The key to debugging JavaScript is to master tools and ideas. First, use browser developer tools to locate problems, open Console to view error information, and use Sources to set breakpoints to observe variable changes; second, use log printing reasonably, such as console.log({ variableName }), console.warn, console.error and console.table; then, step-by-step code execution by setting breakpoints to troubleshoot hidden problems, view variable values ??in Scope, and use Step Over / Step Into to control the execution process; finally, pay attention to common traps, such as variable declaration, parameter order, asynchronous processing and DOM loading timing, and improve code quality with ESLint, type annotation, unit testing and modularity.

Debugging JavaScript code is actually not mysterious, the key is to master the tools and ideas. Many people's first reaction when encountering bugs is "Where did I write it wrong", but the more effective way is: locate the problem in an orderly manner, observe the flow of data, and gradually troubleshoot.

Using browser developer tools is the most basic method

Modern browsers (such as Chrome and Edge) all come with developer tools, among which Sources and Console are the two most commonly used panels.

• Console can output logs, view error information, and execute temporary codes.
• Sources can set breakpoints, perform them in a single step, and observe variable changes.

It is recommended to press F12 or Ctrl Shift I to open the tool after opening the page. First, see if there is any obvious error message, such as "Uncaught ReferenceError", which can usually directly point out which line is wrong.

Log printing should be carefully considered, don't rely solely on console.log

Although console.log is very convenient, if you just type a few logs, you may not see too many problems. You can try:

• Add log to the key function entry and exit to confirm whether the process enters normally
• Use console.log({ variableName }) when outputting objects, so that variable names can be seen in the console, making it easier to identify.
• Use console.warn or console.error to distinguish different levels of information
• It will be clearer to view the data of the array structure with console.table

However, please remember to clean these debugging statements before going online, or use a special log library to manage them.

Setting breakpoints is the key to a deep understanding of code operation

If you want to know what the state of a variable is when a function is executed halfway, you should use breakpoints at this time.

The operation steps are roughly as follows:

• Open Sources panel
• Find the corresponding JS file
• Set a break point next to the line number
• Trigger the corresponding logic, the program will pause at the breakpoint
• Use Scope on the right to view the variable values ??in the current scope
• You can use the Step Over / Step Into button to execute step by step

This process is a bit like "slow-motion playback", which is especially suitable for troubleshooting hidden problems in asynchronous or loops.

Pay attention to common pitfalls and avoid low-level errors

Sometimes bugs are not logical issues, but syntax or call methods are wrong. for example:

• Forgot to add var / let / const causes variables to pollute globally
• The function parameters are written in reverse order, but the type is compatible without error
• The asynchronous function does not have await or handles promises, causing subsequent logic to be executed in advance
• DOM element is operated before it is loaded

Such problems can be prevented by:

• Check for potential syntax problems using tools like ESLint
• Add type annotations to key functions (such as TypeScript)
• Write unit test coverage boundary situation
• Try to organize code in a modular way to reduce side effects

Basically all that is it. Debugging is a meticulous job. If you practice it a few more times, you will naturally feel it.

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