{"id":78503,"date":"2024-09-20T15:57:20","date_gmt":"2024-09-20T08:57:20","guid":{"rendered":"https:\/\/itviec.com\/blog\/?p=78503"},"modified":"2024-09-20T15:57:20","modified_gmt":"2024-09-20T08:57:20","slug":"khai-bao-bien-trong-javascript","status":"publish","type":"post","link":"https:\/\/itviec.com\/blog\/khai-bao-bien-trong-javascript\/","title":{"rendered":"Khai b\u00e1o bi\u1ebfn trong JavaScript: Chi ti\u1ebft c\u00e1c c\u00e1ch ph\u1ed5 bi\u1ebfn"},"content":{"rendered":"<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_84 counter-hierarchy ez-toc-counter ez-toc-grey ez-toc-container-direction\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\" style=\"cursor:inherit\">N\u1ed9i dung b\u00e0i vi\u1ebft<\/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\"><span class=\"ez-toc-js-icon-con\"><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><\/span><\/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:\/\/itviec.com\/blog\/khai-bao-bien-trong-javascript\/#Dinh_nghia_ve_khai_bao_bien_trong_JavaScript\" >\u0110\u1ecbnh ngh\u0129a v\u1ec1 khai b\u00e1o bi\u1ebfn trong JavaScript<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/itviec.com\/blog\/khai-bao-bien-trong-javascript\/#Khai_bao_bien_trong_JavaScript_bang_tu_khoa_var\" >Khai b\u00e1o bi\u1ebfn trong JavaScript b\u1eb1ng t\u1eeb kh\u00f3a var<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/itviec.com\/blog\/khai-bao-bien-trong-javascript\/#Khai_bao_bien_trong_JavaScript_bang_tu_khoa_let\" >Khai b\u00e1o bi\u1ebfn trong JavaScript b\u1eb1ng t\u1eeb kh\u00f3a let<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/itviec.com\/blog\/khai-bao-bien-trong-javascript\/#Khai_bao_bien_trong_JavaScript_bang_tu_khoa_const\" >Khai b\u00e1o bi\u1ebfn trong JavaScript b\u1eb1ng t\u1eeb kh\u00f3a const<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/itviec.com\/blog\/khai-bao-bien-trong-javascript\/#Khai_bao_lai_bien_trong_JavaScript_bang_var\" >Khai b\u00e1o l\u1ea1i bi\u1ebfn trong JavaScript b\u1eb1ng var<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"https:\/\/itviec.com\/blog\/khai-bao-bien-trong-javascript\/#Khai_bao_lai_bien_trong_JavaScript_bang_let_va_const\" >Khai b\u00e1o l\u1ea1i bi\u1ebfn trong JavaScript b\u1eb1ng let v\u00e0 const<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-7\" href=\"https:\/\/itviec.com\/blog\/khai-bao-bien-trong-javascript\/#Cau_hoi_thuong_gap_ve_khai_bao_bien_trong_JavaScript\" >C\u00e2u h\u1ecfi th\u01b0\u1eddng g\u1eb7p v\u1ec1 khai b\u00e1o bi\u1ebfn trong JavaScript<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-8\" href=\"https:\/\/itviec.com\/blog\/khai-bao-bien-trong-javascript\/#Tong_ket_ve_cach_khai_bao_bien_trong_JavaScript\" >T\u1ed5ng k\u1ebft v\u1ec1 c\u00e1ch khai b\u00e1o bi\u1ebfn trong JavaScript<\/a><\/li><\/ul><\/nav><\/div>\n<p><em><strong>Bi\u1ebfn \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng \u0111\u1ec3 l\u01b0u tr\u1eef d\u1eef li\u1ec7u v\u00e0 gi\u00e1 tr\u1ecb c\u00f3 th\u1ec3 t\u00e1i s\u1eed d\u1ee5ng trong JavaScript. N\u1eafm r\u00f5 v\u1ec1 c\u00e1ch khai b\u00e1o bi\u1ebfn s\u1ebd gi\u00fap b\u1ea1n c\u00f3 \u0111\u1ee7 t\u1ef1 tin \u0111\u1ec3 th\u1ef1c hi\u1ec7n vi\u1ebft m\u1ed9t \u0111o\u1ea1n m\u00e3 JavaScript tuy\u1ec7t v\u1eddi. V\u1eady c\u00e1ch khai b\u00e1o bi\u1ebfn trong JavaScript nh\u01b0 th\u1ebf n\u00e0o?<\/strong><\/em><\/p>\n<p><span style=\"font-weight: 400;\">\u0110\u1ecdc b\u00e0i vi\u1ebft sau \u0111\u00e2y \u0111\u1ec3 \u0111\u01b0\u1ee3c gi\u1ea3i \u0111\u00e1p chi ti\u1ebft h\u01a1n v\u1ec1:<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">\u0110\u1ecbnh ngh\u0129a c\u1ee7a khai b\u00e1o bi\u1ebfn trong JavaScript<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">C\u00e1c c\u00e1ch khai b\u00e1o bi\u1ebfn v\u00e0 c\u00e1ch ho\u1ea1t \u0111\u1ed9ng<\/span><\/li>\n<\/ul>\n<h2><span class=\"ez-toc-section\" id=\"Dinh_nghia_ve_khai_bao_bien_trong_JavaScript\"><\/span><b>\u0110\u1ecbnh ngh\u0129a v\u1ec1 khai b\u00e1o bi\u1ebfn trong JavaScript<\/b><span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><span style=\"font-weight: 400;\">Trong JavaScript, bi\u1ebfn l\u00e0 m\u1ed9t container d\u00f9ng \u0111\u1ec3 l\u01b0u tr\u1eef c\u00e1c gi\u00e1 tr\u1ecb d\u1eef li\u1ec7u nh\u01b0 s\u1ed1, chu\u1ed7i, boolean, \u0111\u1ed1i t\u01b0\u1ee3ng,&#8230; Vi\u1ec7c khai b\u00e1o bi\u1ebfn l\u00e0 b\u01b0\u1edbc \u0111\u1ea7u ti\u00ean trong qu\u00e1 tr\u00ecnh l\u1eadp tr\u00ecnh, n\u00f3 cho ph\u00e9p l\u01b0u tr\u1eef t\u1ea1m th\u1eddi c\u00e1c gi\u00e1 tr\u1ecb c\u1ea7n thi\u1ebft \u0111\u1ec3 th\u1ef1c hi\u1ec7n c\u00e1c ph\u00e9p t\u00ednh, so s\u00e1nh v\u00e0 \u0111i\u1ec1u khi\u1ec3n lu\u1ed3ng c\u1ee7a ch\u01b0\u01a1ng tr\u00ecnh.