{"id":683,"date":"2012-03-14T23:51:06","date_gmt":"2012-03-14T23:51:06","guid":{"rendered":"http:\/\/brain.dailyon.net\/?p=683"},"modified":"2012-03-14T23:51:06","modified_gmt":"2012-03-14T23:51:06","slug":"javascript-basic-null-undefined","status":"publish","type":"post","link":"https:\/\/blog.goorigo.com\/?p=683","title":{"rendered":"JavaScript basic : null &#038; undefined"},"content":{"rendered":"<p>\ud38c:<a href=\"http:\/\/luvstudy.tistory.com\/13\">http:\/\/luvstudy.tistory.com\/13<\/a><\/p>\n<p><strong>\uae30\ubcf8 \ub370\uc774\ud130 \ud615\uc2dd<\/strong><br \/>\nJavaScript\uc5d0\ub294 \ub370\uc774\ud130 \ud615\uc2dd\uc73c\ub85c \uc544\ub798\uc758 \ud615\uc2dd\uc774 \uc788\ub2e4.<\/p>\n<ul>\n<li>\uae30\ubcf8 \ud615\uc2dd<\/li>\n<ul>\n<li>String<\/li>\n<li>Number<\/li>\n<li>Boolean<\/li>\n<\/ul>\n<li>\ud569\uc131(\ucc38\uace0) \ud615\uc2dd<\/li>\n<ul>\n<li>Object<\/li>\n<li>Array<\/li>\n<\/ul>\n<li>\ud2b9\uc815 \ud615\uc2dd<\/li>\n<ul>\n<li>null<\/li>\n<li>undefined<\/li>\n<\/ul>\n<\/ul>\n<p>\uc774\uc911 null\uacfc undefined\ub77c\ub294 \ud2b9\uc815 \ud615\uc2dd\uc774 \ub298 \uc0ac\ub78c\uc744 \ud63c\ub780\uc2a4\ub7fd\uac8c \ub9cc\ub4e0\ub2e4.<\/p>\n<p><strong>null<\/strong><br \/>\nnull\uc740 \ubcf4\ud1b5 \uac1d\uccb4 \ud0c0\uc785\uc758 \ud2b9\uc218\ud55c \uac12, \uc989 \uc5b4\ub5a0\ud55c \uac1d\uccb4\ub3c4 \ub098\ud0c0\ub0b4\uc9c0 \uc54a\ub294 \uac12\uc73c\ub85c \ucde8\uae09\ub41c\ub2e4.<br \/>\nnull\uc740 \ub2e4\ub978 \ubaa8\ub4e0 \uac12\ub4e4\uacfc \uad6c\ubd84\ub418\ub294 \uace0\uc720\ud55c \uac12\uc774\ub2e4.<\/p>\n<p>\uc5b4\ub5a4 \ubcc0\uc218\uac00 null\uac12\uc744 \uac00\uc9c0\uba74 \uadf8 \ubcc0\uc218\ub294 \uae30\ubcf8 \ud615\uc2dd(String, Number, Boolean)\uc774\ub098 \ud569\uc131 \ud615\uc2dd(Object, Array)\uc758 \uac12\uc744 \ub2f4\uace0 \uc788\uc9c0 \uc54a\ub2e4\ub294 \uac83\uc744 \uc758\ubbf8\ud55c\ub2e4.<\/p>\n<p>\ub530\ub77c\uc11c \uc5b4\ub5a4 \ubcc0\uc218\uc758 \uac12\uc744 \ucd08\uae30\ud654 \ud558\uace0 \uc2f6\uc744 \uacbd\uc6b0\uc5d4 null\uc744 \uc0ac\uc6a9\ud55c\ub2e4.<\/p>\n<p>&nbsp;<\/p>\n<p>var a = &#8220;\ubb38\uc790\uac12&#8221;; \/\/\uae30\ubcf8 \ud615\uc2dd\uc758 \uac12\uc774 \ud560\ub2f9\ub428.<br \/>\na = null; \/\/\ud574\ub2f9 \uac12\uc774 a \ubcc0\uc218\uc5d0\uc11c \uc81c\uac70\ub428.