This code is based on some real production code we have at my day job, but simplified so we can focus on debugging, rather than understanding what the code is doing. Look for any variables which are being passed around with incorrect values. This means that every exception that is thrown results in Chrome pausing the execution.
The red icon on the line indicates an exception was thrown. Occurs when attempting to call a value like a function, where the value is not a function. To do so, we can resume script execution using the button on the right that looks like a Play button. Means the code called a function that you should not call at the current state. You should use step into whenever you want to know what a function is doing.
You can play around with each of these options to get a better understanding. His clients include companies like Nokia and hot super secret startups. Step out executes the remainder of the code in the current function, and returns you one step out of the call stack. This is especially useful for objects which may contain many properties. In the past, I would use console.
You can use step out to go back the where the function was called and continue from there. Most of the time it works. We can also see the line of code which called our original function. Discussion Sergey Gospodarets.
As developers, much of our time is spent fixing problems. Attempting to write null or undefined as if it was an object. Definitely a topic I might write about in the future! When this is enabled, we can also optionally make Chrome pause on caught exceptions. This code has some logic to generate a list of objects based on any element with a class in the document.
Maximum recursion depth exceeded, too much recursion, Stack overflow. This too is usually caused by typos. These are the main features of the debugger, and all the other functionality builds on top of them.
The line number is now highlighted to indicate it has a breakpoint. Head banging time finished.
We can now use either step into or step over to run if-clause and see what happens. However, there are times that our api server is so heavily busy. One of the secrets to writing robust code is to understand why something happens.
Attempting to read null or undefined as if it was an object. In this example, the programmer accidentally used a single equals instead of two. Awesome article, well done. We can also further inspect any variables by hovering the cursor over their names.
This time, the code pauses at the breakpoint we set from the debugger. The line highlighted in blue is where the code is paused. To pause code execution anywhere we like, we can use a breakpoint. It looks like the type value comes from the getType function, so we need to go into it and find out what goes wrong. Others like Flow and TypeScript could also be useful, though may require a bit more work to set up.
On the highlighted line, we can see the code is trying to call forEach. This means the condition did not result in a truthy value. Our current function is highlighted, Loader. Ensure the function name is correct. Step into will go into any function called on the current line.
We can use step into to go inside the function to take a look. In the code view, Chrome gives us readouts on variable values. It causes an exception to be thrown. We can add breakpoints from the script debugger by clicking on the line numbers on the left side of the code view. Just sprinkle them in my code, and hope that they give me enough information to fix the problem.
Check recursive functions for bugs that could cause them to keep recursing forever. Typically caused by mismatched parentheses or brackets. We can see that the iteration of the forEach is currently in the div element.
No need to add random logging and try to guess what went wrong. We want to see what happens for the image element. We can access all the previous functions by clicking them in the call stack list. We need to identify the root cause of the problem before we fix it.
It still executes the code inside the function as normal. Looks like we found our problem. The code execution jumps over the if-statement. This is just scratching the surface. The approach an experienced developer takes to fix bugs is to find their root cause.
Wow, I thought they were standard practice in the industry nowadays, does anyone really still run their code without checking it with some tool first? The developer tools have many more useful features in them. We can find the script debugger from the Sources tab of the developer tools. The nodeName value is in uppercase, run while our condition checks for a lowercase value.
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