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HAR files, the console, hard refreshes and bug reports, step by step, with a free tool wherever one saves the trouble.
Free toolHAR analyzerFree toolBrowser infoConsole and dev toolsWhat is a stack trace? How to read one in JavaScriptConsole and dev toolsWhat is a source map? How browsers find them and when to ship themConsole and dev toolsHow to open the console in Chrome, Edge, Firefox and SafariConsole errorsTypeError: Failed to fetch: what it means and how to find the causeConsole errorsCannot read properties of undefined: causes and fixesBrowser and deviceWhat is a user agent? How to read a user agent stringBrowser and deviceHow to hard refresh in Chrome, Edge, Firefox and SafariHAR filesWhat is a HAR file? How to make, read and share one safelyBug reportsThe bug report template that actually gets bugs fixed