{"id":1230,"date":"2022-11-18T10:54:17","date_gmt":"2022-11-18T02:54:17","guid":{"rendered":"https:\/\/fdream.net\/blog\/?p=1230"},"modified":"2022-11-18T10:59:31","modified_gmt":"2022-11-18T02:59:31","slug":"can-a-1-a-2-a-3-ever-evaluate-to-true","status":"publish","type":"post","link":"https:\/\/fdream.net\/blog\/article\/1230","title":{"rendered":"\u770b\u5230\u4e00\u4e2a\u5201\u94bb\u95ee\u9898\uff1a\u00a0(a==1 && a==2 && a==3)\u00a0\u53ef\u4ee5\u4e3atrue\u5417\uff1f"},"content":{"rendered":"\n

\u4e00\u770b\u5230\u8fd9\u4e2a\u5201\u94bb\u95ee\u9898\uff0c\u6211\u7adf\u7136\u6709\u70b9\u61f5\uff1a (a==1 && a==2 && a==3) \u662f\u5426\u53ef\u4ee5\u4e3atrue\uff1f\u65e2\u7136\u80fd\u63d0\u51fa\u8fd9\u4e2a\u95ee\u9898\uff0c\u8bf4\u660e\u90a3\u80af\u5b9a\u662f\u53ef\u4ee5\u4e3atrue\u7684\uff0c\u90a3\u4e48\u600e\u4e48\u6837\u5b9e\u73b0\u5462\uff1f<\/p>\n\n\n\n

\u5728js\u91cc\u9762\uff0c\u6bd4\u8f83\u8fd0\u7b97\u7b26\u5bf9\u4e8e\u4e0d\u540c\u7c7b\u578b\u7684\u503c\u4f1a\u505a\u7c7b\u578b\u8f6c\u6362\uff0c\u6240\u4ee5\u8fd9\u91cc\u5e94\u8be5\u6709\u53ef\u4ee5\u64cd\u4f5c\u7684\u53ef\u80fd\u3002\u6211\u4eec\u5148\u770b\u4e00\u4e0b\u975e\u4e25\u683c\u76f8\u7b49\u5728js\u91cc\u9762\u7684\u7c7b\u578b\u8f6c\u6362\u5173\u7cfb\uff1a<\/p>\n\n\n\n

\n \n \n \n \n \n \n \n \n \n \n
<\/th>\n \u88ab\u6bd4\u8f83\u503c B<\/th>\n <\/tr>\n <\/thead>\n
<\/th>\n <\/td>\n Undefined<\/td>\n Null<\/td>\n Number<\/td>\n String<\/td>\n Boolean<\/td>\n Object<\/td>\n <\/tr>\n
\u88ab\u6bd4\u8f83\u503c A<\/th>\n Undefined<\/td>\n true<\/code><\/td>\n true<\/code><\/td>\n false<\/code><\/td>\n false<\/code><\/td>\n false<\/code><\/td>\n IsFalsy(B)<\/code><\/td>\n <\/tr>\n
Null<\/td>\n true<\/code><\/td>\n true<\/code><\/td>\n false<\/code><\/td>\n false<\/code><\/td>\n false<\/code><\/td>\n IsFalsy(B)<\/code><\/td>\n <\/tr>\n
Number<\/td>\n false<\/code><\/td>\n false<\/code><\/td>\n A === B<\/code><\/td>\n A === ToNumber(B)<\/code><\/td>\n A=== ToNumber(B)<\/code><\/td>\n A== ToPrimitive(B)<\/code><\/td>\n <\/tr>\n
String<\/td>\n false<\/code><\/td>\n false<\/code><\/td>\n ToNumber(A) === B<\/code><\/td>\n A === B<\/code><\/td>\n ToNumber(A) === ToNumber(B)<\/code><\/td>\n ToPrimitive(B) == A<\/code><\/td>\n <\/tr>\n
Boolean<\/td>\n false<\/code><\/td>\n false<\/code><\/td>\n ToNumber(A) === B<\/code><\/td>\n ToNumber(A) === ToNumber(B)<\/code><\/td>\n A === B<\/code><\/td>\n ToNumber(A) == ToPrimitive(B)<\/code><\/td>\n <\/tr>\n
Object<\/td>\n false<\/code><\/td>\n false<\/code><\/td>\n ToPrimitive(A) == B<\/code><\/td>\n ToPrimitive(A) == B<\/code><\/td>\n ToPrimitive(A) == ToNumber(B)<\/code><\/td>\n \n A === B<\/code>\n <\/td>\n <\/tr>\n <\/tbody>\n<\/table>\n<\/figure>\n\n\n\n

