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How to Debug Javascript Code

JavaScript debugging is the process of finding and fixing errors (or bugs) in your code.

Here are some common debugging techniques and tools you can use:

Console logging​

You can use the console.log()method to print out values or messages to the browser console. This can help you understand what's happening in your code and where things might be going wrong.

Debuggers​

Most modern browsers have built-in JavaScript debuggers that allow you to step through your code line by line, set breakpoints, and inspect variables and objects at different points in the code. You can access the debugger in your browser's developer tools.

Syntax checkers​

There are various online tools and IDEs that can help you catch syntax errors in your JavaScript code before you run it. These can save you time by highlighting problems before you even try to debug your code.

Linters​

Linters are tools that analyze your code for common errors and potential problems, such as undefined variables, unused code, or incorrect variable scoping. You can use a linter like ESLint in your code editor to help catch errors and improve the quality of your code.

Try/Catch blocks​

You can use try/catch blocks to catch and handle errors in your code. This can prevent your code from crashing if an error occurs, and can help you identify where the problem is happening.

Code review​

Having another developer review your code can help you catch errors that you might have missed, as well as provide feedback on ways to improve your code.

Debugging can be a time-consuming process, but using these techniques and tools can help you identify and fix problems more quickly and efficiently.

The debugger Keyword​

In addition to using browser dev tools, console logging, and other debugging techniques, JavaScript also provides a built-in keyword that can be used to help debug your code - the debugger keyword.

The debugger keyword is a special keyword that can be inserted into your code at a particular point where you want to pause execution and inspect the state of your code. When the JavaScript interpreter encounters the debugger keyword, it will stop execution at that point and allow you to interactively debug your code.

Here's an example of how you can use the debugger keyword in your code:

function multiply(a, b) {
  debugger;
  return a * b;
}

let result = multiply(5, 10);
console.log(result);

In this example:

  • We have a simple function that multiplies two numbers together.
  • We've inserted the debugger keyword at the beginning of the function, which will cause the JavaScript interpreter to pause execution when it reaches that point.

When you run this code in your browser and open the dev tools, you should see the debugger pause execution at the line with the debugger keyword. You can now use the dev tools to inspect the state of your code, set breakpoints, step through the code line by line, and more.

Setting Breakpoints​

Setting breakpoints is a common debugging technique used to pause execution of your JavaScript code at specific lines or points in your code. This allows you to inspect the state of your code, evaluate expressions, and step through the code line by line to identify and fix errors.

To set a breakpoint in your code using your browser's developer tools,

follow these steps:

  • Open your browser's developer tools by pressing F12 or right-clicking on the page and selecting "Inspect".

  • Navigate to the "Sources" tab.

  • Locate the JavaScript file you want to debug in the file tree.

  • Click on the line number where you want to set the breakpoint. A blue marker should appear indicating that a breakpoint has been set.

  • Run your code and execution will pause at the breakpoint.

Once you've set a breakpoint, you can use the debugging tools to inspect the state of your code, evaluate expressions, and step through the code line by line. You can also add additional breakpoints or remove existing ones as needed.

caution

The breakpoints can significantly slow down the execution of your code, so it's a good idea to only use them when necessary and remove them when you're done debugging.