The Definitive Guide to Debugging JavaScript

Debugging is a crucial skill for programmers. Even when we put our best effort into writing code, there can still be issues like crashes, slow performance, or incorrect output. When faced with these situations, the first step is to identify where the problem may be originating from. Is it an environmental issue? Is it related to the input data? Is it a one-time crash caused by excessive memory usage? Or is it happening consistently?

Having a clue about where the error may be coming from allows us to focus our debugging efforts on that specific part of the code. One simple way to start debugging is by reading the code aloud, as hearing it in our own voice can often reveal problems that were not apparent while reading silently.

If reading the code doesn’t provide any insight, the next logical step is to add some lines of code that can provide more information. In JavaScript, we commonly use alert() and console.log() to achieve this. For example, if the calculation of a result is not correct, we can add alert(a) and alert(b) to display the values of a and b before the calculation. Alternatively, we can use console.log() to print the values to the JavaScript console of the browser’s developer tools.

The browser’s developer tools are a powerful set of tools that include a JavaScript console for debugging. It allows us to print complex objects or arrays and inspect their properties. One of the most powerful features of the developer tools is the debugger. It can be found in the Sources panel and allows us to set breakpoints in our code. When a breakpoint is encountered, the execution of the code is paused, and we can inspect the current state of our program, including variable values. We can then resume the execution step-by-step or jump to specific functions.

In addition to breakpoints, there are other types of breakpoints available, such as XHR/fetch breakpoints triggered by network requests, DOM breakpoints triggered by changes to DOM elements, and event listener breakpoints triggered by specific events like mouse clicks.

The developer tools also provide a scope panel that displays all the variables in the current scope and allows us to edit their values. We can add watch expressions to monitor the value of variables or the result of expressions. The tools also allow us to edit scripts on the fly, even while the script execution is halted.

If you’re working with libraries whose code you don’t want to step into during debugging, you can mark them as “blackboxed” to exclude them from the call stack. This is useful when you trust the library and don’t need to see its code during debugging.

In addition to debugging JavaScript in the browser, you can also use the Chrome DevTools to debug Node.js applications. By running Node.js with the --inspect flag and opening the dedicated DevTools for Node, you can debug Node.js applications using the familiar tools provided by the browser’s developer tools.

Debugging JavaScript is an essential skill for any developer. By familiarizing yourself with the browser’s developer tools and using techniques like breakpoints, console.log(), and watch expressions, you can efficiently identify and fix issues in your code.

Tags: JavaScript, debugging, browser devtools, breakpoints, console.log, scope, call stack, blackboxing, Node.js debugging.