\u4ece\u4e0a\u8868\u53ef\u4ee5\u770b\u51fa\uff0c\u5982\u679c\u548c\u6570\u5b57\u8fdb\u884c\u6bd4\u8f83\uff0c\u90a3\u4e48\u5b57\u7b26\u4e32\u548c\u5e03\u5c14\u503c\u90fd\u8bb2\u8f6c\u6362\u4e3a\u6570\u5b57\u4e4b\u540e\u8fdb\u884c\u6bd4\u8f83\uff0c\u663e\u7136\u8fd9\u91cc\u64cd\u4f5c\u7a7a\u95f4\u4e0d\u5927\u3002\u90a3\u5269\u4e0b\u7684\u5c31\u662fObject<\/code>\u4e86\uff0c\u8fd9\u91ccObject<\/code>\u9700\u8981\u8f6c\u6362\u4e3a\u539f\u59cb\u7c7b\u578b\u3002<\/p>\n\n\n\n

\u65b9\u6cd5\u4e00\uff1a<\/h2>\n\n\n\n

\u6211\u4eec\u5148\u8bd5\u8bd5toPrimitive<\/code>\uff1a<\/p>\n\n\n\n

let a = {\n  i: 1,\n  [Symbol.toPrimitive]() {\n    return this.i++\n  }\n}\n\/\/ \u6bcf\u6b21\u6267\u884c\u6bd4\u8f83\u8fd0\u7b97\u65f6, \"Symbol.toPrimitive\"\u4f1a\u88ab\u6267\u884c\u4e00\u6b21\uff0c\u6bcf\u6b21+1\nconsole.log(a == 1 && a == 2 && a == 3)  \/\/ true<\/code><\/pre>\n\n\n\n
\u65b9\u6cd5\u4e8c\uff1a<\/h2>\n\n\n\n
\u4e0a\u9762\u8fd9\u4e2a\u65b9\u6cd5\u662f\u76f4\u63a5\u628aObject<\/code>\u8f6c\u6362\u4e3a\u539f\u59cb\u7c7b\u578bNumber<\/code>\uff0c\u53ef\u4ee5\u76f4\u63a5\u8fdb\u884c\u6bd4\u8f83\uff0c\u5982\u679c\u8fd9\u4e2aObject<\/code>\u6ca1\u6709toPrimitive()<\/code>\u65b9\u6cd5\u5462\uff1f\u90a3\u4e48\u4f1a\u987a\u5e8f\u8c03\u7528valueOf()<\/code>\u4ee5\u53catoString()<\/code>\u65b9\u6cd5\uff0c\u53c2\u8003\uff1a<\/p>\n\n\n\n
\n
Objects are converted to primitives by calling its [@@toPrimitive]()<\/code><\/a> (with \"default\"<\/code> as hint), valueOf()<\/code>, and toString()<\/code> methods, in that order. Note that primitive conversion calls valueOf()<\/code> before toString()<\/code>, which is similar to the behavior of number coercion<\/a> but different from string coercion<\/a>.<\/p>\n\u6765\u6e90\uff1aMDN JavaScript Reference<\/a><\/cite><\/blockquote>\n\n\n\n
\u65e2\u7136\u5982\u6b64\uff0c\u54b1\u5148\u8bd5\u8bd5valueOf()<\/code>\u65b9\u6cd5\uff1a<\/p>\n\n\n\n
let a = {\n  i: 1,\n  valueOf() {\n    return this.i++\n  }\n}\nconsole.log(a == 1 && a == 2 && a == 3)  \/\/ true<\/code><\/pre>\n\n\n\n
\u65b9\u6cd5\u4e09\uff1a<\/h2>\n\n\n\n
\u540c\u7406\uff0c\u4e0a\u9762\u7684valueOf()<\/code>\u8fd8\u53ef\u4ee5\u66ff\u6362\u4e3atoString()<\/code>\uff1a<\/p>\n\n\n\n
