{"id":30565,"date":"2023-01-15T15:56:22","date_gmt":"2023-01-15T10:26:22","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-has-something-changed-in-javascript-implementations-recently-or-is-it-me\/"},"modified":"2023-01-15T15:56:22","modified_gmt":"2023-01-15T10:26:22","slug":"solved-has-something-changed-in-javascript-implementations-recently-or-is-it-me","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-has-something-changed-in-javascript-implementations-recently-or-is-it-me\/","title":{"rendered":"[Solved] Has something changed in Javascript implementations recently (or is it me)?"},"content":{"rendered":"

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No, this hasn’t changed (there’s something similar<\/em> that’s changed, but not this; more below). Here’s the difference: If item<\/code> isn’t declared<\/em>, then you’ll get the error about item<\/code>. If item<\/code> is declared but has the value undefined<\/code>, you’ll get the error about reading a property of undefined<\/code>. This has always been the case.<\/p>\n

You mentioned Firefox. Firefox’s error messages for these two different conditions are confusingly similar. If the variable is undeclared<\/em>, it says:<\/p>\n

ReferenceError: item is not defined<\/pre>\n
But if the variable is declared and has the value undefined<\/code>, it says:<\/p>\n
TypeError: item is undefined<\/pre>\n
Note the difference, but the extreme<\/em> similarity. I would say Firefox’s error messages are poor in this regard. Chrome’s are more clearly distinct:<\/p>\n
ReferenceError: item is not defined\nTypeError: Cannot read property 'property' of undefined<\/pre>\n
But that’s just the error message. The underyling issue, trying to read the value of an undeclared symbol, has always been an error.<\/p>\n
Re the two answers you linked to: I’m sorry, but they were always wrong for the undeclared case<\/em> (and right for the undefined<\/code> case), this is just something that people don’t understand well. If item<\/code> may be completely undeclared<\/em>, you can’t use that idiom. You can<\/em> use typeof<\/code>, though:<\/p>\n
if (typeof item !== \"undefined\" && item.property)\n<\/code><\/pre>\n
typeof<\/code> is fine with undeclared symbols, but reading the value of an undeclared symbol is an error, as has been for as long as we’ve had JavaScript.<\/p>\n
Here’s a question from 2011 about this. The accepted answer with a score of 1,016 talks about typeof<\/code>.<\/p>\n
Here are examples of both cases:<\/p>\n
Undeclared:<\/h3>\n<\/p>\n
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\/\/ Note that `item` isn't declared anywhere at all\r\ntry {\r\n  if (item.property) {\r\n    snippet.log(\"true\");\r\n  }\r\n}\r\ncatch (e) {\r\n  snippet.log((e.name || \"Exception\") + \": \" + (e.message || String(e)));\r\n}<\/code><\/pre>\n
<!-- Script provides the `snippet` object, see http:\/\/meta.stackexchange.com\/a\/242144\/134069 -->\r\n<script src=\"http:\/\/tjcrowder.github.io\/simple-snippets-console\/snippet.js\"><\/script><\/code><\/pre>\n<\/div>\n<\/div>\n
Declared but has the value undefined<\/code>:<\/h3>\n<\/p>\n
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var item;\r\ntry {\r\n  if (item.property) {\r\n    snippet.log(\"true\");\r\n  }\r\n}\r\ncatch (e) {\r\n  snippet.log((e.name || \"Exception\") + \": \" + (e.message || String(e)));\r\n}<\/code><\/pre>\n
<!-- Script provides the `snippet` object, see http:\/\/meta.stackexchange.com\/a\/242144\/134069 -->\r\n<script src=\"http:\/\/tjcrowder.github.io\/simple-snippets-console\/snippet.js\"><\/script><\/code><\/pre>\n<\/div>\n<\/div>\n
The similar thing that’s changed is how assigning<\/em> to something that isn’t declared at all is handled:<\/p>\n
item = \"foo\";\n<\/code><\/pre>\n
In loose mode, that’s an implicit global variable<\/em><\/a>, even if the assignment is inside a function. In ES5’s strict mode, though, it’s an error.<\/p>\n
But that doesn’t relate to reading<\/em> the value of an undeclared variable, which was always an error, just writing<\/em> to it.<\/p>\n<\/p><\/div>\n
<\/div>\n<\/div>\n
            1<\/span> <\/p><\/div>\n<\/div>\n
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solved Has something changed in Javascript implementations recently (or is it me)? <\/p>\n","protected":false},"excerpt":{"rendered":"
[ad_1] No, this hasn’t changed (there’s something similar that’s changed, but not this; more below). Here’s the difference: If item isn’t declared, then you’ll get the error about item. If item is declared but has the value undefined, you’ll get the error about reading a property of undefined. This has always been the case. You … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[333],"class_list":["post-30565","post","type-post","status-publish","format-standard","hentry","category-solved","tag-javascript"],"yoast_head":"\n[Solved] Has something changed in Javascript implementations recently (or is it me)? - JassWeb<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jassweb.com\/solved\/solved-has-something-changed-in-javascript-implementations-recently-or-is-it-me\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"[Solved] Has something changed in Javascript implementations recently (or is it me)? - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] No, this hasn’t changed (there’s something similar that’s changed, but not this; more below). 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