Harry Cresswell

Practical JavaScript - Working with the Debugger

The process of going through your JavaScript and fixing issues with your code is called ‘debugging’.

The ‘debugger’ is a powerful tool we can use to assist us in the debugging process. Debugger is built into the browser and can be found in the chrome dev tools.

We can use the debugger to step through our methods in order to see what’s happening line by line.

To do this we need to add debugger; to the first line within our method.

Now when we call the method, Debugger will pause the program as it’s running, so we can step though and see what’s happening, at every stage of the method.


In our displayTodos method, we can add debugger; to the first line of the function:

displayTodos: function() {
if (this.todos.length === 0) {
  console.log('Your todo list is empty!')
} else {
  console.log('My Todos:');
  for (var i = 0; i < this.todos.length; i++) {
    if (this.todos[i].completed === true) {
      console.log('(x)', this.todos[i].todoText);
    } else {
        console.log('( )', this.todos[i].todoText);

Now when we run todoList.displayTodos(); in the console, the program will pause and debugger will launch in the Sources panel of Inspect.

At the top of the screen we see ‘Paused in debugger’ and notice the script in highlighted on the debugger; statement.

From here we can inspect the different elements of our program to help us debug our code.

On the right side, we can see the different variables in our program. Expand this and see that it’s an Object, with all our different methods on it.

Stepping through the code

On the right side you will see a button ‘Step over next function call’, when pressed this will go to the next line of our program, you’ll see the highlighted line changes too the one below.

If you hover over the values in the program, you can see what they are.

We can hit the Step button again to run the next line and step through the program to the end. Hit the blue button; ‘Resume script execution’ to get back to the console and run the script again.

Remember to remove debugger; from your program so the code works as it did before.


For our addTodo method we can do the exact same thing as before and add debugger; to the first line inside the method.

addTodo: function(todoText) {
    todoText: todoText,
    completed: false

Step 1: run debugger

Now run the method todoList.addTodo('runnin the debugger');

The debugger will pause the program and launch. We can see debugger has paused on the line we added debugger;.

Step 2: Stepping over

Step over the line with the ‘Step over next function call’ button. Here we can hover over this.todos and see the value is currently 0.

Step over once more and here we’re pushing an item to the todos array. You can also see the value of the parameter todoText which equals ‘runnin the debugger’.

Step over again. Now we’ve pushed an item to todos. on the right side we expand the this object, and the todos property will now show us an array with 1 item. If you expand the first item, you will see ‘runnin the debugger’ as the value of todoText.

Step over to complete the function. Then hit ‘resume script execution’ to exit the debugger.

Now you will see ‘runnin the debugger’ printed to the console.

Step 3: Stepping into a function call

Now add another todo item.

todoList.addTodo('adding something extra');

The debugger will pause as expected. Next, keep stepping through until you reach the last section of the method; this.displayTodos();.

Here we can click the down arrow; ’step into the next function call’, to inspect displayTodos. Now we’re in the displayTodos.

As we now have 2 items in our todos array, todos.length is not equal to 0, so when we step through the function it skips the next line. Instead it enters our else statement.

Here we can see step through our for loop to see how it works.

Step 4: Stepping through a for loop

If we step through, we can see how the value of i (our initialisation variable) increases each time the loop is run.

It does this until we reach the todos.length condition, which in this case is 2.

When it hits 2, and the condition is no longer true, so we exit the for loop. And our script ends.

We can hit the blue button to resume our script. Don’t forget to remove debugger; for the script to run as normal.


Now let’a do the same with the changeTodo method by adding debugger to our script.

Add a new item so we can change it todoList.addTodo('something to change');

Next, run the changeTodo method, todoList.changeTodo(0, 'changed');. Now the debugger will pause the program.

From here you can see the arguments we entered in yellow; position is 0 and todoText is ‘changed’.

Now step over to the next line. Then if we go into Scope on the right hand side, expand the this object, then expand the todos array and you will see the object in the 0 position has a todoText property with the value 'something to change'.

Step over again and we will see this value update to 'changed'.

We’ve now reach the displayTodos method.


Move debugger; to the deleteTodos method, then add a todo item:

todoList.addTodo('delete this');

Next run todoList.deleteTodo(0); to pause the script and run the debugger.

Now we’re in the debugger and we can see the position is 0, as that’s what we set. Step over to the next line.

this.todos.splice(postion, 1);

This is going to delete whatever is at position 0 and delete 1 item, in our case the value 'delete this'.

In the Scope panel on the right, watch the value of todos Array go from todos: Array[1] to todos: Array[0], as you step over to the next line.

Now we can resume our script.


Move the debugger statement into toggleCompleted.

Add a new todo item; todoList.addTodo('to be toggled'); then toggle the item to enter debugger.


Step over to see we’re grabbing this.todos item in position 0. Now todo is undefined.

Step over again, now todo is defined. And if you hover over you can see completed: false and todoText: 'to be toggled'. This is the item we’re adding.

When we step over the next line you’ll see completed: true.


Start by moving the debugger; statement to the beginning of the toggleAll method, as we’ve done before.

Next we’ll need to add some data, so we have something to toggle.

todoList.addTodo('this is true');

Next make the item true. We can do this using the toggleCompleted method, parsing in 0, for the position of our new item.


The console should return:

My Todos:
(x) this is true

Now we can use todoList.toggleAll(); to pause the script and run the debugger.

Stepping through code with debugger

Step over the first line and you’ll see we’re saving the length of our todos array to the variable totalTodos. This should show our 1 item.

Next we start completedTodos at 0, so we can count up the number. In other words, each time we find a completed item in the array, we can increase completedTodos by 1.

This time when we step over we reach the for loop. Step over again to set the undefined i to 0. As 0 is less than totalTodos of 1, we go into the for loop when we step over.

Here we have a condition that states if the completed property of our todos item is set to true (which in our case it is), we enter the if statement and increase completedTodos by 1. We’re using the value of i to grab the todo item at position 0.

Step over once more and you’ll see completedTodos is now set to 1.

At this point we return to the start of the for loop, where our final expression i++ increases i by 1. As 1 is no longer less than 1, but equal to 1, we leave the for loop and arrive at Case 1.

In Case 1 we check if everything is true, in which case we will make everything false.

We do this by checking if completedTodos is equals to totalTodos. In our case they are both 1, so the condition is true.

As the condition is true, we go into the if statement and we reach a for loop. Here i is reset to 0. Which is less than totalTodos, currently 1. So we go into the for loop when we step over again.

Now we grab the only item we have, in position 0, and set completed to false.

Finally, we return to the start of the for loop and increment i by 1. When we evaluate the condition again it’s no longer true, as 1 is not less than our totalTodos which is 1. Now we exit the for loop.

This time we skip our else statement in case 2 because case 1 was true.

Use the debugger all the time

Debugger helps you solve problems with your code before you start asking questions. Use debugger to understand your code even if theres nothing wrong with it.

If you have a question like what is this line of code doing, use the debugger.

Don’t ask questions that the debugger can answer.

Focus on understanding, not building from scratch

Focus on understanding code, not building from scratch. Use the debugger to understand the code deeply. Then forget about it.

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