u0111\u1ebfn \u01b0u \u0111i\u1ec3m c\u1ee7a <em>defer<\/em>, n\u00f3 d\u01b0\u1eddng nh\u01b0 l\u00e0 l\u1ef1a ch\u1ecdn t\u1ed1t h\u01a1n <em>async<\/em> trong nhi\u1ec1u b\u1ed1i c\u1ea3nh \u0111a d\u1ea1ng.<\/p>\n\n\n<p>Tr\u1eeb khi b\u1ea1n \u1ed5n th\u1ecfa v\u1edbi vi\u1ec7c tr\u00ec ho\u00e3n trong l\u1ea7n render \u0111\u1ea7u ti\u00ean c\u1ee7a trang, c\u00f2n kh\u00f4ng h\u00e3y \u0111\u1ea3m b\u1ea3o r\u1eb1ng khi trang \u0111\u01b0\u1ee3c ph\u00e2n t\u00edch c\u00fa ph\u00e1p, JavaScript b\u1ea1n mu\u1ed1n \u0111\u00e3 \u0111\u01b0\u1ee3c th\u1ef1c thi (c\u00e2u n\u00e0y c\u00f3 \u00fd n\u00f3i r\u1eb1ng n\u1ebfu b\u1ea1n c\u1ea7n JS th\u1ef1c thi trong khi ph\u00e2n t\u00edch trang th\u00ec async m\u1edbi l\u00e0 l\u1ef1a ch\u1ecdn t\u1ed1t h\u01a1n, v\u00ec n\u00f3 kh\u00f4ng b\u1ecb tr\u00ec ho\u00e3n nh\u01b0 v\u1edbi defer, m\u1ed9t v\u00ed d\u1ee5 \u0111i\u1ec3n h\u00ecnh trong v\u1ea5n \u0111\u1ec1 n\u00e0y l\u00e0 jQuery, nhi\u1ec1u trang s\u1ebd g\u1eb7p l\u1ed7i khi defer jQuery).<\/p>\n\n\n<p><em>(D\u1ecbch t\u1eeb b\u00e0i vi\u1ebft: Efficiently load JavaScript with defer and async, website: flaviocopes)<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Khi b\u1ea1n t\u1ea3i JavaScript tr\u00ean trang HTML, b\u1ea1n c\u1ea7n c\u1ea9n th\u1eadn \u0111\u1ec3 kh\u00f4ng l\u00e0m \u1ea3nh h\u01b0\u1edfng x\u1ea5u \u0111\u1ebfn hi\u1ec7u su\u1ea5t (t\u1ed1c \u0111\u1ed9) c\u1ee7a trang. Ph\u1ee5 thu\u1ed9c v\u00e0o vi\u1ec7c b\u1ea1n \u0111\u1eb7t \u0111o\u1ea1n m\u00e3 JavaScript \u1edf \u0111\u00e2u v\u00e0 c\u00e1ch \u0111\u1eb7t n\u00f3 tr\u00ean trang HTML, s\u1ef1 k\u1ebft h\u1ee3p n\u00e0y s\u1ebd \u1ea3nh h\u01b0\u1edfng \u0111\u1ebfn th\u1eddi gian t\u1ea3i trang (loading &hellip;<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[272],"tags":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v21.6 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>T\u1ea3i JavaScript hi\u1ec7u qu\u1ea3 v\u1edbi defer v\u00e0 async &#8226; Ki\u1ebfn c\u00e0ng<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/kiencang.net\/tai-javascript-defer-async\/\" \/>\n<meta property=\"og:locale\" content=\"vi_VN\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"T\u1ea3i JavaScript hi\u1ec7u qu\u1ea3 v\u1edbi defer v\u00e0 async &#8226; Ki\u1ebfn c\u00e0ng\" \/>\n<meta property=\"og:description\" content=\"Khi b\u1ea1n t\u1ea3i JavaScript tr\u00ean trang HTML, b\u1ea1n c\u1ea7n c\u1ea9n th\u1eadn \u0111\u1ec3 kh\u00f4ng l\u00e0m \u1ea3nh h\u01b0\u1edfng x\u1ea5u \u0111\u1ebfn hi\u1ec7u su\u1ea5t (t\u1ed1c \u0111\u1ed9) c\u1ee7a trang. 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