<\/p>\n<p>var B = function(name) {<br \/>\nthis.name = name;<br \/>\n}<br \/>\nvar b = new B(&#8220;b\uc774\ub984&#8221;); \/\/B \uc0dd\uc131\uc790\ub97c \uc0ac\uc6a9\ud558\uc5ec \uac1c\uccb4\uac00 \uc0dd\uc131-&gt; b\ub77c\ub294 \ubcc0\uc218\uc5d0 \ud574\ub2f9 \uac1d\uccb4\uac00 \ucc38\uc870\ub428.<br \/>\nb = null; \/\/b\uc5d0 \ucc38\uc870 \uc815\ubcf4\uac00 \uc81c\uac70\ub428. (\uac1d\uccb4\uac00 \uc81c\uac70\ub418\ub294 \uac83\uc774 \uc544\ub2c8\ub77c \ucc38\uc870 \uc815\ubcf4\uac00 \uc81c\uac70\ub418\ub294 \uac83.)<\/p>\n<p>null\uc774 \uc22b\uc790 \ubb38\ub9e5\uc5d0\uc11c \uc0ac\uc6a9\ub418\uba74 0\uc73c\ub85c \ubcc0\ud658\ub418\uba70, \ubb38\uc790\uc5f4 \ubb38\ub9e5\uc5d0\uc11c \uc0ac\uc6a9\ub418\uba74 &#8220;null&#8221;\ub85c \ubcc0\ud658\ub41c\ub2e4.<br \/>\n\ubd88\ub9ac\uc5b8 \ubb38\ub9e5\uc5d0\uc11c \uc0ac\uc6a9\ub418\uba74 false\ub85c \ubcc0\ud658\ub41c\ub2e4.<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>\/\/\uc22b\uc790 \ubb38\ub9e5\uc5d0\uc11c \uc0ac\uc6a9\ud558\ub294 \uacbd\uc6b0<br \/>\ndocument.writeln(3 + null); \/\/ 3 + 0\uc774\ubbc0\ub85c \uacb0\uacfc : 3<br \/>\ndocument.writeln(3 &#8211; null); \/\/ 3 &#8211; 0\uc774\ubbc0\ub85c \uacb0\uacfc : 3<br \/>\ndocument.writeln(3 * null); \/\/ 3 * 0\uc774\ubbc0\ub85c \uacb0\uacfc : 0<br \/>\ndocument.writeln(3 \/ null); \/\/ 3 \/ 0\uc774\ubbc0\ub85c \uacb0\uacfc : Infinity(\ubb34\ud55c\ub300)<br \/>\ndocument.writeln(3 % null); \/\/ 3 % 0\uc774\ubbc0\ub85c \uacb0\uacfc : NaN(Not a Number. \uc22b\uc790\uac00 \uc544\ub2d8)<\/p>\n<p>\/\/\ubb38\uc790 \ubb38\ub9e5\uc5d0\uc11c \uc0ac\uc6a9\ud558\ub294 \uacbd\uc6b0<br \/>\ndocument.writeln(&#8220;\uc774\uac83\uc740 &#8221; + null + &#8220;\uc785\ub2c8\ub2e4.&#8221;) \/\/\uacb0\uacfc : \uc774\uac83\uc740 null \uc785\ub2c8\ub2e4.<\/p>\n<p>\/\/boolean \ubb38\ub9e5\uc5d0\uc11c \uc0ac\uc6a9\ud558\ub294 \uacbd\uc6b0.<br \/>\nvar a=null;<br \/>\nif(!a) document.writeln(&#8220;null \uc785\ub2c8\ub2e4.&#8221;);<\/p>\n<p>&nbsp;<\/p>\n<p><strong>undefined<br \/>\n<\/strong>undefined\ub294 \uc120\uc5b8\uc740 \ub418\uc5c8\uc9c0\ub9cc \uac12\uc774 \ud560\ub2f9\ub41c \uc801\uc774 \uc5c6\ub294 \ubcc0\uc218\uc5d0 \uc811\uadfc\ud558\uac70\ub098, \uc874\uc7ac\ud558\uc9c0 \uc54a\ub294 \uac1d\uccb4 \ud504\ub85c\ud37c\ud2f0\uc5d0 \uc811\uadfc\ud560 \uacbd\uc6b0 \ubc18\ud690\ub418\ub294 \uac12\uc774\ub2e4.