let a = {\n  i: 1,\n  toString() {\n    return this.i++\n  }\n}\nconsole.log(a == 1 && a == 2 && a == 3)  \/\/ true<\/code><\/pre>\n\n\n\n
\u65b9\u6cd5\u56db\uff1a<\/h2>\n\n\n\n
\u8bf4\u5230toString()<\/code>\u7684\u8bdd\uff0c\u6211\u4eec\u8fd8\u6709\u4e2a\u7279\u6b8a\u7684\u5bf9\u8c61\u4e5f\u53ef\u4ee5toString()<\/code>\u2014\u2014\u6570\u7ec4\u3002\u6570\u7ec4\u5728toString()<\/code>\u7684\u65f6\u5019\u9ed8\u8ba4\u4f1a\u8c03\u7528join()<\/code>\u65b9\u6cd5\uff0c\u8fd9\u4e2ajoin()<\/code>\u65b9\u6cd5\u6211\u4eec\u4e5f\u53ef\u4ee5\u9a9a\u64cd\u4f5c\u4e00\u4e0b\uff1a<\/p>\n\n\n\n
let a = [1, 2, 3]\n\na.join = a.shift\n\nconsole.log(a == 1 && a == 2 && a == 3)  \/\/ true<\/code><\/pre>\n\n\n\n
\u4f3c\u4e4e\u5230\u8fd9\u91cc\u5df2\u7ecf\u65e0\u8def\u53ef\u8d70\u4e86\uff0c\u6bd5\u7adf\u5df2\u7ecf\u53d1\u73b0\u4e86\u56db\u79cd\u65b9\u6cd5\u3002\u8fd8\u6709\u6ca1\u6709\u4e00\u4e9b\u975e\u5e38\u89c4\u65b9\u6cd5\u5462\uff1f<\/p>\n\n\n\n
\u65b9\u6cd5\u4e94\uff1a<\/h2>\n\n\n\n
\u5f88\u591a\u6846\u67b6\u91cc\u9762\u6709\u7528\u5230\u4e00\u4e2a\u65b9\u6cd5Object.defineProperty()<\/code>\uff0c\u8fd9\u4e2a\u4f3c\u4e4e\u4e5f\u53ef\u4ee5\u64cd\u4f5c\u4e00\u4e0b\uff1a<\/p>\n\n\n\n
let _a = 1\nObject.defineProperty(window, 'a', {\n  get() {\n    return _a++\n  }\n})\n\nconsole.log(a == 1 && a == 2 && a == 3)  \/\/ true<\/code><\/pre>\n\n\n\n
\u65b9\u6cd5\u516d\uff1a<\/h2>\n\n\n\n
\u8fd8\u6709\u975e\u5e38\u706b\u7684Proxy\uff1a<\/p>\n\n\n\n
let a = new Proxy({ i: 1 }, {\n  get(target) {\n    return () => target.i++\n  }\n})\n\nconsole.log(a == 1 && a == 2 && a == 3)  \/\/ true<\/code><\/pre>\n\n\n\n
\u65b9\u6cd5\u4e03\uff1a<\/h2>\n\n\n\n
\u8fd8\u6709\u4e00\u4e2ajs\u4e2d\u201c\u9519\u8bef\u7684\u5b58\u5728\u201dwith<\/code>\u5173\u952e\u8bcd\uff0c\u5e73\u5e38\u4e5f\u51e0\u4e4e\u4e0d\u4f1a\u7528\u5230\u7684\uff1a<\/p>\n\n\n\n
let i = 1\nwith ({\n  get a() {\n    return i++\n  }\n}) {\n  console.log(a == 1 && a == 2 && a == 3)  \/\/ true\n}<\/code><\/pre>\n\n\n\n
\n
\u6ce8\u610f\uff1awith\u8fd9\u4e2a\u5173\u952e\u8bcd\u5df2\u7ecf\u88ab\u6807\u8bb0\u4e3a\u8fc7\u65f6\u7684\uff0c\u867d\u7136\u90e8\u5206\u6d4f\u89c8\u5668\u4ecd\u7136\u652f\u6301\u5b83\u3002<\/p>\n\u6765\u6e90\uff1aMDN JavaScript Reference<\/a><\/cite><\/blockquote>\n","protected":false},"excerpt":{"rendered":"