\u00a0<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Hi\u1ec7n t\u1ea1i, JavaScript cung c\u1ea5p ba c\u00e1ch khai b\u00e1o bi\u1ebfn ch\u00ednh l\u00e0 var, let v\u00e0 const. M\u1ed7i c\u00e1ch c\u00f3 nh\u1eefng \u0111\u1eb7c \u0111i\u1ec3m v\u00e0 ph\u1ea1m vi ho\u1ea1t \u0111\u1ed9ng kh\u00e1c nhau.<\/span><\/p>\n<blockquote><p><i>Xem th\u00eam: <\/i><strong><a href=\"https:\/\/itviec.com\/blog\/javascript-la-gi\/\" target=\"_blank\" rel=\"noopener\"><i>JavaScript l\u00e0 g\u00ec? H\u1ecdc JavaScript c\u01a1 b\u1ea3n v\u1edbi l\u1ed9 tr\u00ecnh d\u1ec5 hi\u1ec3u nh\u1ea5t<\/i><\/a><\/strong><\/p><\/blockquote>\n<h2><span class=\"ez-toc-section\" id=\"Khai_bao_bien_trong_JavaScript_bang_tu_khoa_var\"><\/span><b>Khai b\u00e1o bi\u1ebfn trong JavaScript b\u1eb1ng t\u1eeb kh\u00f3a var<\/b><span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><span style=\"font-weight: 400;\">T\u1eeb kh\u00f3a var trong JavaScript \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng \u0111\u1ec3 khai b\u00e1o c\u00e1c bi\u1ebfn c\u00f3 ph\u1ea1m vi h\u00e0m (function) ho\u1eb7c to\u00e0n c\u1ee5c (global). Tr\u01b0\u1edbc ES6, var l\u00e0 t\u1eeb kh\u00f3a duy nh\u1ea5t \u0111\u1ec3 khai b\u00e1o m\u00e0 kh\u00f4ng c\u00f3 ph\u1ea1m vi kh\u1ed1i (block) nh\u01b0 let v\u00e0 const.\u00a0<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Hi\u1ec7n nay, c\u00e1c l\u1eadp tr\u00ecnh vi\u00ean th\u01b0\u1eddng khuy\u00ean b\u1ea1n n\u00ean s\u1eed d\u1ee5ng let ho\u1eb7c const khi l\u00e0m vi\u1ec7c c\u00f9ng JavaScript thay v\u00ec var. B\u1edfi v\u00ec let v\u00e0 const cung c\u1ea5p ph\u1ea1m vi kh\u1ed1i (block scope) v\u00e0 tr\u00e1nh \u0111\u01b0\u1ee3c m\u1ed9t s\u1ed1 l\u1ed7i ph\u1ed5 bi\u1ebfn li\u00ean quan \u0111\u1ebfn vi\u1ec7c khai b\u00e1o bi\u1ebfn.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">C\u00fa ph\u00e1p c\u01a1 b\u1ea3n c\u1ee7a var nh\u01b0 sau:\u00a0<\/span><\/p>\n<pre>var variableName = valueOfVar;<\/pre>\n<p><span style=\"font-weight: 400;\">Khi b\u1ea1n khai b\u00e1o m\u1ed9t bi\u1ebfn b\u1eb1ng var, bi\u1ebfn \u0111\u00f3 s\u1ebd \u0111\u01b0\u1ee3c \u201choisted\u201d v\u00e0 kh\u1edfi t\u1ea1o trong b\u1ed9 nh\u1edb l\u00e0 undefined tr\u01b0\u1edbc khi m\u00e3 \u0111\u01b0\u1ee3c th\u1ef1c thi. V\u00ec v\u1eady, b\u1ea1n c\u00f3 th\u1ec3 truy c\u1eadp bi\u1ebfn tr\u01b0\u1edbc khi khai b\u00e1o, nh\u01b0ng k\u1ebft qu\u1ea3 tr\u1ea3 v\u1ec1 s\u1ebd l\u00e0 undefined.\u00a0<\/span><\/p>\n<p><span style=\"font-weight: 400;\">Khi qu\u00e1 tr\u00ecnh th\u1ef1c thi \u0111\u01b0\u1ee3c b\u1eaft \u0111\u1ea7u v\u00e0 \u0111\u1ebfn d\u00f2ng n\u01a1i bi\u1ebfn \u0111\u01b0\u1ee3c khai b\u00e1o, n\u00f3 s\u1ebd thay th\u1ebf gi\u00e1 tr\u1ecb trong b\u1ed9 nh\u1edb b\u1eb1ng gi\u00e1 tr\u1ecb c\u1ee7a bi\u1ebfn. V\u00ed d\u1ee5 nh\u01b0:\u00a0<\/span><\/p>\n<pre><span style=\"font-weight: 400;\">var<\/span><span style=\"font-weight: 400;\"> strawberry;<\/span>\r\n<span style=\"font-weight: 400;\">console<\/span><span style=\"font-weight: 400;\">.log(strawberry); <\/span><span style=\"font-weight: 400;\">\/\/ undefined<\/span>\r\n<span style=\"font-weight: 400;\">strawberry = <\/span><span style=\"font-weight: 400;\">'\ud83c\udf53'<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n<span style=\"font-weight: 400;\">console<\/span><span style=\"font-weight: 400;\">.log(strawberry); <\/span><span style=\"font-weight: 400;\">\/\/ \ud83c\udf53<\/span><\/pre>\n<p><span style=\"font-weight: 400;\">V\u1edbi v\u00ed d\u1ee5 tr\u00ean, b\u1ea1n c\u00f3 th\u1ec3 s\u1eed d\u1ee5ng bi\u1ebfn strawberry tr\u01b0\u1edbc khi khai b\u00e1o, nh\u01b0ng k\u1ebft qu\u1ea3 s\u1ebd tr\u1ea3 v\u1ec1 undefined. \u0110i\u1ec1u n\u00e0y s\u1ebd khi\u1ebfn ch\u01b0\u01a1ng tr\u00ecnh ch\u1ea1y m\u00e0 kh\u00f4ng c\u00f3 l\u1ed7i, tuy nhi\u00ean trong m\u1ed9t s\u1ed1 tr\u01b0\u1eddng h\u1ee3p c\u00f3 th\u1ec3 d\u1eabn \u0111\u1ebfn k\u1ebft qu\u1ea3 kh\u00f4ng mu\u1ed1n.\u00a0<\/span><\/p>\n<h2><span class=\"ez-toc-section\" id=\"Khai_bao_bien_trong_JavaScript_bang_tu_khoa_let\"><\/span><b>Khai b\u00e1o bi\u1ebfn trong JavaScript b\u1eb1ng t\u1eeb kh\u00f3a let<\/b><span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><span style=\"font-weight: 400;\">T\u1eeb kh\u00f3a let trong JavaScript \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng \u0111\u1ec3 t\u1ea1o c\u00e1c bi\u1ebfn c\u00f3 ph\u1ea1m vi kh\u1ed1i (block scope) m\u00e0 ch\u00fang \u0111\u01b0\u1ee3c khai b\u00e1o. Sau khi \u0111\u00e3 s\u1eed d\u1ee5ng let \u0111\u1ec3 \u0111\u1ecbnh ngh\u0129a m\u1ed9t bi\u1ebfn, b\u1ea1n kh\u00f4ng th\u1ec3 khai b\u00e1o l\u1ea1i bi\u1ebfn \u0111\u00f3 trong c\u00f9ng m\u1ed9t kh\u1ed1i. \u0110i\u1ec1u quan tr\u1ecdng l\u00e0 ph\u1ea3i khai b\u00e1o c\u00e1c bi\u1ebfn let tr\u01b0\u1edbc khi s\u1eed d\u1ee5ng ch\u00fang.