<\/p>\n<p>\uc27d\uac8c \uc0dd\uac01\ud558\uba74 null\uc740 \uc0ac\uc6a9\uc790\uac00 \uace0\uc758\ub85c \uac12\uc744 \ud560\ub2f9\ud558\uc5ec \uac12\uc774 \uc5c6\ub2e4\ub294 \uac83\uc744 \uba85\uc2dc\ud55c \uac83\uc774\uace0,<br \/>\nundefined\ub294 \uc0ac\uc6a9\uc790\uac00 \uc544\ub2cc \ud504\ub85c\uadf8\ub7a8\uc774 \uc2e4\ud589\uc744 \ud558\uba74\uc11c \uc874\uc7ac\ud558\uc9c0 \uc54a\ub294 \uac12\uc758 \ubcc0\uc218\ub97c \ud638\ucd9c\ud558\ub3c4\ub85d(\uc8fc\ub85c \uc624\ud0c0\ub098 \ub204\ub77d\ub41c \ubcc0\uc218) \ud55c \uacbd\uc6b0\uc640 \uac19\uc774 \ud638\ucd9c\uc0c1 \uc624\ub958\ub97c \ub098\ud0c0\ub0b4\ub294 \ud2b9\uc815\ud615\uc2dd\uc774\ub77c\uace0 \uc0dd\uac01\ud558\uba74 \uc27d\ub2e4.<\/p>\n<p>\ub2e4\uc74c\uc758 \uc608\uc81c\ub97c \ubcf4\ub3c4\ub85d \ud558\uc790.<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>var a;<br \/>\ndocument.writeln(a); \/\/undefined a\uac00 \uc120\uc5b8\uc740 \ub418\uc5c8\uc73c\ub098 \uac12\uc774 \ud560\ub2f9\ub41c \uc801\uc774 \uc5c6\uc74c.<\/p>\n<p>\/\/document.writeln(b); \/\/\uc5c6\ub294 \uac12\uc758 \ud638\ucd9c\ub85c \uc778\ud574 \uc774\ud6c4 JavaScript\uad6c\uac04 \ud638\ucd9c \uc911\uc9c0<br \/>\ndocument.writeln(typeof b); \/\/undefined<\/p>\n<p>document.writeln(&#8220;\uae00\uc4f0\uae30&#8221;) \/\/\uc704\uc5d0 \uc8fc\uc11d\ucc98\ub9ac\ub41c b\ud638\ucd9c \uad6c\ubb38\uc774 \uc2e4\ud589\ub418\ub294 \uacbd\uc6b0(b\uac00 \uc5c6\ub294\ub370\ub3c4 \ud638\ucd9c\ub418\ub294 \uacbd\uc6b0) \uc774\ud6c4 \uc2a4\ud06c\ub9bd\ud2b8\ub294 \uc2e4\ud589\ub418\uc9c0 \uc54a\uc74c.<\/p>\n<p>a\uc758 \uacbd\uc6b0\ub294 \ubcc0\uc218\ub294 \uc788\uc9c0\ub9cc \uac12\uc744 \ud560\ub2f9\ud558\uc9c0 \uc54a\uc544 \uc0dd\uae34 \ubcc0\uc218 a\uc5d0 \ub300\ud55c \uac12\uc774 undefined\uc778 \uacbd\uc6b0\uc774\uace0<br \/>\nb\uc758 \uacbd\uc6b0\ub294 b\ub77c\ub294 \ubcc0\uc218\ub97c \ud560\ub2f9\ud558\uc9c0 \uc54a\uace0 \ud638\ucd9c\ud558\uc5ec \uc0ac\uc6a9\ub41c \ubcc0\uc218\uc5d0 \ub300\ud574 undefined\uc778 \uacbd\uc6b0\uc774\ub2e4.<br \/>\n\uc608\uc81c\uc5d0\uc120?\ub610\ub2e4\ub978 JS \uc601\uc5ed\uc774 \uc874\uc7ac\ud558\uc9c0 \uc54a\uc544 \ubcc4 \ubb38\uc81c \uc5c6\uc5b4\ubcf4\uc77c\uc9c0 \ubab0\ub77c\ub3c4,?