Created
September 17, 2021 01:21
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One works and the other freezes the browser... Why?
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| <!DOCTYPE html> | |
| <html> | |
| <head> | |
| <script> | |
| let _openCVPromise = null; | |
| function getOpenCV() { | |
| if (_openCVPromise === null) { | |
| _openCVPromise = new Promise(function(resolve, reject) { | |
| const script = document.createElement("script"); | |
| script.setAttribute("src", "https://shmeppy-external-js.sfo3.cdn.digitaloceanspaces.com/opencv-4.5.3.js"); | |
| script.addEventListener("load", function() { | |
| resolve(window.cv); // !! Only difference | |
| }); | |
| document.head.appendChild(script); | |
| }); | |
| } | |
| return _openCVPromise; | |
| } | |
| </script> | |
| </head> | |
| <body> | |
| <button onClick="console.log(getOpenCV())">load</button> | |
| <button onClick="console.log('OpenCV loaded', !!window.cv)">Beep</button> | |
| </body> | |
| </html> |
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| <!DOCTYPE html> | |
| <html> | |
| <head> | |
| <script> | |
| let _openCVPromise = null; | |
| function getOpenCV() { | |
| if (_openCVPromise === null) { | |
| _openCVPromise = new Promise(function(resolve, reject) { | |
| const script = document.createElement("script"); | |
| script.setAttribute("src", "https://shmeppy-external-js.sfo3.cdn.digitaloceanspaces.com/opencv-4.5.3.js"); | |
| script.addEventListener("load", function() { | |
| resolve(); // !! Only difference | |
| }); | |
| document.head.appendChild(script); | |
| }); | |
| } | |
| return _openCVPromise; | |
| } | |
| </script> | |
| </head> | |
| <body> | |
| <button onClick="console.log(getOpenCV())">load</button> | |
| <button onClick="console.log('OpenCV loaded', !!window.cv)">Beep</button> | |
| </body> | |
| </html> |
Ahhhh that makes sense. Thank you for solving this mystery.
Though I have a new mystery. I forget why I made this gist. Where did you find a link to this?
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After many hours of bug hunting I found this out for my own sake, feeling quite triumphant!
The reason that
resolve(window.cv);freezes the browser is thatcvis infinitely thenable, so an infinite recursion is started.resolvechecks if the provided argument is thenable, and if so hooks up to it via itsthenmethod, passingresolveas argument I suppose. And if OpenCV has been initialized, then itsthenmethod calls the provided function with OpenCV itself as a argument, which has athenmethod, which is called and calls back with itself, which has athenmethod, which is called and calls back with itself, and so on...My solution was to wrap it:
resolve({cv: window.cv}).