<\/span><\/p>\n<pre><span style=\"font-weight: 400;\">let<\/span> <span style=\"font-weight: 400;\">variable<\/span><span style=\"font-weight: 400;\">_n<\/span><span style=\"font-weight: 400;\">ame = <\/span><span style=\"font-weight: 400;\">value<\/span><span style=\"font-weight: 400;\">;<\/span><\/pre>\n<p><span style=\"font-weight: 400;\">V\u00ed d\u1ee5 nh\u01b0:\u00a0<\/span><\/p>\n<pre><span style=\"font-weight: 400;\">let<\/span><span style=\"font-weight: 400;\"> x = <\/span><span style=\"font-weight: 400;\">5<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n<span style=\"font-weight: 400;\">let<\/span><span style=\"font-weight: 400;\"> y = <\/span><span style=\"font-weight: 400;\">6<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n<span style=\"font-weight: 400;\">let<\/span><span style=\"font-weight: 400;\"> z = x + y;<\/span>\r\n<span style=\"font-weight: 400;\">document.getElementById(<\/span><span style=\"font-weight: 400;\">\"demo\"<\/span><span style=\"font-weight: 400;\">).innerHTML =<\/span>\r\n<span style=\"font-weight: 400;\">\"The value of z is: \"<\/span><span style=\"font-weight: 400;\"> + z;<\/span><\/pre>\n<p><span style=\"font-weight: 400;\">K\u1ebft qu\u1ea3 hi\u1ec3n th\u1ecb:<\/span><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-78856\" src=\"https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-1-vippro.jpg\" alt=\"khai b\u00e1o bi\u1ebfn trong javascript - itviec blog\" width=\"523\" height=\"55\" srcset=\"https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-1-vippro.jpg 523w, https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-1-vippro-300x32.jpg 300w, https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-1-vippro-200x21.jpg 200w, https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-1-vippro-100x11.jpg 100w\" sizes=\"auto, (max-width: 523px) 100vw, 523px\" \/><\/p>\n<p><span style=\"font-weight: 400;\">V\u00ed d\u1ee5 kh\u00e1c v\u1ec1 ph\u1ea1m vi kh\u1ed1i c\u1ee7a let, bi\u1ebfn num c\u00f3 ph\u1ea1m vi kh\u1ed1i v\u00e0 kh\u00f4ng th\u1ec3 truy c\u1eadp b\u00ean ngo\u00e0i kh\u1ed1i. N\u1ebfu b\u1ea1n c\u1ed1 g\u1eafng truy c\u1eadp bi\u1ebfn b\u00ean ngo\u00e0i kh\u1ed1i, n\u00f3 s\u1ebd n\u00e9m ra k\u1ebft qu\u1ea3 l\u1ed7i tham chi\u1ebfu.\u00a0<\/span><\/p>\n<pre><span style=\"font-weight: 400;\">{<\/span>\r\n<span style=\"font-weight: 400;\">\u00a0 \u00a0 <\/span><span style=\"font-weight: 400;\">let<\/span> <span style=\"font-weight: 400;\">num<\/span><span style=\"font-weight: 400;\"> = <\/span><span style=\"font-weight: 400;\">10<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n<span style=\"font-weight: 400;\">\u00a0 \u00a0 console.<\/span><span style=\"font-weight: 400;\">log<\/span><span style=\"font-weight: 400;\">(<\/span><span style=\"font-weight: 400;\">num<\/span><span style=\"font-weight: 400;\">)<\/span>\r\n<span style=\"font-weight: 400;\">}<\/span>\r\n<span style=\"font-weight: 400;\">console.<\/span><span style=\"font-weight: 400;\">log<\/span><span style=\"font-weight: 400;\">(<\/span><span style=\"font-weight: 400;\">num<\/span><span style=\"font-weight: 400;\">)<\/span><\/pre>\n<p><span style=\"font-weight: 400;\">K\u1ebft qu\u1ea3 hi\u1ec3n th\u1ecb:<\/span><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-78855\" src=\"https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-2-vippro.jpg\" alt=\"khai b\u00e1o bi\u1ebfn trong javascript - itviec blog\" width=\"778\" height=\"99\" srcset=\"https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-2-vippro.jpg 778w, https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-2-vippro-300x38.jpg 300w, https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-2-vippro-700x89.jpg 640w, https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-2-vippro-200x25.jpg 200w, https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-2-vippro-100x13.jpg 100w\" sizes=\"auto, (max-width: 778px) 100vw, 778px\" \/><\/p>\n<h2><span class=\"ez-toc-section\" id=\"Khai_bao_bien_trong_JavaScript_bang_tu_khoa_const\"><\/span><b>Khai b\u00e1o bi\u1ebfn trong JavaScript b\u1eb1ng t\u1eeb kh\u00f3a const<\/b><span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><span style=\"font-weight: 400;\">T\u1eeb kh\u00f3a const trong JavaScript \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng \u0111\u1ec3 \u0111\u1ecbnh ngh\u0129a c\u00e1c bi\u1ebfn kh\u00f4ng th\u1ec3 thay \u0111\u1ed5i sau khi ch\u00fang \u0111\u01b0\u1ee3c g\u00e1n gi\u00e1 tr\u1ecb. \u0110i\u1ec1u n\u00e0y s\u1ebd ng\u0103n ch\u1eb7n s\u1ef1 s\u1eeda \u0111\u1ed5i \u0111\u1ed1i v\u1edbi gi\u00e1 tr\u1ecb c\u1ee7a bi\u1ebfn. Ngo\u00e0i ra, const kh\u00f4ng cho ph\u00e9p khai b\u00e1o l\u1ea1i c\u00f9ng m\u1ed9t bi\u1ebfn trong c\u00f9ng m\u1ed9t kh\u1ed1i v\u00e0 n\u00f3 cung c\u1ea5p ph\u1ea1m vi kh\u1ed1i (block scope), \u0111\u01b0\u1ee3c gi\u1edbi thi\u1ec7u trong ES2015 \u0111\u1ec3 t\u1ea1o c\u00e1c bi\u1ebfn kh\u00f4ng th\u1ec3 thay \u0111\u1ed5i.