DevTools includes a full source level debugger, supporting breakpoints, stepping, and variable inspection.
When you open the debugger tab, you should see all the libraries for you application listed in bottom left of the screen (under the Scripts area), and the source for the main entry-point for your app in the loaded in the main source area.
In order to browse around more of your application sources, you can scroll through the Scripts area and select other source files to display.
To set a breakpoint, click on the left margin (the line number ruler) in the source area. Clicking once will set a breakpoint, which should also show up in the Breakpoints area on the left. Clicking again will remove the breakpoint.
When your application encounters a breakpoint, it'll pause there, and the DevTools debugger will show the paused execution location in the source area. In addition, the Call stack and Variables areas will populate with the current call stack for the paused isolate, and the local variables for the selected frame. Selecting other frames in the Call stack area will change the contents of the Variables area.
Within the Variables area, you can inspect individual objects by toggling them open to see their fields. Hovering over an object in the Variables area will call the toString() method for that object and display the result.
When paused, the three stepping buttons become active.
Step in to step into a method invocation, stopping at the first executable line in that invoked methodStep over to step over a method invocation; this steps through source lines in the current methodStep out to step out of the current method, without stopping at any intermediary linesIn addition, the Resume button will continue regular execution of the application.
Console output for the running app (stdout and stderr) is displayed in the console, below the source code area.
To adjust the break on exceptions behavior, toggle the Break on unhandled exceptions and Break on all exceptions checkboxes in the upper right of the debugger UI.
Breaking on unhandled exceptions will only pause execution if the breakpoint is considered uncaught by the application code. Breaking on all exceptions will cause the debugger to pause whether or not the breakpoint was caught by application code.