{"id":1421,"date":"2015-09-28T10:57:02","date_gmt":"2015-09-28T02:57:02","guid":{"rendered":"http:\/\/www.daliane.com\/?p=1421"},"modified":"2015-09-28T11:04:46","modified_gmt":"2015-09-28T03:04:46","slug":"undefined_yu_null_de_qu_bie_qian_duan_mian_shi_jing_dian_wen_ti","status":"publish","type":"post","link":"http:\/\/www.daliane.com\/category\/jinjiaoyou\/undefined_yu_null_de_qu_bie_qian_duan_mian_shi_jing_dian_wen_ti\/","title":{"rendered":"undefined\u4e0enull\u7684\u533a\u522b(\u524d\u7aef\u9762\u8bd5\u7ecf\u5178\u95ee\u9898)"},"content":{"rendered":"

\u5927\u591a\u6570\u8ba1\u7b97\u673a\u8bed\u8a00\uff0c\u6709\u4e14\u4ec5\u6709\u4e00\u4e2a\u8868\u793a”\u65e0”\u7684\u503c\uff0c\u6bd4\u5982\uff0cC\u8bed\u8a00\u7684NULL\uff0cJava\u8bed\u8a00\u7684null\uff0cPython\u8bed\u8a00\u7684None\uff0cRuby\u8bed\u8a00\u7684nil\u3002<\/p>\n

\u6709\u70b9\u5947\u602a\u7684\u662f\uff0cJavaScript\u8bed\u8a00\u5c45\u7136\u6709\u4e24\u4e2a\u8868\u793a”\u65e0”\u7684\u503c\uff1aundefined\u548cnull\u3002\u8fd9\u662f\u4e3a\u4ec0\u4e48\uff1f<\/p>\n

\"undefined<\/p>\n

<\/a>\u4e00\u3001\u76f8\u4f3c\u6027<\/h2>\n

\u5728JavaScript\u4e2d\uff0c\u5c06\u4e00\u4e2a\u53d8\u91cf\u8d4b\u503c\u4e3aundefined\u6216null\uff0c\u8001\u5b9e\u8bf4\uff0c\u51e0\u4e4e\u6ca1\u533a\u522b\u3002<\/p>\n

\n
\r\nvar<\/span> a =<\/span> undefined;<\/span>\r\n\r\nvar<\/span> a =<\/span> null<\/span>;<\/span>\r\n\r\n<\/code><\/pre>\n<\/blockquote>\n
\u4e0a\u9762\u4ee3\u7801\u4e2d\uff0ca\u53d8\u91cf\u5206\u522b\u88ab\u8d4b\u503c\u4e3aundefined\u548cnull\uff0c\u8fd9\u4e24\u79cd\u5199\u6cd5\u51e0\u4e4e\u7b49\u4ef7\u3002<\/p>\n
undefined\u548cnull\u5728if\u8bed\u53e5\u4e2d\uff0c\u90fd\u4f1a\u88ab\u81ea\u52a8\u8f6c\u4e3afalse\uff0c\u76f8\u7b49\u8fd0\u7b97\u7b26\u751a\u81f3\u76f4\u63a5\u62a5\u544a\u4e24\u8005\u76f8\u7b49\u3002<\/p>\n
\n
\r\nif<\/span> (<\/span>!<\/span>undefined)<\/span> \r\n    console.<\/span>log(<\/span><\/span>'undefined is false'<\/span>)<\/span>;<\/span>\r\n\/\/ undefined is false\r\n<\/span>\r\nif<\/span> (<\/span>!<\/span>null<\/span>)<\/span> \r\n    console.<\/span>log(<\/span><\/span>'null is false'<\/span>)<\/span>;<\/span>\r\n\/\/ null is false\r\n<\/span>\r\nundefined ==<\/span> null<\/span>\r\n\/\/ true\r\n<\/span>\r\n<\/code><\/pre>\n<\/blockquote>\n
\u4e0a\u9762\u4ee3\u7801\u8bf4\u660e\uff0c\u4e24\u8005\u7684\u884c\u4e3a\u662f\u4f55\u7b49\u76f8\u4f3c\uff01<\/p>\n