\uc2e4\uc81c \ud398\uc774\uc9c0\uc5d0\uc11c b\uc640 \uac19\uc774 \ubcc0\uc218 \uc790\uccb4\uc5d0 \ub300\ud55c undefined \ud638\ucd9c?\uc624\ub958\uac00 \ubc1c\uc0dd\ud558\uac8c \ub418\uba74?\ud574\ub2f9 \uc624\ub958\uac00 \ubc1c\uc0dd\ub41c &lt;script&gt;~&lt;\/script&gt;\ub85c \uac10\uc2f8\uc9c4 \ube14\ub7ed \ub0b4\uc758 \uc774\ud6c4 \uad6c\ubb38\ub4e4\uc740 \uc2e4\ud589\uc774 \ub418\uc9c0 \uc54a\ub294\ub2e4.<\/p>\n<p>b\uc758 \uacbd\uc6b0\ub294 \ud558\ub098\uc758 \ubcc0\uc218\uac00?\uc5c6\uc74c\uc73c\ub85c \uc778\ud574 \uc774\ud6c4\uc758 \uc2a4\ud06c\ub9bd\ud2b8\uc5d0 \ub300\ud574\uc11c\ub3c4 \uc601\ud5a5\uc744 \ubbf8\uce58\uac8c \ub418\ub294 \uce58\uba85\uc801\uc778 \uc624\ub958\uc774\ub2e4.<br \/>\n\ub530\ub77c\uc11c \uc774\ub7f0 undefined\uc5d0 \ub300\ud55c \uccb4\ud06c\uac00 \uc120\ud589\ub418\uc5b4\uc57c \ud55c\ub2e4.<br \/>\n\uc774\ub7f4 \ub54c \uc0ac\uc6a9\ud560 \uc218 \uc788\ub294 \uac83\uc774 typeof \uc5f0\uc0b0\uc790\uc774\ub2e4.<\/p>\n<p><strong>typeof \uc5f0\uc0b0\uc790<\/strong><br \/>\ntypeof \uc5f0\uc0b0\uc790\ub294 \ud615\uc2dd \uc815\ubcf4\ub97c \ub2e4\uc74c \uc911 \ud558\ub098\uc758 \ubb38\uc790\uc5f4\ub85c \ubc18\ud658\ud574\uc900\ub2e4.<br \/>\nnumber, string, boolean, object, function, undefined<\/p>\n<div>\n<div><code>if<\/code>?<code>(<\/code><code>typeof<\/code>?<code>a ==?<\/code><code>\"undefined\"<\/code><code>) document.writeln(<\/code><code>\"a\uac00 \uc874\uc7ac\ud558\uc9c0 \uc54a\uc74c.\"<\/code><code>)<\/code><\/div>\n<\/div>\n<p>\ud568\uc218\uc758 \uacbd\uc6b0\ub3c4?\uc801\uc808\ud788 \uccb4\ud06c \ud574\uc900\ub2e4.<\/p>\n<div>\n<div><\/div>\n<div>\n<div><code>1.<\/code><code>\/\/\ud568\uc218\uc758 \uacbd\uc6b0<\/code><\/div>\n<div><code>2.<\/code><code>function<\/code>?<code>test() {<\/code><\/div>\n<div><code>3.<\/code><code>if<\/code>?<code>(<\/code><code>typeof<\/code>?<code>a ==<\/code><code>\"undefined\"<\/code><code>)?<\/code><code>return<\/code>?<code>false<\/code><code>;???<\/code><code>\/\/\uc5c6\uc73c\uba74 \ud568\uc218 \ud638\ucd9c \uc911\uc9c0<\/code><\/div>\n<div><code>4.<\/code><code>\/\/\uc788\ub294 \uacbd\uc6b0 \uc870\uac74 \uccb4\ud06c<\/code><\/div>\n<div><code>5.