To debug JavaScript in Chrome, open DevTools, select Sources, set a breakpoint in the relevant script, and reproduce the problem. When execution pauses, inspect values in Scope and Watch, check the Call Stack, and step through the code to see what happens. Chrome DevTools is built into Chrome; you do not need a separate debugger product.
Open Sources and find the code
Open Chrome DevTools and select the Sources panel. It contains a file tree, a code editor, and debugger controls. Locate the script that appears to handle the behavior you want to investigate. The precise pane arrangement can vary with the DevTools window width.
The Chrome DevTools documentation describes Sources as the place to debug JavaScript: Debug JavaScript.
Set a breakpoint and reproduce the problem
- In Sources, open the relevant script.
- Click the line number beside the statement where you want execution to stop. A breakpoint should appear on that line.
- Return to the page and repeat the action that triggers the behavior.
- When execution reaches the breakpoint, Chrome pauses and highlights the current line.
A line breakpoint is most useful when you have a reasonable idea which statement is involved. If the trigger is clearer than the code location, use an event, DOM-change, or exception breakpoint instead.
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Inspect state while execution is paused
Scope: values available at this point
Use Scope to inspect values available to the paused code, including local, closure, and global properties. These values reflect the selected call frame, so selecting a different frame in the Call Stack can show a different scope.
Watch: expressions you want to track
Add valid JavaScript expressions to Watch when you want to follow particular values while stepping. Watch expressions refresh as execution advances. This is useful for checking whether a value changes before or after a particular statement.
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Console: test an expression in the paused context
The Console can evaluate expressions while execution is paused. Use it to inspect a value or test a focused expression against the current context. Evaluation can affect page state if the expression has side effects, so prefer read-only expressions unless changing state is intentional.
Step through the code and trace the call path
- Step into enters a function called by the current statement, letting you inspect its execution.
- Step over runs the current call without entering it, which is useful when that function is not the focus.
- Step out finishes the current function and returns to its caller.
- Resume continues execution until another breakpoint or pause condition is reached.
- Continue to here runs execution to a selected later line in the current function, handy when the intervening statements are not relevant.
Read the Call Stack to see which functions led to the pause. Select a frame to inspect its call site and associated scope. Async frames may appear when the framework supports async stack tagging; their availability is not guaranteed for every application.
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Choose a breakpoint for the kind of failure
| Breakpoint type | Use it when | What it reveals |
|---|---|---|
| Line breakpoint | You suspect a particular statement or section of a script. | Execution stops when it reaches the selected line. |
| Event-listener breakpoint | A browser or interface event, such as a user interaction, appears to trigger the bug. | Execution pauses when a selected event handler runs. |
| DOM-change breakpoint | You know which node is changing but not which code changes it. | Execution pauses when the selected node, its attributes, or its children change. |
| Exception breakpoint | You need to find where an error is thrown. | Execution can pause on caught or uncaught exceptions, depending on the selected option. The documented page notes a Node.js limitation for caught exceptions. |
| Function breakpoint | You want to stop whenever an in-scope function is called. | Enter debug(functionName) in the Console to pause on calls to that function. |
Pick the stopping point that best matches the clue you have: code location, event, node mutation, exception, or function call. Once paused, step into only the calls relevant to the issue and step over unrelated ones.
Debug bundled or minified code with source maps
Chrome runs the deployed JavaScript, which may be bundled or minified. If the build generates source maps and the server makes them available, DevTools can map the deployed code back to authored files. Breakpoints, errors, and logs can then point to those authored files rather than only the transformed output.
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If authored files do not appear, check that source maps are generated by the build and can be loaded from the server. Without usable maps, DevTools cannot show the original source through this mapping workflow.
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- The breakpoint never pauses: Confirm the page action actually runs the script and reaches that line. If you are looking at bundled output, check whether source maps are available and whether you set the breakpoint in the code that is executing.
- The file is difficult to read: Check for source maps from the build and verify the server serves them. Otherwise, you may be viewing deployed bundled or minified code.
- Scope shows unexpected values: Confirm you selected the intended Call Stack frame. Scope displays values available to that frame, not necessarily the frame you were inspecting previously.
- A DOM-change breakpoint does not stop where expected: Verify that the selected node is the one being mutated and that the chosen breakpoint covers the relevant change, such as attributes or children.
- An exception breakpoint does not catch the case you expect: Check whether the exception is caught or uncaught and which exception option is enabled. The documented guide identifies a Node.js limitation for caught exceptions.
- Async history is missing: Async stack visibility can depend on framework support for async stack tagging; the full history may not be available.
- You cannot edit a paused function live: Live editing has restrictions. The function must meet DevTools’ Call Stack requirements, including being the top-most function; recursive calls and some function types also have limits. Use normal source changes for durable fixes.
DevTools labels and keyboard shortcuts can vary by version and platform. When a shortcut or control is not where expected, use the current interface labels rather than relying on a shortcut from another setup.
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