\u00a0<\/span><\/p>\n<p><span style=\"font-weight: 400;\">C\u00fa ph\u00e1p c\u01a1 b\u1ea3n c\u1ee7a t\u1eeb kh\u00f3a const nh\u01b0 sau:\u00a0<\/span><\/p>\n<pre><span style=\"font-weight: 400;\">const const_name<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n<span style=\"font-weight: 400;\">const x<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n\r\n<span style=\"font-weight: 400;\">const fruit = \u201capple\u201d;<\/span>\r\n\r\n<span style=\"font-weight: 400;\">const flower = \u201crose\u201d;<\/span><\/pre>\n<p><span style=\"font-weight: 400;\">Ngo\u00e0i ra, kh\u00f4ng th\u1ec3 g\u00e1n l\u1ea1i gi\u00e1 tr\u1ecb cho bi\u1ebfn \u0111\u01b0\u1ee3c khai b\u00e1o b\u1eb1ng const. N\u1ebfu b\u1ea1n c\u1ed1 t\u00ecnh g\u00e1n l\u1ea1i gi\u00e1 tr\u1ecb, ch\u00fang s\u1ebd g\u00e2y ra l\u1ed7i \u201cAssignment to constant variable\u201d<\/span><\/p>\n<pre><span style=\"font-weight: 400;\">const x = 12;<\/span>\r\n<span style=\"font-weight: 400;\">x = 13;<\/span>\r\n<span style=\"font-weight: 400;\">x += 1;<\/span><\/pre>\n<p><span style=\"font-weight: 400;\">K\u1ebft qu\u1ea3 hi\u1ec3n th\u1ecb khi g\u00e1n l\u1ea1i gi\u00e1 tr\u1ecb const:<\/span><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-78894\" src=\"https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-4-vippro.jpg\" alt=\"khai b\u00e1o bi\u1ebfn trong javascript - itviec blog\" width=\"730\" height=\"61\" srcset=\"https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-4-vippro.jpg 730w, https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-4-vippro-300x25.jpg 300w, https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-4-vippro-700x58.jpg 640w, https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-4-vippro-200x17.jpg 200w, https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-4-vippro-100x8.jpg 100w\" sizes=\"auto, (max-width: 730px) 100vw, 730px\" \/><\/p>\n<p><span style=\"font-weight: 400;\">M\u1ed9t v\u00ed d\u1ee5 m\u00f4 t\u1ea3 const ch\u1ee9a trong block scope nh\u01b0 sau:<\/span><\/p>\n<pre><span style=\"font-weight: 400;\">const x = <\/span><span style=\"font-weight: 400;\">22<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n<span style=\"font-weight: 400;\">{<\/span>\r\n<span style=\"font-weight: 400;\">\u00a0 \u00a0 const x = <\/span><span style=\"font-weight: 400;\">90<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n<span style=\"font-weight: 400;\">\u00a0 \u00a0 console.log(x)<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n<span style=\"font-weight: 400;\">\u00a0 \u00a0 {<\/span>\r\n<span style=\"font-weight: 400;\">\u00a0 \u00a0 \u00a0 \u00a0 const x = <\/span><span style=\"font-weight: 400;\">77<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n<span style=\"font-weight: 400;\">\u00a0 \u00a0 \u00a0 \u00a0 console.log(x)<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n<span style=\"font-weight: 400;\">\u00a0 \u00a0 }<\/span>\r\n<span style=\"font-weight: 400;\">\u00a0 \u00a0 {<\/span>\r\n<span style=\"font-weight: 400;\">\u00a0 \u00a0 \u00a0 \u00a0 const x = <\/span><span style=\"font-weight: 400;\">45<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n<span style=\"font-weight: 400;\">\u00a0 \u00a0 \u00a0 \u00a0 console.log(x)<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n<span style=\"font-weight: 400;\">\u00a0 \u00a0 }<\/span>\r\n<span style=\"font-weight: 400;\">}<\/span>\r\n<span style=\"font-weight: 400;\">console.log(x)<\/span><span style=\"font-weight: 400;\">;<\/span><\/pre>\n<p><span style=\"font-weight: 400;\">K\u1ebft qu\u1ea3 hi\u1ec3n th\u1ecb:<\/span><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-78854\" src=\"https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-3-vippro.jpg\" alt=\"khai b\u00e1o bi\u1ebfn trong javascript - itviec blog\" width=\"610\" height=\"169\" srcset=\"https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-3-vippro.jpg 610w, https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-3-vippro-300x83.jpg 300w, https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-3-vippro-200x55.jpg 