Once you've paused execution on a breakpoint, you may want to follow execution line-by-line to observe what happens. Open your browser's Developer Tools and look for the Execution Control icons. (This example uses the icons in Google Chrome, but they'll be similar in other browsers.)
Step Over: Run the next line of code. If that line contains a function call, run the whole function and move to the next line, rather than jumping to wherever the function is defined. Shortcut : F10(Chrome, Firefox, IE/Edge), F6(Safari)
Use these in conjunction with the Call Stack, which will tell you which function you're currently inside of, which function called that function, and so forth.
See Google's guide on "How to Step Through the Code" for more details and advice.
Links to browser shortcut key documentation: