Node.js Debug Application

As a developer, while implementing the applications we will debug the code which we write to make sure that the functionality is working fine by using different tools. In node.js also we have different tools to debug the applications.

 

Following are the different ways to debug node.js applications.

 

  • Using node.js built-in debugging client
  • By inserting a debugger statement
  • Node Inspector

Node.js Built-in Debugging Client

Node.js has provided a built-in debugging client to debug an application. To debug an application with debugging client, we need to start node.js with an inspect argument followed by the path of the script to debug like as shown in the following statement.

 

node inspect script_file_name.js 

Here, inspect is an argument to enable debugging and script_file_name.js is the path of a script file which we want to debug.

 

Following is the example of enabling a debugger for simpleisexample.js script file which we created in a previous node.js NPM article.

 

node inspect simpleisexample.js

When we execute the above statement, we will get a successful launch of the debugger and the debugging will always start from the starting point of the script like as shown below.

 

Node.js Debugger Statement Example Result

 

If you observe the above image, we have a greater than (>) symbol to indicate the current debugging statement.

 

Now, we can continue the script file debugging in node.js by using following commands.

 

CommandDescription
next / n The debugging will be moved to next statement.
cont / c Continue exection and stops at debugger statement if any.
step / s Step in function.
out / o Step out function.
pause Pause running code
watch Add expression or variable to watch while debugging.
watchers It will print the values of expressions and variables which are added to the watch.
unwatch Remove expression or variable from watch.

Following is the example of moving the debugging to the next statement using next command in node.js.

 

Node.js debug application with next line command

 

This is how we can debug the code in node.js using built-in debugging client.

Use Debugger Statement

Actually, the node.js debugging client is not a full-featured debugger and the code debugging will always start from the starting of the script. In case, if we want to debug from the specific position of code statements, then it’s better to use debugger statement.

 

By inserting debugger; statement in the source code of script will enable a breakpoint at the specified position of code.

 

Now, create a script file called mathoperations.js and write the code like as shown following with debugger; statement.

 

var add = 25 + 50;

var sub = 50 - 25;

debugger;

var mul = 10 * 50;

var divsn = 50 / 25;

console.log("Addition Is :" , add);

console.log("Subtraction Is :" , sub);

console.log("Multiplication Is :" , mul);

console.log("Division Is :" , divsn);

If you observe above code, we are performing some arithmetic operations and added a debugger; statement to enable the breakpoint at required position of code.

Now, to enable the debugging for the above application, run the following command.

 

node inspect mathoperations.js

Once we execute the above command, the debugger will start and stops at the first line of the code like as shown below.

 

Node.js enable debugging with debugger statement

 

If you observe the above image, we have a greater than (>) symbol to indicate the current debugging statement.

 

Now, use cont or c statement to stop the execution directly at the position where we defined a debugger; statement like as shown below.

 

Node.js debug application with cont line command

 

If you observe the above image, after we execute cont statement the execution has stopped at the position where defined a debugger; statement.

 

Now, if we use the next statement the debugger will be moved to the next statement like as shown below.

 

Node.js debug application with next line command

 

This is how we can move the debugging to the next line of code in node.js based on our requirements.

Node.js Watch & Watchers Commands

In node.js, we can watch the expression and variable values while debugging using watch('expression') & watchers commands. On every breakpoint, the expressions in the watcher's list will be evaluated in the current context and displayed immediately before the breakpoint’s source code listing.

 

Following is the example of using watch command to add variables to an active watchers list.

 

Node.js debugger add items to watch list

If you observe the above example, we added three variables (add, sub, mul) to the watcher's list by using watch command.

 

Now, we can watch the active watchers list by using watchers command like as shown below.

 

Node.js debug application with watchers list

 

Now if we use cont statement, the execution will stop at next debugger; statement, if it found otherwise the code execution will be done like as shown below.

 

Node.js debugger after completion of application execution

 

If you observe the above result, the code execution has been completed because we didn’t mention any other debugger; statements in our code except at line 3.

Node.js Exit Application Debugging 

Once we are done with the debugging tasks and if we want to exit from debug mode, then we need to use a .exit command like as shown below.

 

Node.js exit debugging application

 

This is how we can do the debugging in node.js to cross verify the functionality of node.js applications based on our requirements.

 

In the next chapters, we will learn how to use the Node Inspector to debug the node.js applications.