200w, https:\/\/itviec.com\/blog\/wp-content\/uploads\/2024\/09\/khai-bao-bien-trong-javascript-3-vippro-100x28.jpg 100w\" sizes=\"auto, (max-width: 610px) 100vw, 610px\" \/><\/p>\n<h2><span class=\"ez-toc-section\" id=\"Khai_bao_lai_bien_trong_JavaScript_bang_var\"><\/span><b>Khai b\u00e1o l\u1ea1i bi\u1ebfn trong JavaScript b\u1eb1ng var<\/b><span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><span style=\"font-weight: 400;\">B\u1ea1n c\u00f3 th\u1ec3 khai b\u00e1o l\u1ea1i m\u1ed9t bi\u1ebfn \u0111\u01b0\u1ee3c khai b\u00e1o b\u1eb1ng var trong c\u00f9ng ph\u1ea1m vi. Bi\u1ebfn s\u1ebd b\u1ecb \u1ea3nh h\u01b0\u1edfng trong global scope ho\u1eb7c functional scope n\u1ebfu n\u00f3 \u0111\u01b0\u1ee3c khai b\u00e1o trong m\u1ed9t h\u00e0m.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">V\u00ec v\u1eady, ngay c\u1ea3 khi b\u1ea1n khai b\u00e1o l\u1ea1i m\u1ed9t bi\u1ebfn trong ph\u1ea1m vi nh\u1ecf h\u01a1n, bi\u1ebfn s\u1ebd thay \u0111\u1ed5i trong t\u1ea5t c\u1ea3 c\u00e1c ph\u1ea1m vi m\u00e0 bi\u1ebfn \u0111\u00f3 kh\u1ea3 d\u1ee5ng.<\/span><\/p>\n<pre><span style=\"font-weight: 400;\">\/\/ Global Scope<\/span>\r\n<span style=\"font-weight: 400;\">var<\/span><span style=\"font-weight: 400;\"> pepper = <\/span><span style=\"font-weight: 400;\">\"\ud83c\udf36\ufe0f\"<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n\r\n<span style=\"font-weight: 400;\">console<\/span><span style=\"font-weight: 400;\">.log(pepper); <\/span><span style=\"font-weight: 400;\">\/\/ \ud83c\udf36\ufe0f<\/span>\r\n\r\n<span style=\"font-weight: 400;\">\/\/ Block scope<\/span>\r\n<span style=\"font-weight: 400;\">{<\/span>\r\n<span style=\"font-weight: 400;\">\u00a0 <\/span><span style=\"font-weight: 400;\">var<\/span><span style=\"font-weight: 400;\"> pepper = <\/span><span style=\"font-weight: 400;\">\"\ud83c\udf3d\"<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n\r\n<span style=\"font-weight: 400;\">\u00a0 <\/span><span style=\"font-weight: 400;\">console<\/span><span style=\"font-weight: 400;\">.log(pepper); <\/span><span style=\"font-weight: 400;\">\/\/ \ud83c\udf3d<\/span>\r\n<span style=\"font-weight: 400;\">}<\/span>\r\n\r\n<span style=\"font-weight: 400;\">console<\/span><span style=\"font-weight: 400;\">.log(pepper); <\/span><span style=\"font-weight: 400;\">\/\/ \ud83c\udf3d<\/span><\/pre>\n<p><span style=\"font-weight: 400;\">\u1ede v\u00ed d\u1ee5 tr\u00ean, b\u1ea1n \u0111\u00e3 khai b\u00e1o m\u1ed9t bi\u1ebfn l\u00e0 pepper trong block scope v\u00e0 g\u00e1n cho n\u00f3 m\u1ed9t gi\u00e1 tr\u1ecb kh\u00e1c. \u0110i\u1ec1u n\u00e0y s\u1ebd \u1ea3nh h\u01b0\u1edfng \u0111\u1ebfn bi\u1ebfn trong global scope v\u00e0 b\u1ea1n m\u1ea5t quy\u1ec1n truy c\u1eadp v\u00e0o bi\u1ebfn tr\u01b0\u1edbc \u0111\u00f3.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">\u0110\u1ec3 tr\u00e1nh t\u00ecnh tr\u1ea1ng n\u00e0y, b\u1ea1n c\u00f3 th\u1ec3 th\u1ef1c hi\u1ec7n khai b\u00e1o l\u1ea1i trong m\u1ed9t h\u00e0m, bi\u1ebfn b\u00ean ngo\u00e0i h\u00e0m v\u1eabn gi\u1eef nguy\u00ean v\u00e0 bi\u1ebfn b\u00ean trong s\u1ebd kh\u00f4ng th\u1ec3 \u1ea3nh h\u01b0\u1edfng \u0111\u1ebfn n\u00f3. V\u1edbi v\u00ed d\u1ee5 tr\u00ean, b\u1ea1n c\u00f3 th\u1ec3 khai b\u00e1o m\u1ed9t bi\u1ebfn m\u1edbi l\u00e0 melon b\u00ean trong h\u00e0m, nh\u01b0 v\u1eady bi\u1ebfn melon \u0111\u01b0\u1ee3c khai b\u00e1o trong global scope v\u1eabn s\u1ebd \u0111\u01b0\u1ee3c gi\u1eef nguy\u00ean.<\/span><\/p>\n<pre><span style=\"font-weight: 400;\">\/\/ Global scope<\/span>\r\n<span style=\"font-weight: 400;\">var<\/span><span style=\"font-weight: 400;\"> melon = <\/span><span style=\"font-weight: 400;\">\"\ud83c\udf49\"<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n\r\n<span style=\"font-weight: 400;\">\/\/ Functional scope<\/span>\r\n<span style=\"font-weight: 400;\">function<\/span> <span style=\"font-weight: 400;\">redeclareMelon<\/span><span style=\"font-weight: 400;\">() {<\/span>\r\n<span style=\"font-weight: 400;\">\u00a0 <\/span><span style=\"font-weight: 400;\">var<\/span><span style=\"font-weight: 400;\"> melon = <\/span><span style=\"font-weight: 400;\">\"\ud83c\udf48\"<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n\r\n<span style=\"font-weight: 400;\">\u00a0 <\/span><span style=\"font-weight: 400;\">console<\/span><span style=\"font-weight: 400;\">.log(melon); <\/span><span style=\"font-weight: 400;\">\/\/ \ud83c\udf48<\/span>\r\n<span style=\"font-weight: 400;\">}<\/span>\r\n\r\n<span style=\"font-weight: 400;\">console<\/span><span style=\"font-weight: 400;\">.log(melon); <\/span><span style=\"font-weight: 400;\">\/\/ \ud83c\udf49<\/span><\/pre>\n<h2><span class=\"ez-toc-section\" id=\"Khai_bao_lai_bien_trong_JavaScript_bang_let_va_const\"><\/span><b>Khai b\u00e1o l\u1ea1i bi\u1ebfn trong JavaScript b\u1eb1ng let v\u00e0 const<\/b><span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><span style=\"font-weight: 400;\">B\u1ea1n kh\u00f4ng th\u1ec3 khai b\u00e1o l\u1ea1i c\u00e1c bi\u1ebfn \u0111\u01b0\u1ee3c khai b\u00e1o b\u1eb1ng let ho\u1eb7c const trong c\u00f9ng ph\u1ea1m vi. N\u1ebfu b\u1ea1n c\u1ed1 g\u1eafng th\u1ef1c hi\u1ec7n quy tr\u00ecnh, ch\u01b0\u01a1ng tr\u00ecnh s\u1ebd tr\u1ea3 v\u1ec1 k\u1ebft qu\u1ea3 l\u00e0 SyntaxError.