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To debug JavaScript in a web page, start in your browser’s Developer Tools Console: find the first relevant error and note its file and line. Open that location in the source debugger, set a breakpoint, reproduce the problem, and inspect the values, scope, and call stack while execution is paused. The Console helps you spot errors and test small expressions; the debugger helps you trace how execution reached a failure.
Start with the Console
Open your browser’s Developer Tools and select the Console panel. You can usually open Developer Tools from the browser’s menu; exact menu names, shortcuts, and panel layouts vary by browser and operating system. If you need a shortcut, check your browser’s current help rather than relying on one that may not apply to your setup. MDN’s browser developer-tools guide explains what these tools are and why their interfaces differ.
- Reproduce the problem with the Console open.
- Look for the first error that appears to relate to the behavior. Later errors may be consequences of the first failure.
- Read the error and note its script file and line reference, if provided. Wording and details vary by browser.
- If the failure happens only after a particular action, repeat that action and watch for errors at that moment.
The Console can also evaluate JavaScript in the context of the loaded page. Use it to check a small expression or inspect current DOM state. For example, document.title returns the page title, and document.querySelector('h1') checks whether the page has a matching heading. These are quick observations of the current page, not a substitute for examining the code path that produced the behavior. Avoid running unfamiliar code in a page’s Console: it executes with access to that page’s context.
Move from an error to the debugger
Use the error’s file and line reference as your starting point. Chrome names its source debugger Sources; Firefox calls it Debugger. Other browsers have their own layouts and labels. The underlying workflow is similar: open the relevant script, set a breakpoint, and reproduce the problem. MDN’s JavaScript debugging guide covers this workflow, and Chrome DevTools’ JavaScript debugging guide documents Chrome’s interface.
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- Open the script identified by the Console error in the debugger. If the browser displays a linked file-and-line reference, select it to navigate there.
- Set a breakpoint on the failing statement or just before it. In source panels, this is commonly done by selecting the line number; control placement can vary with the browser.
- Reload the page or repeat the action that triggers the code. Execution pauses when it reaches the breakpoint.
- Inspect the paused state before stepping: check the values of relevant variables, the current scope, and the call stack.
- Step through statements one at a time. Compare what the code expects with the actual values, and note the first point where execution diverges from the expected path.
A breakpoint is useful because it freezes execution at a specific point, letting you inspect the state rather than infer it only from the final error. The call stack shows how execution arrived at that point; scope shows which variables are available there. Together, they can reveal whether a value is missing, unexpected, or coming from a different caller than you assumed.
Pause with a debugger; statement
If a clickable source breakpoint is awkward to place, add debugger; at the point where you want execution to pause, then reproduce the behavior with Developer Tools attached:
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function updateTotal(items) {
debugger;
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}When debugging functionality is available, this statement can pause execution like a breakpoint. Without an attached debugger, it has no effect. Remove it after investigating, or leave it only when an intentional pause is part of your development workflow; an unexpected pause is disruptive for anyone debugging that code later. See MDN’s debugger reference.
Debug minified or bundled code with source maps
Production pages often run generated, bundled, or minified scripts whose names and formatting differ from the original source. A source map can connect the deployed code in the browser to the original files, making it easier to set breakpoints and inspect the code you recognize. Chrome documents this behavior in its source maps guide.
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If the original source does not appear in the debugger, check whether the deployed script points to a source map and whether that map is available to the browser. Without an accessible, working map, you may need to inspect the generated script itself. A map helps relate generated code to its sources; it does not guarantee that every deployment exposes the original files.
Choose the right tool for the question
| Tool | Use it to | What to inspect |
|---|---|---|
| Console | Find errors and test small expressions against the loaded page. | Error details, file and line references, and the result of an expression. |
| Source debugger | Pause execution and trace the code around a failure. | Variable values, scope, call stack, and the statements executed next. |
Use the Console to find a useful starting point; use the debugger when you need to see what happened before or at the failure. The panels work together, but they answer different questions.
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Troubleshoot common debugging problems
- No error appears: Keep the Console open while reproducing the issue, including the action that triggers it. A behavior can be wrong without producing a JavaScript error, so inspect the relevant code with a breakpoint as well.
- The error points to a line you do not recognize: The page may be running bundled or minified code. Look for an available source map; if the original source is not shown, check that the deployed script references an accessible map.
- The breakpoint does not pause: Confirm that the breakpoint is on the code path being run, then reload or repeat the action that reaches it. If generated code makes placement difficult, try a temporary
debugger;statement. - A variable is unavailable or has a surprising value: Make sure execution is paused in the expected function and at the expected point. Check the current scope and call stack; a variable may not exist in that scope or may not yet have been assigned the value you expected.
- The error message differs from an example: Browser wording can vary. Use the file and line reference, the point at which the error occurs, and the paused execution state to investigate rather than relying on an exact phrase.
- You cannot find a panel or control: Panel names and layouts differ across browsers and may change. Look for the current Console and source-debugging tools in your browser’s Developer Tools or consult its current official help.
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curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpEquivalent Python and Node.js requests:
import requests
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