{"id":10970,"date":"2023-11-21T20:25:00","date_gmt":"2023-11-21T19:25:00","guid":{"rendered":"https:\/\/monodes.com\/predaelli\/?p=10970"},"modified":"2023-11-21T11:32:06","modified_gmt":"2023-11-21T10:32:06","slug":"javascript-can-a-1-a-2-a3-ever-evaluate-to-true-stack-overflow","status":"publish","type":"post","link":"https:\/\/monodes.com\/predaelli\/2023\/11\/21\/javascript-can-a-1-a-2-a3-ever-evaluate-to-true-stack-overflow\/","title":{"rendered":"javascript – Can (a== 1 && a ==2 && a==3) ever evaluate to true? – Stack Overflow"},"content":{"rendered":"\n

javascript – Can (a== 1 && a ==2 && a==3) ever evaluate to true? – Stack Overflow<\/a><\/em><\/p>\n\n\n\n

Yes, it can. IMHO it is one of the several undesirable consequences of loosely typed languages. In fact, according to an almost anonymous user<\/a>:<\/p>\n\n\n\n

\n

If you take advantage of how ==<\/code> works<\/a>, you could simply create an object with a custom toString<\/code> (or valueOf<\/code>) function that changes what it returns each time it is used such that it satisfies all three conditions.<\/p>\n\n\n\n

const a = {\n  i: 1,\n  toString: function () {\n    return a.i++;\n  }\n}\n\nif(a == 1 && a == 2 && a == 3) {\n  console.log('Hello World!');\n}<\/code><\/pre>\n\n\n\n
The reason this works is due to the use of the loose equality operator. When using loose equality, if one of the operands is of a different type than the other, the engine will attempt to convert one to the other. In the case of an object on the left and a number on the right, it will attempt to convert the object to a number by first calling valueOf<\/code> if it is callable, and failing that, it will call toString<\/code>. I used toString<\/code> in this case simply because it’s what came to mind, valueOf<\/code> would make more sense. If I instead returned a string from toString<\/code>, the engine would have then attempted to convert the string to a number giving us the same end result, though with a slightly longer path.<\/p>\n<\/blockquote>\n\n\n\n
Worst, it is also possible using the === operator!<\/p>\n\n\n\n
\n
var i = 0;\n\nwith({\n  get a() {\n    return ++i;\n  }\n}) {\n  if (a == 1 && a == 2 && a == 3)\n    console.log("wohoo");\n}<\/code><\/pre>\n\n\n\n
This uses a getter inside of a with<\/code> statement to let a<\/code> evaluate to three different values.<\/p>\n\n\n\n
… this still does not mean this should be used in real code…<\/p>\n\n\n\n
Even worse, this trick will also work with the use of ===<\/code>.<\/p>\n\n\n\n
  var i = 0;\n\n  with({\n    get a() {\n      return ++i;\n    }\n  }) {\n    if (a !== a)\n      console.log("yep, this is printed.");\n  }<\/code><\/pre>\n<\/blockquote>\n","protected":false},"excerpt":{"rendered":"
javascript – Can (a== 1 && a ==2 && a==3) ever evaluate to true? – Stack Overflow Yes, it can. IMHO it is one of the several undesirable consequences of loosely typed languages. In fact, according to an almost anonymous user: Worst, it is also possible using the === operator!<\/p>\n
Read more →<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"inline_featured_image":false,"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":4,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"","footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[72,50,278],"tags":[],"class_list":["post-10970","post","type-post","status-publish","format-standard","hentry","category-documentations","category-javascript","category-tricks"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p6daft-2QW","jetpack-related-posts":[{"id":9317,"url":"https:\/\/monodes.com\/predaelli\/2022\/04\/28\/7-more-killer-one-liners-in-javascript-by-tapajyoti-bose-apr-2022-medium\/","url_meta":{"origin":10970,"position":0},"title":"7 More Killer One-Liners in JavaScript | by Tapajyoti Bose | Apr, 2022 | Medium","author":"Paolo Redaelli","date":"2022-04-28","format":false,"excerpt":"7 More Killer One-Liners in JavaScript | by Tapajyoti Bose | Apr, 2022 | Medium This is a continuation of the previous list of JavaScript one-liners. 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