\u00a0<\/span><\/p>\n<p><span style=\"font-weight: 400;\">B\u1ea1n c\u00f3 th\u1ec3 th\u1ef1c hi\u1ec7n khai b\u00e1o l\u1ea1i c\u00e1c bi\u1ebfn b\u1eb1ng let v\u00e0 const trong ph\u1ea1m vi con. B\u1edfi v\u00ec c\u00e1c bi\u1ebfn \u0111\u01b0\u1ee3c khai b\u00e1o b\u1eb1ng let v\u00e0 const l\u00e0 block scope v\u00e0 kh\u00f4ng \u1ea3nh h\u01b0\u1edfng \u0111\u1ebfn ph\u1ea1m vi l\u1edbn h\u01a1n.\u00a0<\/span><\/p>\n<pre><span style=\"font-weight: 400;\">\/\/ Global scope<\/span>\r\n<span style=\"font-weight: 400;\">const<\/span><span style=\"font-weight: 400;\"> banana = <\/span><span style=\"font-weight: 400;\">\"\ud83c\udf4c\"<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n\r\n<span style=\"font-weight: 400;\">\/\/ Block scope<\/span>\r\n<span style=\"font-weight: 400;\">{<\/span>\r\n<span style=\"font-weight: 400;\">\u00a0 <\/span><span style=\"font-weight: 400;\">const<\/span><span style=\"font-weight: 400;\"> banana = <\/span><span style=\"font-weight: 400;\">\"\ud83c\udf52\"<\/span><span style=\"font-weight: 400;\">;<\/span>\r\n\r\n<span style=\"font-weight: 400;\">\u00a0 console.<\/span><span style=\"font-weight: 400;\">log<\/span><span style=\"font-weight: 400;\">(banana); <\/span><span style=\"font-weight: 400;\">\/\/ \ud83c\udf52<\/span>\r\n<span style=\"font-weight: 400;\">}<\/span>\r\n\r\n<span style=\"font-weight: 400;\">console.<\/span><span style=\"font-weight: 400;\">log<\/span><span style=\"font-weight: 400;\">(banana); <\/span><span style=\"font-weight: 400;\">\/\/ \ud83c\udf4c<\/span><\/pre>\n<p><span style=\"font-weight: 400;\">Ch\u1eb3ng h\u1ea1n v\u1edbi v\u00ed d\u1ee5 tr\u00ean, b\u1ea1n khai b\u00e1o hai bi\u1ebfn banana v\u1edbi bi\u1ebfn \u0111\u1ea7u ti\u00ean n\u1eb1m trong global scope v\u00e0 bi\u1ebfn th\u1ee9 hai trong block scope. Bi\u1ebfn trong global scope v\u1eabn gi\u1eef nguy\u00ean v\u00e0 bi\u1ebfn trong block scope \u0111\u00e3 \u0111\u01b0\u1ee3c thay \u0111\u1ed5i l\u00e0 m\u1ed9t bi\u1ebfn m\u1edbi \u0111\u1ed9c l\u1eadp.<\/span><\/p>\n<p><span style=\"font-weight: 400;\">C\u00e1ch n\u00e0y c\u00f3 m\u1ed9t nh\u01b0\u1ee3c \u0111i\u1ec3m l\u00e0 b\u1ea1n s\u1ebd kh\u00f4ng \u0111\u01b0\u1ee3c ch\u1ec9nh s\u1eeda bi\u1ebfn banana trong global scope v\u00e0 block scope.\u00a0<\/span><\/p>\n<h2><span class=\"ez-toc-section\" id=\"Cau_hoi_thuong_gap_ve_khai_bao_bien_trong_JavaScript\"><\/span><b> C\u00e2u h\u1ecfi th\u01b0\u1eddng g\u1eb7p v\u1ec1 khai b\u00e1o bi\u1ebfn trong JavaScript<\/b><span class=\"ez-toc-section-end\"><\/span><\/h2>\n<h3><b>Var, let, const c\u00f3 g\u00ec kh\u00e1c nhau?<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Nh\u00ecn chung, c\u1ea3 ba t\u1eeb kh\u00f3a \u0111\u1ec1u \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng \u0111\u1ec3 khai b\u00e1o bi\u1ebfn trong JavaScript. Tuy nhi\u00ean, ch\u00fang v\u1eabn c\u00f3 c\u00e1ch ho\u1ea1t \u0111\u1ed9ng ri\u00eang bi\u1ec7t v\u1edbi nhau, c\u1ee5 th\u1ec3 nh\u01b0 sau:<\/span><\/p>\n<table>\n<tbody>\n<tr>\n<td><\/td>\n<td><b>var<\/b><\/td>\n<td><b>let<\/b><\/td>\n<td><b>const<\/b><\/td>\n<\/tr>\n<tr>\n<td><strong>Ph\u1ea1m vi<\/strong><\/td>\n<td><span style=\"font-weight: 400;\">Ph\u1ea1m vi ch\u1ee9c n\u0103ng\u00a0 (function scoped)<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Ph\u1ea1m vi kh\u1ed1i\u00a0 (block scope) ho\u1eb7c global scope<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Ph\u1ea1m vi kh\u1ed1i (block scope) ho\u1eb7c global scope<\/span><\/td>\n<\/tr>\n<tr>\n<td><strong>T\u00ednh ch\u1ea5t bi\u1ebfn<\/strong><\/td>\n<td><span style=\"font-weight: 400;\">C\u00f3 th\u1ec3 thay \u0111\u1ed5i<\/span><\/td>\n<td><span style=\"font-weight: 400;\">C\u00f3 th\u1ec3 thay \u0111\u1ed5i<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Kh\u00f4ng th\u1ec3 thay \u0111\u1ed5i<\/span><\/td>\n<\/tr>\n<tr>\n<td><strong>Khai b\u00e1o l\u1ea1i<\/strong><\/td>\n<td><span style=\"font-weight: 400;\">C\u00f3 th\u1ec3 khai b\u00e1o l\u1ea1i<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Kh\u00f4ng khai b\u00e1o l\u1ea1i bi\u1ebfn trong c\u00f9ng ph\u1ea1m