Live Javascript edit in Safari?

Currently this is not possible.

The closest option is to pause JavaScript and execute commands from the console.

To open Safari dev tools, press CtrlAltC on Windows or commandoptionC Mac. Or enable Safari dev commands in the menubar in Safari Preferences -> Advanced -> Show Develop Menu.

See Apple's docs on how to use its dev tools.

Unlike in Chrome, the Safari debugger does not currently support the ability to click on a script file and edit it in place. However, you can still stop execution using break points or the pause button, then execute code in the console to alter values, then resume execution.

For example, if you have the code:

var t = 1;
(function(){
    var t = 2;
    console.log(t);  //* put break point on this line..    
})();
console.log(t);

And you but a break point where indicated, then run t = 4 in the console, the value 4 then 1 are printed to the console.