\u4e00\u770b\u5230\u8fd9\u4e2a\u5201\u94bb\u95ee\u9898\uff0c\u6211\u7adf\u7136\u6709\u70b9\u61f5\uff1a (a==1 && a==2 && a==3) \u662f\u5426\u53ef\u4ee5\u4e3atrue\uff1f\u65e2\u7136\u80fd\u63d0\u51fa\u8fd9\u4e2a\u95ee\u9898\uff0c\u8bf4\u660e\u90a3\u80af\u5b9a\u662f\u53ef\u4ee5\u4e3atrue\u7684\uff0c\u90a3\u4e48\u600e\u4e48\u6837\u5b9e\u73b0\u5462\uff1f \u5728js\u91cc\u9762\uff0c\u6bd4\u8f83\u8fd0\u7b97\u7b26\u5bf9\u4e8e\u4e0d\u540c\u7c7b\u578b\u7684\u503c\u4f1a\u505a\u7c7b\u578b\u8f6c\u6362\uff0c\u6240\u4ee5\u8fd9\u91cc\u5e94\u8be5\u6709\u53ef\u4ee5\u64cd\u4f5c\u7684\u53ef\u80fd\u3002\u6211\u4eec\u5148\u770b\u4e00\u4e0b\u975e\u4e25\u683c\u76f8\u7b49\u5728js\u91cc\u9762\u7684\u7c7b\u578b\u8f6c\u6362\u5173\u7cfb\uff1a \u88ab\u6bd4\u8f83\u503c B Undefined Null Number String Boolean Object \u88ab\u6bd4\u8f83\u503c A Undefined true true false false false IsFalsy(B) Null true true false false false IsFalsy(B) Number false false A === B A === ToNumber(B) A=== ToNumber(B) A== ToPrimitive(B) String false false ToNumber(A) === B A === B ToNumber(A) === ToNumber(B) … <\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[104,105,20],"class_list":["post-1230","post","type-post","status-publish","format-standard","hentry","category-coding","tag-JavaScript","tag-JS","tag-20"],"views":350,"_links":{"self":[{"href":"https:\/\/fdream.net\/blog\/wp-json\/wp\/v2\/posts\/1230","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/fdream.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/fdream.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/fdream.net\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/fdream.net\/blog\/wp-json\/wp\/v2\/comments?post=1230"}],"version-history":[{"count":0,"href":"https:\/\/fdream.net\/blog\/wp-json\/wp\/v2\/posts\/1230\/revisions"}],"wp:attachment":[{"href":"https:\/\/fdream.net\/blog\/wp-json\/wp\/v2\/media?parent=1230"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/fdream.net\/blog\/wp-json\/wp\/v2\/categories?post=1230"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/fdream.net\/blog\/wp-json\/wp\/v2\/tags?post=1230"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}