\u65e2\u7136undefined\u548cnull\u7684\u542b\u4e49\u4e0e\u7528\u6cd5\u90fd\u5dee\u4e0d\u591a\uff0c\u4e3a\u4ec0\u4e48\u8981\u540c\u65f6\u8bbe\u7f6e\u4e24\u4e2a\u8fd9\u6837\u7684\u503c\uff0c\u8fd9\u4e0d\u662f\u65e0\u7aef\u589e\u52a0JavaScript\u7684\u590d\u6742\u5ea6\uff0c\u4ee4\u521d\u5b66\u8005\u56f0\u6270\u5417\uff1fGoogle\u516c\u53f8\u5f00\u53d1\u7684JavaScript\u8bed\u8a00\u7684\u66ff\u4ee3\u54c1Dart\u8bed\u8a00\uff0c\u5c31\u660e\u786e\u89c4\u5b9a\u53ea\u6709null\uff0c\u6ca1\u6709undefined\uff01<\/p>\n
<\/a>\u4e8c\u3001\u5386\u53f2\u539f\u56e0<\/h2>\n
\u6700\u8fd1\uff0c\u6211\u5728\u8bfb\u65b0\u4e66\u300aSpeaking JavaScript\u300b<\/a>\u65f6\uff0c\u610f\u5916\u53d1\u73b0\u4e86\u8fd9\u4e2a\u95ee\u9898\u7684\u7b54\u6848\uff01<\/p>\n
\u539f\u6765\uff0c\u8fd9\u4e0eJavaScript\u7684\u5386\u53f2\u6709\u5173\u30021995\u5e74JavaScript\u8bde\u751f<\/a>\u65f6\uff0c\u6700\u521d\u50cfJava\u4e00\u6837\uff0c\u53ea\u8bbe\u7f6e\u4e86null\u4f5c\u4e3a\u8868\u793a”\u65e0”\u7684\u503c\u3002<\/p>\n
\u6839\u636eC\u8bed\u8a00\u7684\u4f20\u7edf\uff0cnull\u88ab\u8bbe\u8ba1\u6210\u53ef\u4ee5\u81ea\u52a8\u8f6c\u4e3a0\u3002<\/p>\n
\n
\r\nNumber(<\/span><\/span>null<\/span>)<\/span>\r\n\/\/ 0\r\n<\/span>\r\n5<\/span> +<\/span> null<\/span>\r\n\/\/ 5\r\n<\/span>\r\n<\/code><\/pre>\n<\/blockquote>\n
\u4f46\u662f\uff0cJavaScript\u7684\u8bbe\u8ba1\u8005Brendan Eich\uff0c\u89c9\u5f97\u8fd9\u6837\u505a\u8fd8\u4e0d\u591f\uff0c\u6709\u4e24\u4e2a\u539f\u56e0\u3002<\/p>\n
\u9996\u5148\uff0cnull\u50cf\u5728Java\u91cc\u4e00\u6837\uff0c\u88ab\u5f53\u6210\u4e00\u4e2a\u5bf9\u8c61\u3002\u4f46\u662f\uff0cJavaScript\u7684\u6570\u636e\u7c7b\u578b\u5206\u6210\u539f\u59cb\u7c7b\u578b\uff08primitive\uff09\u548c\u5408\u6210\u7c7b\u578b\uff08complex\uff09\u4e24\u5927\u7c7b\uff0cBrendan Eich\u89c9\u5f97\u8868\u793a”\u65e0”\u7684\u503c\u6700\u597d\u4e0d\u662f\u5bf9\u8c61\u3002<\/p>\n
\u5176\u6b21\uff0cJavaScript\u7684\u6700\u521d\u7248\u672c\u6ca1\u6709\u5305\u62ec\u9519\u8bef\u5904\u7406\u673a\u5236\uff0c\u53d1\u751f\u6570\u636e\u7c7b\u578b\u4e0d\u5339\u914d\u65f6\uff0c\u5f80\u5f80\u662f\u81ea\u52a8\u8f6c\u6362\u7c7b\u578b\u6216\u8005\u9ed8\u9ed8\u5730\u5931\u8d25\u3002Brendan Eich\u89c9\u5f97\uff0c\u5982\u679cnull\u81ea\u52a8\u8f6c\u4e3a0\uff0c\u5f88\u4e0d\u5bb9\u6613\u53d1\u73b0\u9519\u8bef\u3002<\/p>\n
