Browser developer tools
Major browsers like Chrome, Firefox, Safari have built in developer tools that are accessible from the Inspect menu option.
Console
Quick commands can be executed into the DOM of the current page through the console tab.
Network
The network tab allows visiblity of all network requests made by the browser for that DOM instance.
DOM Inspect
Allows visiblity into the DOM tree and element picker and CSS inspection.
Sources
Debugger breakpoints can be set on the JS source files to step through the client side JavaScript code.