<\/code><code>}<\/code><\/div>\n<\/div>\n<\/div>\n<p>JavaScript\uc5d0\uc11c \uc0ac\uc6a9\ud558\ub294 \ubcc0\uc218\ub294 \uc704\uc640 \uac19\uc774 \uc0ac\uc6a9\uc774 \uac00\ub2a5\ud558\uc9c0\ub9cc HTML\uc758 DOM\uc694\uc18c\uc5d0 \ub300\ud574\uc11c\ub294 \uc880 \ub2e4\ub974\ub2e4.<br \/>\ngetElementbyId \uc694\uc18c\ub97c \uc0ac\uc6a9\ud558\uba74 \ud638\ucd9c\ud55c id\uac12\uc744 \uac00\uc9c4 object\uac00 \ub9ac\ud134\ub41c\ub2e4.<br \/>\n\uc874\uc7ac\ud558\uc9c0 \uc54a\ub294 id\uc758 DOM\uc694\uc18c\ub97c \ud638\ucd9c\ud558\uba74 \uac12\uc774 null\uc778 object\uac00 \ud638\ucd9c\ub41c\ub2e4.<br \/>\n\ub530\ub77c\uc11c typeof\uc758 \uccb4\ud06c\uac00 \ud544\uc694\uc5c6\ub2e4.<\/p>\n<div>\n<div><\/div>\n<div>\n<div><code>1.<\/code><code>if<\/code><code>(document.getElementById(<\/code><code>\"a\"<\/code><code>) ==?<\/code><code>null<\/code><code>) document.writeln(<\/code><code>\"a\uac00 \uc874\uc7ac\ud558\uc9c0 \uc54a\uc74c.\"<\/code><code>)<\/code><\/div>\n<\/div>\n<\/div>\n<p>JSON\uac1d\uccb4\uc758 \uacbd\uc6b0 var\ub85c \ub9cc\ub4e4\uace0 \ub09c \uc774\ud6c4 \ud638\ucd9c\uc2dc \uc874\uc7ac\ud558\uc9c0 \uc54a\ub294 \uac12\uc778 \uacbd\uc6b0 \uc790\ub3d9\uc73c\ub85c \ud574\ub2f9 \uba64\ubc84\ubcc0\uc218\uac00 \uc0dd\uc131\uc774 \ub418\ubbc0\ub85c \ubcc0\uc218 \uc790\uccb4\uc758 \uc874\uc7ac\ud558\uc9c0 \uc54a\uc74c\uc73c\ub85c \uc778\ud55c \uc624\ub958 \ubc1c\uc0dd\uc740 \ubc1c\uc0dd\ud558\uc9c0 \uc54a\uc73c\uba70 \ud574\ub2f9 \uba64\ubc84\ubcc0\uc218\uc758 \uac12\uc740 undefined\uac00 \ub41c\ub2e4.<\/p>\n<div>\n<div><\/div>\n<div>\n<div><code>1.<\/code><code>var<\/code>?<code>objectA = {};<\/code><\/div>\n<div><code>2.<\/code><code>document.writeln(<\/code><code>typeof<\/code>?<code>objectA);???<\/code><code>\/\/\ub2f9\uc5f0\ud788 object<\/code><\/div>\n<div><code>3.<\/code><code>document.writeln(<\/code><code>typeof<\/code>?<code>objectA.a);?<\/code><code>\/\/\uacb0\uacfc : undefined, \ubcc0\uc218\ub294 \ud568\ucd95\uc801\uc120\uc5b8\uc73c\ub85c \uc778\ud574 \uc874\uc7ac<\/code><\/div>\n<div><code>4.<\/code><code>document.writeln(objectA.a);????<\/code><code>\/\/\uacb0\uacfc : undefined<\/code><\/div>\n<\/div>\n<\/div>\n<p>null\uacfc undefined\ub294 \uc11c\ub85c \uad6c\ubcc4\ub418\ub294 \uac12\uc774\uc9c0\ub9cc \ub3d9\ub4f1 \uc5f0\uc0b0\uc790(equality operator) == \ub294 \ub458\uc744 \uac19\uc740 \uac83\uc73c\ub85c \uac04\uc8fc\ud55c\ub2e4.