\u56e0\u6b64\uff0cBrendan Eich\u53c8\u8bbe\u8ba1\u4e86\u4e00\u4e2aundefined\u3002<\/p>\n
<\/a>\u4e09\u3001\u6700\u521d\u8bbe\u8ba1<\/h2>\n
JavaScript\u7684\u6700\u521d\u7248\u672c\u662f\u8fd9\u6837\u533a\u5206\u7684\uff1anull\u662f\u4e00\u4e2a\u8868\u793a”\u65e0”\u7684\u5bf9\u8c61\uff0c\u8f6c\u4e3a\u6570\u503c\u65f6\u4e3a0\uff1bundefined\u662f\u4e00\u4e2a\u8868\u793a”\u65e0”\u7684\u539f\u59cb\u503c\uff0c\u8f6c\u4e3a\u6570\u503c\u65f6\u4e3aNaN\u3002<\/p>\n
\n
\r\nNumber(<\/span><\/span>undefined)<\/span>\r\n\/\/ NaN\r\n<\/span>\r\n5<\/span> +<\/span> undefined\r\n\/\/ NaN\r\n<\/span>\r\n<\/code><\/pre>\n<\/blockquote>\n
<\/a>\u56db\u3001\u76ee\u524d\u7684\u7528\u6cd5<\/h2>\n
\u4f46\u662f\uff0c\u4e0a\u9762\u8fd9\u6837\u7684\u533a\u5206\uff0c\u5728\u5b9e\u8df5\u4e2d\u5f88\u5feb\u5c31\u88ab\u8bc1\u660e\u4e0d\u53ef\u884c\u3002\u76ee\u524d\uff0cnull\u548cundefined\u57fa\u672c\u662f\u540c\u4e49\u7684\uff0c\u53ea\u6709\u4e00\u4e9b\u7ec6\u5fae\u7684\u5dee\u522b\u3002<\/p>\n
null\u8868\u793a”\u6ca1\u6709\u5bf9\u8c61”\uff0c\u5373\u8be5\u5904\u4e0d\u5e94\u8be5\u6709\u503c\u3002\u5178\u578b\u7528\u6cd5\u662f\uff1a<\/p>\n
\uff081\uff09 \u4f5c\u4e3a\u51fd\u6570\u7684\u53c2\u6570\uff0c\u8868\u793a\u8be5\u51fd\u6570\u7684\u53c2\u6570\u4e0d\u662f\u5bf9\u8c61\u3002<\/p>\n
\uff082\uff09 \u4f5c\u4e3a\u5bf9\u8c61\u539f\u578b\u94fe\u7684\u7ec8\u70b9\u3002<\/p><\/blockquote>\n
\n
\r\nObject.<\/span>getPrototypeOf(<\/span><\/span>Object.<\/span>prototype)<\/span>\r\n\/\/ null\r\n<\/span>\r\n<\/code><\/pre>\n<\/blockquote>\n
undefined\u8868\u793a”\u7f3a\u5c11\u503c”\uff0c\u5c31\u662f\u6b64\u5904\u5e94\u8be5\u6709\u4e00\u4e2a\u503c\uff0c\u4f46\u662f\u8fd8\u6ca1\u6709\u5b9a\u4e49\u3002\u5178\u578b\u7528\u6cd5\u662f\uff1a<\/p>\n
\uff081\uff09\u53d8\u91cf\u88ab\u58f0\u660e\u4e86\uff0c\u4f46\u6ca1\u6709\u8d4b\u503c\u65f6\uff0c\u5c31\u7b49\u4e8eundefined\u3002<\/p>\n
\uff082) \u8c03\u7528\u51fd\u6570\u65f6\uff0c\u5e94\u8be5\u63d0\u4f9b\u7684\u53c2\u6570\u6ca1\u6709\u63d0\u4f9b\uff0c\u8be5\u53c2\u6570\u7b49\u4e8eundefined\u3002<\/p>\n
\uff083\uff09\u5bf9\u8c61\u6ca1\u6709\u8d4b\u503c\u7684\u5c5e\u6027\uff0c\u8be5\u5c5e\u6027\u7684\u503c\u4e3aundefined\u3002<\/p>\n
\uff084\uff09\u51fd\u6570\u6ca1\u6709\u8fd4\u56de\u503c\u65f6\uff0c\u9ed8\u8ba4\u8fd4\u56deundefined\u3002<\/p><\/blockquote>\n
\n