vi kh\u1ed1i<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Kh\u00f4ng th\u1ec3 khai b\u00e1o l\u1ea1i m\u1ed9t bi\u1ebfn \u0111\u00e3 \u0111\u01b0\u1ee3c khai b\u00e1o b\u1eb1ng const<\/span><\/td>\n<\/tr>\n<tr>\n<td><strong>Ngu\u1ed3n<\/strong><\/td>\n<td><span style=\"font-weight: 400;\">Tr\u01b0\u1edbc phi\u00ean b\u1ea3n ES2015<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Phi\u00ean b\u1ea3n ES2015 (ES6)<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Phi\u00ean b\u1ea3n ES2015 (ES6)<\/span><\/td>\n<\/tr>\n<tr>\n<td><strong>H\u1ed7 tr\u1ee3<\/strong><\/td>\n<td><span style=\"font-weight: 400;\">H\u1ed7 tr\u1ee3 tr\u00ean t\u1ea5t c\u1ea3 c\u00e1c tr\u00ecnh duy\u1ec7t, bao g\u1ed3m c\u00e1c phi\u00ean b\u1ea3n c\u0169 nh\u01b0 Internet Explorer 5 v\u00e0 6<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Kh\u00f4ng h\u1ed7 tr\u1ee3 c\u00e1c tr\u00ecnh duy\u1ec7t b\u1ea3n c\u0169<\/span><\/td>\n<td><span style=\"font-weight: 400;\">Kh\u00f4ng h\u1ed7 tr\u1ee3 c\u00e1c tr\u00ecnh duy\u1ec7t b\u1ea3n c\u0169<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h3><b>Khi n\u00e0o c\u1ea7n s\u1eed d\u1ee5ng var, let ho\u1eb7c const?<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">T\u00f9y v\u00e0o m\u1ed7i tr\u01b0\u1eddng h\u1ee3p v\u00e0 m\u1ee5c \u0111\u00edch s\u1eed d\u1ee5ng m\u00e0 b\u1ea1n c\u00f3 th\u1ec3 l\u1ef1a ch\u1ecdn khai b\u00e1o bi\u1ebfn b\u1eb1ng var, let ho\u1eb7c const. V\u1eady khi n\u00e0o d\u00f9ng var, let, const \u0111\u1ec3 khai b\u00e1o bi\u1ebfn?<\/span><\/p>\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">B\u1ea1n l\u1ef1a ch\u1ecdn khai b\u00e1o bi\u1ebfn b\u1eb1ng const n\u1ebfu gi\u00e1 tr\u1ecb kh\u00f4ng thay \u0111\u1ed5i, ho\u1eb7c ki\u1ec3u c\u1ee7a bi\u1ebfn kh\u00f4ng thay \u0111\u1ed5i nh\u01b0 khi l\u00e0m vi\u1ec7c v\u1edbi m\u1ea3ng (array) v\u00e0 \u0111\u1ed1i t\u01b0\u1ee3ng (object).<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">S\u1eed d\u1ee5ng let n\u1ebfu gi\u00e1 tr\u1ecb c\u00f3 th\u1ec3 thay \u0111\u1ed5i ho\u1eb7c khi kh\u00f4ng d\u00f9ng \u0111\u01b0\u1ee3c const.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Var v\u1eabn \u0111\u01b0\u1ee3c h\u1ed7 tr\u1ee3 cho c\u00e1c tr\u00ecnh duy\u1ec7t hi\u1ec7n \u0111\u1ea1i. Tuy nhi\u00ean do c\u00e1c v\u1ea5n \u0111\u1ec1 li\u00ean quan \u0111\u1ebfn ph\u1ea1m vi v\u00e0 hosting n\u00ean c\u00e1c l\u1eadp tr\u00ecnh vi\u00ean th\u01b0\u1eddng \u01b0u ti\u00ean s\u1eed d\u1ee5ng let ho\u1eb7c const \u0111\u1ec3 thay th\u1ebf cho var.\u00a0<\/span><\/li>\n<\/ul>\n<h3><b>Block, Global v\u00e0 Functional scope c\u1ee7a bi\u1ebfn c\u00f3 g\u00ec kh\u00e1c nhau?<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">Scope (ph\u1ea1m vi) l\u00e0 b\u1ed1i c\u1ea3nh th\u1ef1c thi hi\u1ec7n t\u1ea1i trong \u0111\u00f3 c\u00e1c gi\u00e1 tr\u1ecb ho\u1eb7c bi\u1ec3u th\u1ee9c \u201cc\u00f3 th\u1ec3 nh\u00ecn th\u1ea5y\u201d ho\u1eb7c c\u00f3 th\u1ec3 \u0111\u01b0\u1ee3c tham chi\u1ebfu.\u00a0<\/span><\/p>\n<p><span style=\"font-weight: 400;\">V\u1ec1 bi\u1ebfn, ph\u1ea1m vi l\u00e0 n\u01a1i m\u1ed9t s\u1ed1 bi\u1ebfn nh\u1ea5t \u0111\u1ecbnh c\u00f3 s\u1eb5n, c\u00f3 th\u1ec3 truy c\u1eadp c\u00e1c bi\u1ebfn \u0111\u00e3 \u0111\u01b0\u1ee3c khai b\u00e1o trong ph\u1ea1m vi l\u1edbn ho\u1eb7c nh\u1ecf h\u01a1n. S\u1ef1 kh\u00e1c nhau gi\u1eefa ba ph\u1ea1m vi n\u00e0y nh\u01b0 sau:<\/span><\/p>\n<table>\n<tbody>\n<tr>\n<td><b>Block scope<\/b><\/td>\n<td><b>Functional scope<\/b><\/td>\n<td><b>Global scope<\/b><\/td>\n<\/tr>\n<tr>\n<td>\n<ul>\n<li style=\"font-weight: 400; text-align: left;\" aria-level=\"1\"><span style=\"font-weight: 400;\">\u0110\u01b0\u1ee3c t\u1ea1o b\u1eb1ng m\u1ed9t c\u1eb7p d\u1ea5u ngo\u1eb7c nh\u1ecdn {}<\/span><\/li>\n<li style=\"font-weight: 400; text-align: left;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Ch\u1ec9 h\u1ee3p l\u1ec7 v\u1edbi let v\u00e0 const<\/span><\/li>\n<li style=\"font-weight: 400; text-align: left;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Khi khai b\u00e1o m\u1ed9t bi\u1ebfn b\u1eb1ng var, n\u00f3 s\u1ebd t\u1ef1 \u0111\u1ed9ng chuy\u1ec3n \u0111\u1ebfn Global ho\u1eb7c Functional n\u1ebfu 2 ph\u1ea1m vi n\u00e0y c\u00f3 t\u1ed3n t\u1ea1i trong m\u00e3.<\/span><\/li>\n<\/ul>\n<\/td>\n<td style=\"text-align: left;\">\n<ul>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Ph\u1ea1m vi \u0111\u01b0\u1ee3c t\u1ea1o b\u1eb1ng khai b\u00e1o h\u00e0m.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Ch\u1ec9 kh\u1ea3 d\u1ee5ng trong ph\u1ea1m vi \u0111\u00f3 v\u00e0 kh\u00f4ng th\u1ec3 truy c\u1eadp b\u00ean ngo\u00e0i.