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created at
2025-12-04 10:01:22
updated at
2025-12-04 10:01:24
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https://hillelwayne.com/post/javascript-puzzle/
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2025-08-06 08:16:04
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title
A Perplexing Javascript Parsing Puzzle
excerpt
content

 What does this print? x = 1 x --> 0 Think it through, then try it in a browser console! Answer and explanation in the dropdown. 
 
 
 
 
 
 Show answer
 
 It prints 1. wait wtf At the beginning of a line (and only at the beginning of a line), --> starts a comment. The JavaScript is parsed as x = 1; x; // 0 The browser then displays the value of the last expression, which of course is 1. but why It’s a legacy hack. Netscape Navigator 2 introduced both JavaScript and the <script> tag. Older browsers in common use (like Navigator 1) had no idea that <script> content was anything special and wrote it as regular text on the page. To ensure graceful degradation, webdevs would wrap their scripts in html comment blocks: <script> <!-- console.log("hello world!") --> </script> Old browsers would parse the content as an HTML comment and ignore it, new browsers would parse the content as JavaScrip...
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