<br \/>\n\uc774 \ub450 \uac12\uc744 \uad6c\ubcc4\ud558\uc5ec \ube44\uad50\ud558\ub824\uba74 \uc77c\uce58 \uc5f0\uc0b0\uc790(===) \ub610\ub294 typeof \uc5f0\uc0b0\uc790\ub97c \uc0ac\uc6a9\ud558\uba74 \ub41c\ub2e4.<\/p>\n<p>\uc704\uc5d0\uc11c \uc5b8\uae09\ud55c HTML DOM\uacfc \uc77c\ubc18 \uc790\ubc14 \ubcc0\uc218 \ubc0f \ud568\uc218\uc5d0 \ub300\ud574 \ud655\uc2e4\ud788 \uad6c\ubd84\ud558\uc5ec \uac1c\ubc1c\ud560 \uc218 \uc788\uac8c \ub418\uba74 \ub3d9\ub4f1 \uc5f0\uc0b0\uc790\uac00 \uc544\ub2cc \uc77c\uce58 \uc5f0\uc0b0\uc790\ub85c \ud0c0\uc785\uc744 \uccb4\ud06c\ud558\ub294 \ubc84\ub987\uc744 \ub4e4\uc774\ub3c4\ub85d \ud574\uc57c\ud55c\ub2e4.<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>\ud38c:http:\/\/luvstudy.tistory.com\/13 \uae30\ubcf8 \ub370\uc774\ud130 \ud615\uc2dd JavaScript\uc5d0\ub294 \ub370\uc774\ud130 \ud615\uc2dd\uc73c\ub85c \uc544\ub798\uc758 \ud615\uc2dd\uc774 \uc788\ub2e4. \uae30\ubcf8 \ud615\uc2dd String Number Boolean \ud569\uc131(\ucc38\uace0) \ud615\uc2dd Object Array \ud2b9\uc815 \ud615\uc2dd null undefined \uc774\uc911 null\uacfc undefined\ub77c\ub294 \ud2b9\uc815 \ud615\uc2dd\uc774 \ub298 \uc0ac\ub78c\uc744&hellip; <\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[1],"tags":[],"_links":{"self":[{"href":"https:\/\/blog.goorigo.com\/index.php?rest_route=\/wp\/v2\/posts\/683"}],"collection":[{"href":"https:\/\/blog.goorigo.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blog.goorigo.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blog.goorigo.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/blog.goorigo.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=683"}],"version-history":[{"count":0,"href":"https:\/\/blog.goorigo.com\/index.php?rest_route=\/wp\/v2\/posts\/683\/revisions"}],"wp:attachment":[{"href":"https:\/\/blog.goorigo.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=683"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blog.goorigo.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=683"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blog.goorigo.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=683"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}