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Var, let v\u00e0 const s\u1ebd gi\u1ed1ng nhau trong tr\u01b0\u1eddng h\u1ee3p n\u00e0y.<\/span><\/li>\n<\/ul>\n<\/td>\n<td>\n<ul>\n<li style=\"font-weight: 400; text-align: left;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Ph\u1ea1m vi ch\u00ednh bao g\u1ed3m t\u1ea5t c\u1ea3 c\u00e1c ph\u1ea1m vi trong m\u1ed9t t\u1eadp l\u1ec7nh.\u00a0<\/span><\/li>\n<li style=\"font-weight: 400; text-align: left;\" aria-level=\"1\"><span style=\"font-weight: 400;\">C\u00e1c bi\u1ebfn \u0111\u01b0\u1ee3c khai b\u00e1o trong Global c\u00f3 s\u1eb5n trong t\u1ea5t c\u1ea3 c\u00e1c ph\u1ea1m vi.<\/span><\/li>\n<\/ul>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h2><span class=\"ez-toc-section\" id=\"Tong_ket_ve_cach_khai_bao_bien_trong_JavaScript\"><\/span><b>T\u1ed5ng k\u1ebft v\u1ec1 c\u00e1ch khai b\u00e1o bi\u1ebfn trong JavaScript<\/b><span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><span style=\"font-weight: 400;\">B\u00e0i vi\u1ebft \u0111\u00e3 gi\u1edbi thi\u1ec7u chi ti\u1ebft v\u1ec1 c\u00e1ch <\/span><b>khai b\u00e1o bi\u1ebfn trong JavaScript<\/b><span style=\"font-weight: 400;\"> b\u1eb1ng var, let v\u00e0 const. T\u00f9y theo t\u1eebng tr\u01b0\u1eddng h\u1ee3p v\u00e0 m\u1ee5c \u0111\u00edch m\u00e0 b\u1ea1n c\u00f3 th\u1ec3 l\u1ef1a ch\u1ecdn t\u1eeb kh\u00f3a khai b\u00e1o ph\u00f9 h\u1ee3p. B\u00ean c\u1ea1nh ki\u1ebfn th\u1ee9c n\u1ec1n t\u1ea3ng, b\u1ea1n c\u00f3 th\u1ec3 th\u1ef1c h\u00e0nh th\u00f4ng qua c\u00e1c d\u1ef1 \u00e1n th\u1ef1c t\u1ebf \u0111\u1ec3 n\u00e2ng c\u1ea5p k\u1ef9 n\u0103ng c\u1ee7a b\u1ea3n th\u00e2n c\u0169ng nh\u01b0 c\u01a1 h\u1ed9i l\u00e0m vi\u1ec7c h\u1ea5p d\u1eabn nh\u00e9!<\/span><\/p>\n<p><span style=\"font-weight: 400;\">B\u1ea1n c\u00f3 th\u1ec3 k\u1ebft h\u1ee3p theo d\u00f5i nh\u1eefng b\u00e0i vi\u1ebft c\u01a1 b\u1ea3n v\u1ec1 c\u00e1c ph\u01b0\u01a1ng th\u1ee9c trong JavaScript sau:<\/span><\/p>\n<ul>\n<li><a href=\"https:\/\/itviec.com\/blog\/if-trong-javascript\/\" target=\"_blank\" rel=\"noopener\">H\u01b0\u1edbng d\u1eabn s\u1eed d\u1ee5ng c\u00e1c l\u1ec7nh if trong JavaScript chi ti\u1ebft<\/a><\/li>\n<li><a href=\"https:\/\/itviec.com\/blog\/return-trong-javascript\/\" target=\"_blank\" rel=\"noopener\">Return trong JavaScript: H\u01b0\u1edbng d\u1eabn s\u1eed d\u1ee5ng chi ti\u1ebft<\/a><\/li>\n<li><a href=\"https:\/\/itviec.com\/blog\/vong-lap-trong-javascript\/\" target=\"_blank\" rel=\"noopener\">V\u00f2ng l\u1eb7p trong JavaScript: Chi ti\u1ebft 8 ph\u01b0\u01a1ng th\u1ee9c l\u1eb7p k\u00e8m v\u00ed d\u1ee5<\/a><\/li>\n<li><a href=\"https:\/\/itviec.com\/blog\/foreach-javascript-la-gi\/\" target=\"_blank\" rel=\"noopener\">forEach JavaScript: M\u1ed9t s\u1ed1 thao t\u00e1c c\u01a1 b\u1ea3n v\u00e0 v\u00ed d\u1ee5 chi ti\u1ebft<\/a><\/li>\n<li><a href=\"https:\/\/itviec.com\/blog\/array-javascript-la-gi\/\" target=\"_blank\" rel=\"noopener\">Array JavaScript: T\u1ed5ng h\u1ee3p 12 ph\u01b0\u01a1ng th\u1ee9c quan tr\u1ecdng c\u1ee7a array<\/a><\/li>\n<\/ul>\n<blockquote><p><em>Xem th\u00eam:<\/em> <strong><em><a href=\"https:\/\/itviec.com\/blog\/javascript-developer\/\" target=\"_blank\" rel=\"noopener\">JavaScript Developer l\u00e0 g\u00ec? C\u00e1c h\u01b0\u1edbng ph\u00e1t tri\u1ec3n c\u1ee7a JavaScript Developer<\/a><\/em><\/strong><\/p><\/blockquote>\n","protected":false},"excerpt":{"rendered":"<p>Bi\u1ebfn \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng \u0111\u1ec3 l\u01b0u tr\u1eef d\u1eef li\u1ec7u v\u00e0 gi\u00e1 tr\u1ecb c\u00f3 th\u1ec3 t\u00e1i s\u1eed d\u1ee5ng trong JavaScript. N\u1eafm r\u00f5 v\u1ec1 c\u00e1ch khai b\u00e1o bi\u1ebfn s\u1ebd gi\u00fap b\u1ea1n c\u00f3 \u0111\u1ee7 t\u1ef1 tin \u0111\u1ec3 th\u1ef1c hi\u1ec7n vi\u1ebft m\u1ed9t \u0111o\u1ea1n m\u00e3 JavaScript tuy\u1ec7t v\u1eddi. V\u1eady c\u00e1ch khai b\u00e1o bi\u1ebfn trong JavaScript nh\u01b0 th\u1ebf n\u00e0o? \u0110\u1ecdc b\u00e0i [&hellip;]<\/p>\n","protected":false},"author":207,"featured_media":78893,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_gspb_post_css":"","footnotes":""},"categories":[109],"tags":[],"class_list":["post-78503","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-chuyen-mon-it"],"blocksy_meta":[],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v26.8 (Yoast SEO v27.7) - https:\/\/yoast.com\/product\/yoast-seo-premium-wordpress\/ -->\n<title>Khai b\u00e1o bi\u1ebfn trong JavaScript: Chi ti\u1ebft c\u00e1c c\u00e1ch ph\u1ed5 bi\u1ebfn - ITviec Blog<\/title>\n<meta name=\"description\" content=\"Khai b\u00e1o bi\u1ebfn trong JavaScript b\u1eb1ng c\u00e1ch s\u1eed d\u1ee5ng var, let ho\u1eb7c const. 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