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JavaScript

Errors in the code that runs in your visitors' browsers. 6 answers.

JavaScript error Open the browser console (F12). The red error names the problem, the file and the line. That's the starting point for every JavaScript error. "Cannot read property of undefined" Code tried to read a property of something that's undefined or null, usually data that hasn't loaded yet or an element that isn't on the page. "undefined is not a function" Code tried to call something as a function that isn't one: a misspelled name, a method that doesn't exist on that kind of value, or a library that didn't load. Script not loading Check the browser's Network tab for the script: a 404 means a wrong path, a blocked request means a Content Security Policy, mixed content or an ad blocker. React error If a React app goes blank, an error during rendering removed the UI. The console shows which component failed. Error Boundaries stop one bug blanking it all. Vue error Read the console: Vue warnings name the component. Most Vue errors are reading data before it loads or a template referring to something undefined.

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