\r\nvar<\/span> i;<\/span>\r\ni \/\/ undefined\r\n<\/span>\r\nfunction<\/span> f(<\/span><\/span>x)<\/span>{<\/span>console.<\/span>log(<\/span><\/span>x)<\/span>}<\/span>\r\nf(<\/span><\/span>)<\/span> \/\/ undefined\r\n<\/span>\r\nvar<\/span>  o =<\/span> new<\/span> Object<\/span>(<\/span>)<\/span>;<\/span>\r\no.<\/span>p \/\/ undefined\r\n<\/span>\r\nvar<\/span> x =<\/span> f(<\/span><\/span>)<\/span>;<\/span>\r\nx \/\/ undefined\r\n<\/span>\r\n<\/code><\/pre>\n<\/blockquote>\n
\uff08\u5b8c\uff09<\/p>\n","protected":false},"excerpt":{"rendered":"
\u5927\u591a\u6570\u8ba1\u7b97\u673a\u8bed\u8a00\uff0c\u6709\u4e14\u4ec5\u6709\u4e00\u4e2a\u8868\u793a”\u65e0”\u7684\u503c\uff0c\u6bd4\u5982\uff0cC\u8bed\u8a00\u7684NULL\uff0cJava\u8bed\u8a00\u7684n…<\/p>\n","protected":false},"author":1,"featured_media":228,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_kad_post_transparent":"","_kad_post_title":"","_kad_post_layout":"","_kad_post_sidebar_id":"","_kad_post_content_style":"","_kad_post_vertical_padding":"","_kad_post_feature":"","_kad_post_feature_position":"","_kad_post_header":false,"_kad_post_footer":false,"footnotes":""},"categories":[38],"tags":[112,111],"_links":{"self":[{"href":"http:\/\/www.daliane.com\/category\/jinjiaoyou\/wp-json\/wp\/v2\/posts\/1421"}],"collection":[{"href":"http:\/\/www.daliane.com\/category\/jinjiaoyou\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.daliane.com\/category\/jinjiaoyou\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.daliane.com\/category\/jinjiaoyou\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.daliane.com\/category\/jinjiaoyou\/wp-json\/wp\/v2\/comments?post=1421"}],"version-history":[{"count":1,"href":"http:\/\/www.daliane.com\/category\/jinjiaoyou\/wp-json\/wp\/v2\/posts\/1421\/revisions"}],"predecessor-version":[{"id":1423,"href":"http:\/\/www.daliane.com\/category\/jinjiaoyou\/wp-json\/wp\/v2\/posts\/1421\/revisions\/1423"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/www.daliane.com\/category\/jinjiaoyou\/wp-json\/wp\/v2\/media\/228"}],"wp:attachment":[{"href":"http:\/\/www.daliane.com\/category\/jinjiaoyou\/wp-json\/wp\/v2\/media?parent=1421"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.daliane.com\/category\/jinjiaoyou\/wp-json\/wp\/v2\/categories?post=1421"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.daliane.com\/category\/jinjiaoyou\/wp-json\/wp\/v2\/tags?post=1421"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}